tag:theconversation.com,2011:/fr/topics/particle-physics-1449/articles
Particle physics – The Conversation
2024-03-03T19:19:44Z
tag:theconversation.com,2011:article/224372
2024-03-03T19:19:44Z
2024-03-03T19:19:44Z
Gravity experiments on the kitchen table: why a tiny, tiny measurement may be a big leap forward for physics
<figure><img src="https://images.theconversation.com/files/579074/original/file-20240301-30-ecsdm7.jpg?ixlib=rb-1.1.0&rect=3%2C16%2C2131%2C1536&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/string-theory-physical-processes-quantum-entanglement-733672807">Shutterstock</a></span></figcaption></figure><p>Just over a week ago, European physicists <a href="https://www.science.org/doi/10.1126/sciadv.adk2949">announced</a> they had measured the strength of gravity on the smallest scale ever. </p>
<p>In a clever tabletop experiment, researchers at Leiden University in the Netherlands, the University of Southampton in the UK, and the Institute for Photonics and Nanotechnologies in Italy measured a force of around 30 attonewtons on a particle with just under half a milligram of mass. An attonewton is a billionth of a billionth of a newton, the standard unit of force.</p>
<p>The researchers <a href="https://www.eurekalert.org/news-releases/1035222">say</a> the work could “unlock more secrets about the universe’s very fabric” and may be an important step toward the next big revolution in physics. </p>
<p>But why is that? It’s not just the result: it’s the method, and what it says about a path forward for a branch of science critics say may be trapped in a loop of <a href="https://www.prospectmagazine.co.uk/ideas/technology/38913/is-particle-physics-at-a-dead-end">rising costs and diminishing returns</a>.</p>
<h2>Gravity</h2>
<p>From a physicist’s point of view, gravity is an extremely weak force. This might seem like an odd thing to say. It doesn’t feel weak when you’re trying to get out of bed in the morning!</p>
<p>Still, compared with the other forces that we know about – such as the electromagnetic force that is responsible for binding atoms together and for generating light, and the strong nuclear force that binds the cores of atoms – gravity exerts a relatively weak attraction between objects. </p>
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Read more:
<a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Explainer: Standard Model of Particle Physics</a>
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<p>And on smaller scales, the effects of gravity get weaker and weaker.</p>
<p>It’s easy to see the effects of gravity for objects the size of a star or planet, but it is much harder to detect gravitational effects for small, light objects.</p>
<h2>The need to test gravity</h2>
<p>Despite the difficulty, physicists really want to test gravity at small scales. This is because it could help resolve a century-old mystery in current physics.</p>
<p>Physics is dominated by two extremely successful theories. </p>
<p>The first is general relativity, which describes gravity and spacetime at large scales. The second is quantum mechanics, which is a theory of particles and fields – the basic building blocks of matter – at small scales. </p>
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<strong>
Read more:
<a href="https://theconversation.com/approaching-zero-super-chilled-mirrors-edge-towards-the-borders-of-gravity-and-quantum-physics-162785">Approaching zero: super-chilled mirrors edge towards the borders of gravity and quantum physics</a>
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<p>These two theories are in some ways contradictory, and physicists don’t understand what happens in situations where both should apply. One goal of modern physics is to combine general relativity and quantum mechanics into a theory of “quantum gravity”. </p>
<p>One example of a situation where quantum gravity is needed is to fully understand black holes. These are predicted by general relativity – and we have observed huge ones in space – but tiny black holes may also arise at the quantum scale. </p>
<p>At present, however, we don’t know how to bring general relativity and quantum mechanics together to give an account of how gravity, and thus black holes, work in the quantum realm.</p>
<h2>New theories and new data</h2>
<p>A number of approaches to a potential theory of quantum gravity have been developed, including <a href="https://theconversation.com/explainer-string-theory-2983">string theory</a>, <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5567241/">loop quantum gravity</a> and <a href="https://link.springer.com/article/10.1007/s41114-019-0023-1">causal set theory</a>.</p>
<p>However, these approaches are entirely theoretical. We currently don’t have any way to test them via experiments.</p>
<p><div data-react-class="Tweet" data-react-props="{"tweetId":"1761104918780461222"}"></div></p>
<p>To empirically test these theories, we’d need a way to measure gravity at very small scales where quantum effects dominate.</p>
<p>Until recently, performing such tests was out of reach. It seemed we would need very large pieces of equipment: even bigger than the world’s largest particle accelerator, the Large Hadron Collider, which sends high-energy particles zooming around a 27-kilometre loop before smashing them together. </p>
<h2>Tabletop experiments</h2>
<p>This is why the recent small-scale measurement of gravity is so important.</p>
<p>The experiment conducted jointly between the Netherlands and the UK is a “tabletop” experiment. It didn’t require massive machinery.</p>
<p>The experiment works by floating a particle in a magnetic field and then swinging a weight past it to see how it “wiggles” in response.</p>
<p>This is analogous to the way one planet “wiggles” when it swings past another.</p>
<p>By levitating the particle with magnets, it can be isolated from many of the influences that make detecting weak gravitational influences so hard.</p>
<p>The beauty of tabletop experiments like this is they don’t cost billions of dollars, which removes one of the main barriers to conducting small-scale gravity experiments, and potentially to making progress in physics. (The latest proposal for a bigger successor to the Large Hadron Collider would <a href="https://www.nature.com/articles/d41586-024-00353-9">cost US$17 billion</a>.)</p>
<h2>Work to do</h2>
<p>Tabletop experiments are very promising, but there is still work to do.</p>
<p>The recent experiment comes close to the quantum domain, but doesn’t quite get there. The masses and forces involved will need to be even smaller, to find out how gravity acts at this scale. </p>
<p>We also need to be prepared for the possibility that it may not be possible to push tabletop experiments this far.</p>
<p>There may yet be some technological limitation that prevents us from conducting experiments of gravity at quantum scales, pushing us back toward building bigger colliders.</p>
<h2>Back to the theories</h2>
<p>It’s also worth noting some of the theories of quantum gravity that might be tested using tabletop experiments are very radical.</p>
<p>Some theories, such as loop quantum gravity, suggest <a href="https://theconversation.com/time-might-not-exist-according-to-physicists-and-philosophers-but-thats-okay-181268">space and time may disappear</a> at very small scales or high energies. If that’s right, it may not be possible to carry out experiments at these scales.</p>
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Read more:
<a href="https://theconversation.com/explainer-string-theory-2983">Explainer: String theory</a>
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<p>After all, experiments as we know them are the kind of thing that happen at a particular place, across a particular interval of time. If theories like this are correct, we may need to rethink the very nature of experimentation so we can make sense of it in situations where space and time are absent.</p>
<p>On the other hand, the very fact we can perform straightforward experiments involving gravity at small scales may suggest that space and time are present after all.</p>
<p>Which will prove true? The best way to find out is to keep going with tabletop experiments, and to push them as far as they can go.</p><img src="https://counter.theconversation.com/content/224372/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sam Baron receives funding from the Australian Research Council.</span></em></p>
A new measurement of gravity at small scales hints at an alternative to billion-dollar experiments for the future of physics.
Sam Baron, Associate Professor, Philosophy of Science, The University of Melbourne
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/215414
2023-11-15T13:21:33Z
2023-11-15T13:21:33Z
The universe is expanding faster than theory predicts – physicists are searching for new ideas that might explain the mismatch
<figure><img src="https://images.theconversation.com/files/559383/original/file-20231114-23-g88npv.png?ixlib=rb-1.1.0&rect=8%2C7%2C1189%2C1210&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The James Webb Space Telescope's deep field image shows a universe full of sparkling galaxies.</span> <span class="attribution"><a class="source" href="https://webbtelescope.org/contents/media/images/2022/038/01G7JGTH21B5GN9VCYAHBXKSD1?news=true">NASA/STScI</a></span></figcaption></figure><p>Astronomers have known for decades that the universe is expanding. When they use telescopes to observe faraway galaxies, they see that these <a href="https://theconversation.com/explainer-the-mysterious-dark-energy-that-speeds-the-universes-rate-of-expansion-40224">galaxies are moving away</a> from Earth.</p>
<p>To astronomers, the wavelength of light a galaxy emits is longer the faster the galaxy is moving away from us. The farther away the galaxy is, the more its light has shifted toward the longer wavelengths on the red side of the spectrum – so the higher the “redshift.”</p>
<p>Because the speed of light is finite, fast, but not infinitely fast, seeing something far away means we’re looking at the thing how it looked in the past. With distant, high-redshift galaxies, we’re <a href="https://theconversation.com/looking-back-toward-cosmic-dawn-astronomers-confirm-the-faintest-galaxy-ever-seen-207602">seeing the galaxy</a> when the universe was in a younger state. So “high redshift” corresponds to the early times in the universe, and “low redshift” corresponds to the late times in the universe. </p>
<p>But as astronomers have studied these distances, they’ve learned that the universe is not just expanding – its rate of expansion is accelerating. And that expansion rate is even faster than the leading theory predicts it should be, leaving <a href="https://rekeeley.github.io/">cosmologists like me</a> puzzled and looking for new explanations. </p>
<h2>Dark energy and a cosmological constant</h2>
<p>Scientists call the source of this acceleration <a href="https://theconversation.com/dark-energy-map-gives-clue-about-what-it-is-but-deepens-dispute-about-the-cosmic-expansion-rate-143200">dark energy</a>. We’re not quite sure what drives dark energy or how it works, but we think its behavior could be explained by <a href="https://doi.org/10.1086/300499">a cosmological constant</a>, which is a <a href="https://doi.org/10.1038/d41586-018-05095-z">property of spacetime</a> that contributes to the expansion of the universe. </p>
<p>Albert Einstein originally came up with this constant – he marked it with a lambda in his theory of <a href="https://theconversation.com/why-does-gravity-pull-us-down-and-not-up-162141">general relativity</a>. With a <a href="https://www.livescience.com/cosmological-constant.html">cosmological constant</a>, as the universe expands, the energy density of the cosmological constant stays the same.</p>
<p>Imagine a box full of particles. If the volume of the box increases, the density of particles would decrease as they spread out to take up all the space in the box. Now imagine the same box, but as the volume increases, the density of the particles stays the same. </p>
<p>It doesn’t seem intuitive, right? That the energy density of the cosmological constant does not decrease as the universe expands is, of course, very weird, but this property helps explain the accelerating universe.</p>
<h2>A standard model of cosmology</h2>
<p>Right now, the leading theory, or standard model, of cosmology is <a href="https://lambda.gsfc.nasa.gov/education/graphic_history/univ_evol.html">called “Lambda CDM</a>.” Lambda denotes the cosmological constant describing dark energy, and CDM stands for cold dark matter. This model describes both the acceleration of the universe in its late stages as well as the expansion rate in its early days.</p>
<p>Specifically, the Lambda CDM explains observations of the cosmic microwave background, which is the afterglow of microwave radiation from when the universe <a href="https://doi.org/10.1051/0004-6361/201833910">was in a “hot, dense state</a>” about 300,000 years after the Big Bang. Observations using the <a href="https://www.esa.int/Enabling_Support/Operations/Planck">Planck satellite</a>, which measures the <a href="https://www.esa.int/Science_Exploration/Space_Science/Herschel/Cosmic_Microwave_Background_CMB_radiation">cosmic microwave background</a>, led scientists to create the Lambda CDM model. </p>
<p>Fitting the Lambda CDM model to the cosmic microwave background allows physicists to predict the value of the <a href="https://news.uchicago.edu/explainer/hubble-constant-explained">Hubble constant</a>, which isn’t actually a constant but a measurement describing the universe’s current expansion rate. </p>
<p>But the Lambda CDM model isn’t perfect. The expansion rate scientists have calculated by measuring distances to galaxies, and the expansion rate as described in Lambda CDM using <a href="https://doi.org/10.3847/2041-8213/ac5c5b">observations of the cosmic microwave background</a>, don’t line up. Astrophysicists call that disagreement the Hubble tension.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An illustration showing the progression of the Universe's expansion after the Big Bang. The Universe is depicted as a cylindrical funnel with labels along the bottom showing the first stars, the development of planets, and now the dark energy acceleration" src="https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=390&fit=crop&dpr=1 600w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=390&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=390&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=490&fit=crop&dpr=1 754w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=490&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/553089/original/file-20231010-21-bzoffm.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=490&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The universe is expanding faster than predicted by popular models in cosmology.</span>
<span class="attribution"><a class="source" href="https://www.jpl.nasa.gov/infographics/the-big-bang-and-expansion-of-the-universe">NASA</a></span>
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<h2>The Hubble tension</h2>
<p>Over the past few years, I’ve been <a href="https://doi.org/10.1103/PhysRevLett.131.111002">researching ways</a> to explain this Hubble tension. The tension may be indicating that the Lambda CDM model is incomplete and physicists should modify their model, or it could indicate that it’s time for researchers to come up with new ideas about how the universe works. And new ideas are always the most exciting things for a physicist.</p>
<p>One way to explain the Hubble tension is to modify the Lambda CDM model by changing the expansion rate at low redshift, at late times in the universe. Modifying the model like this can help physicists predict what sort of physical phenomena might be causing the Hubble tension. </p>
<p>For instance, maybe dark energy is not a cosmological constant but instead the result of gravity working in new ways. If this is the case, dark energy would evolve as the universe expands – and the cosmic microwave background, which shows what the universe looked like only a few years after its creation, would have a different prediction for the Hubble constant. </p>
<p>But, <a href="https://doi.org/10.1103/PhysRevLett.131.111002">my team’s latest research</a> has found that physicists can’t explain the Hubble tension just by changing the expansion rate in the late universe – this whole class of solutions falls short.</p>
<h2>Developing new models</h2>
<p>To study what types of solutions could explain the Hubble tension, we <a href="https://doi.org/10.1103/PhysRevLett.131.111002">developed statistical tools</a> that enabled us to test the viability of the entire class of models that change the expansion rate in the late universe. These statistical tools are very flexible, and we used them to match or mimic different models that could potentially fit observations of the universe’s expansion rate and might offer a solution to the Hubble tension.</p>
<p>The models we tested include evolving dark energy models, where dark energy acts differently at different times in the universe. We also tested interacting dark energy-dark matter models, where dark energy interacts with dark matter, and modified gravity models, where gravity acts differently at different times in the universe. </p>
<p>But none of these could fully explain the Hubble tension. These results suggest that physicists should study the early universe to understand the source of the tension.</p><img src="https://counter.theconversation.com/content/215414/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ryan Keeley does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>
The universe is expanding faster than physicists would expect. To figure out what processes underlie this fast expansion rate, some researchers are first trying to rule out what processes can’t.
Ryan Keeley, Postdoctoral Scholar in Physics, University of California, Merced
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/205628
2023-10-17T19:03:53Z
2023-10-17T19:03:53Z
New technique uses near-miss particle physics to peer into quantum world − two physicists explain how they are measuring wobbling tau particles
<figure><img src="https://images.theconversation.com/files/532985/original/file-20230620-21-sf8wvl.jpg?ixlib=rb-1.1.0&rect=464%2C501%2C4206%2C3241&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Large Hadron Collider at CERN can be used to study many kinds of fundamental particles, including mysterious and rare tau particles.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/abstract-neon-circles-digital-fractal-black-royalty-free-image/1191907046?phrase=particle+physics&adppopup=true">Oxygen/Moment via Getty Images</a></span></figcaption></figure><p>One way physicists seek clues to unravel the mysteries of the universe is by smashing matter together and inspecting the debris. But these types of destructive experiments, while incredibly informative, have limits. </p>
<p>We are two scientists who <a href="https://www.colorado.edu/physics/dennis-perepelitsa">study nuclear</a> and <a href="https://www.phy.cam.ac.uk/staff/dr-jesse-liu">particle physics</a> using CERN’s Large Hadron Collider near Geneva, Switzerland. Working with an international group of nuclear and particle physicists, our team realized that hidden in the data from previous studies was a remarkable and innovative experiment. </p>
<p>In a new paper published in Physical Review Letters, we developed a new method with our colleagues for measuring <a href="https://doi.org/10.1103/PhysRevLett.131.151802">how fast a particle called the tau wobbles</a>.</p>
<p>Our novel approach looks at the times incoming particles in the accelerator whiz by each other rather than the times they smash together in head-on collisions. Surprisingly, this approach enables far more accurate measurements of the tau particle’s wobble than previous techniques. This is the first time in nearly 20 years scientists have measured this wobble, known as the <a href="https://doi.org/10.1088/1742-6596/912/1/012001">tau magnetic moment</a>, and it may help illuminate tantalizing cracks <a href="https://theconversation.com/the-standard-model-of-particle-physics-may-be-broken-an-expert-explains-182081">emerging in the known laws of physics</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing a particle wobbling off of a vertical axis." src="https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=319&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=319&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=319&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=401&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=401&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=401&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Electrons, muons and taus all wobble in a magnetic field like a spinning top. Measuring the wobbling speed can provide clues into quantum physics.</span>
<span class="attribution"><span class="source">Jesse Liu</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Why measure a wobble?</h2>
<p>Electrons, the building blocks of atoms, have two heavier cousins called the <a href="https://www.britannica.com/science/subatomic-particle/Charged-leptons-electron-muon-tau">muon and the tau</a>. Taus are the heaviest in this family of three and the most mysterious, as they exist only for minuscule amounts of time.</p>
<p>Interestingly, when you place an electron, muon or tau inside a magnetic field, these particles wobble in a manner similar to how a spinning top wobbles on a table. This wobble is called a particle’s magnetic moment. It is possible to predict how fast these particles should wobble using the <a href="https://home.cern/science/physics/standard-model">Standard Model of particle physics</a> – scientists’ best theory of how particles interact.</p>
<p>Since the 1940s, physicists have been interested in measuring magnetic moments to reveal intriguing <a href="https://doi.org/10.1103/PhysRev.74.250">effects in the quantum world</a>. According to quantum physics, clouds of particles and antiparticles are constantly <a href="https://www.symmetrymagazine.org/article/july-2009/virtual-particles">popping in and out of existence</a>. These fleeting fluctuations slightly alter how fast electrons, muons and taus wobble inside a magnetic field. By measuring this wobble very precisely, physicists can peer into this cloud to uncover possible hints of undiscovered particles. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A chart showing the basic particles." src="https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=574&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=574&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=574&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=722&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=722&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=722&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Electrons, muons and taus are three closely related particles in the Standard Model of particle physics – scientists’ current best description of the fundamental laws of nature.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg#/media/File:Standard_Model_of_Elementary_Particles.svg">MissMJ, Cush/Wikimedia Commons</a></span>
</figcaption>
</figure>
<h2>Testing electrons, muons and taus</h2>
<p>In 1948, theoretical physicist Julian Schwinger first calculated how the quantum cloud <a href="https://doi.org/10.1103/PhysRev.73.416">alters the electron’s magnetic moment</a>. Since then, experimental physicists have measured the speed of the electron’s wobble to an extraordinary <a href="https://doi.org/10.1038/s41586-020-2964-7">13 decimal places</a>. </p>
<p>The heavier the particle, the more its wobble will change because of undiscovered new particles lurking in its quantum cloud. Since electrons are so light, this limits their sensitivity to new particles.</p>
<p>Muons and taus are much heavier but also far shorter-lived than electrons. While muons exist only for mere microseconds, scientists at Fermilab near Chicago measured the muon’s magnetic moment to <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics/">10 decimal places</a> in 2021. They found that muons wobbled noticeably faster than Standard Model predictions, suggesting unknown particles may be appearing in the muon’s quantum cloud.</p>
<p>Taus are the heaviest particle of the family – 17 times more massive than a muon and 3,500 times heavier than an electron. This makes them much more <a href="https://doi.org/10.1103/PhysRevD.64.035003">sensitive to potentially undiscovered particles</a> in the quantum clouds. But taus are also the hardest to see, since they live for just a millionth of the time a muon exists.</p>
<p>To date, the best measurement of the tau’s magnetic moment was made in 2004 using <a href="https://home.cern/science/accelerators/large-electron-positron-collider">a now-retired electron collider</a> at CERN. Though an incredible scientific feat, after multiple years of collecting data that experiment could measure the speed of the tau’s wobble to only <a href="https://doi.org/10.1140/epjc/s2004-01852-y">two decimal places</a>. Unfortunately, to test the Standard Model, physicists would need a measurement <a href="https://doi.org/10.1142/S0217732307022694">10 times as precise</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Diagram showing two particles nearly colliding." src="https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=319&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=319&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=319&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=401&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=401&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=401&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Instead of colliding two nuclei head-on to create tau particles, two lead ions can whiz past each other in a near miss and still produce taus.</span>
<span class="attribution"><span class="source">Jesse Liu</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Lead ions for near-miss physics</h2>
<p>Since the 2004 measurement of the tau’s magenetic moment, physicists have been seeking new ways to measure the tau wobble.</p>
<p>The Large Hadron Collider usually smashes the nuclei of two atoms together – that is why it is called a collider. These head-on collisions create a <a href="https://cds.cern.ch/record/2841509">fireworks display of debris</a> that can include taus, but the noisy conditions preclude careful measurements of the tau’s magnetic moment.</p>
<p>From 2015 to 2018, there was an experiment at CERN that was designed primarily to allow nuclear physicists to study <a href="https://home.cern/science/physics/heavy-ions-and-quark-gluon-plasma">exotic hot matter</a> created in head-on collisions. The particles used in this experiment were lead nuclei that had been stripped of their electrons – called lead ions. Lead ions are electrically charged and produce <a href="https://doi.org/10.1038/nphys4208">strong electromagnetic fields</a>. </p>
<p>The electromagnetic fields of lead ions contain particles of light called photons. When two lead ions collide, their photons can also collide and convert all their energy into a single pair of particles. It was these photon collisions that scientists used to <a href="https://doi.org/10.1103/PhysRevLett.121.212301">measure muons</a>.</p>
<p>These lead ion experiments ended in 2018, but it wasn’t until 2019 that one of us, Jesse Liu, teamed up with particle physicist Lydia Beresford in Oxford, England, and realized the data from the same lead ion experiments could potentially be used to do something new: measure the tau’s magnetic moment. </p>
<p><a href="https://doi.org/10.1103/PhysRevD.102.113008">This discovery was a total surprise</a>. It goes like this: Lead ions are so small that they often miss each other in collision experiments. But occasionally, the ions pass very close to each other without touching. When this happens, their accompanying photons can still smash together while the ions continue flying on their merry way. </p>
<p>These photon collisions can create a variety of particles – like the muons in the previous experiment, and also taus. But without the chaotic fireworks produced by head-on collisions, these near-miss events are far quieter and ideal for measuring traits of the elusive tau.</p>
<p>Much to our excitement, when the team looked back at data from 2018, indeed these lead ion near misses were creating tau particles. There was a new experiment hidden in plain sight!</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A long tube in an underground tunnel." src="https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Large Hadron Collider accelerates particles to incredibly high speeds before trying to smash particles together, but not all attempts result in successful collisions.</span>
<span class="attribution"><a class="source" href="http://cds.cern.ch/record/1211045">Maximilien Brice/CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>First measurement of tau wobble in two decades</h2>
<p>In April 2022, the CERN team announced that we had found <a href="https://atlas.cern/updates/briefing/observation-taupair-heavy-ions">direct evidence of tau particles created</a> during lead ion near misses. Using that data, the team was also able to measure the tau magnetic moment – the first time such a measurement had been done since 2004. The final results were published on Oct. 12, 2023.</p>
<p>This landmark result measured the tau wobble to two decimal places. Much to our astonishment, this method tied the previous best measurement using only one month of data recorded in 2018.</p>
<p>After no experimental progress for nearly 20 years, this result opens an entirely new and important path toward the tenfold improvement in precision needed to test Standard Model predictions. Excitingly, more data is on the horizon. </p>
<p>The Large Hadron Collider just restarted <a href="https://home.cern/news/news/experiments/lhc-lead-ion-collision-run-starts">lead ion data collection on Sept. 28, 2023</a>, after routine maintenance and upgrades. Our team plans to quadruple the sample size of lead ion near-miss data by 2025. This increase in data will double the accuracy of the measurement of the tau magnetic moment, and improvements to analysis methods may go even further.</p>
<p>Tau particles are one of physicists’ best windows to the enigmatic quantum world, and we are excited for surprises that upcoming results may reveal about the fundamental nature of the universe.</p><img src="https://counter.theconversation.com/content/205628/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jesse Liu is supported by a Junior Research Fellowship at Trinity College, University of Cambridge. </span></em></p><p class="fine-print"><em><span>Dennis V. Perepelitsa receives research funding from the U.S. Department of Energy, Office of Science.</span></em></p>
Physicists uncovered a new experiment hidden in old data from the Large Hadron Collider. Using this innovative approach, the team has unlocked an entirely new way to study quantum physics.
Jesse Liu, Research Fellow in Physics, University of Cambridge
Dennis V. Perepelitsa, Associate Professor of Physics, University of Colorado Boulder
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/214285
2023-09-27T15:48:49Z
2023-09-27T15:48:49Z
Antimatter: we cracked how gravity affects it – here’s what it means for our understanding of the universe
<figure><img src="https://images.theconversation.com/files/550580/original/file-20230927-23-ldr3z0.jpeg?ixlib=rb-1.1.0&rect=12%2C8%2C1440%2C944&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Insertion of the ALPHA-g apparatus.</span> <span class="attribution"><span class="source">Cern</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>A substance called antimatter is at the heart of one of the greatest mysteries of the universe. We know that every particle has an antimatter companion that is virtually identical to itself, but with the opposite charge. When a particle and its antiparticle meet, they annihilate each other – disappearing in a burst of light.</p>
<p>Our current understanding of physics predicts that equal quantities of matter and antimatter should have been created during the formation of the universe. But this doesn’t seem to have happened as it would have resulted in all particles annihilating right away. </p>
<p>Instead, there’s plenty of matter around us, yet very little antimatter – even deep in space. This enigma has led to <a href="https://theconversation.com/cern-discovery-sheds-light-on-the-great-mystery-of-why-the-universe-has-less-antimatter-than-matter-147226">a grand search</a> to to find flaws in the theory or otherwise explain the missing antimatter. </p>
<p>One such approach has focused on gravity. Perhaps antimatter behaves differently under gravity, being pulled in the opposite direction to matter? If so, we might simply be in a part of the universe from which it is impossible to observe the antimatter. </p>
<p>Our new study, <a href="https://www.nature.com/articles/s41586-023-06527-1">published in Nature</a>, reveals how antimatter actually behaves under the influence of gravity.</p>
<p>Other approaches to the question of why we observe more matter than antimatter span numerous sub-fields in physics. These range <a href="https://home.cern/science/experiments/ams">from astrophysics</a> – aiming to observe and predict the behaviour of antimatter in the cosmos <a href="https://home.cern/science/experiments/ams">with experiments</a> – to high energy particle physics, <a href="https://home.cern/science/experiments/lhcb">investigating the processes and fundamental particles that form antimatter</a> and govern their lifetime.</p>
<p>While slight differences have been observed in the <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">lifetime of some antimatter particles</a> compared to their matter counterparts, these results are still far from a sufficient explanation of the asymmetry. </p>
<p>The physical properties of antihydrogen – an atom composed of an antimatter electron (the positron) bound to an antimatter proton (antiproton) – are expected to be exactly the same as those of hydrogen. In addition to possessing the same chemical properties as hydrogen, such as colour and energy, we also expect that antihydrogen should behave the same in a gravitational field. </p>
<p>The so-called “weak equivalence principle” in the theory of general relativity states that the motion of bodies in a gravitational field is independent of their composition. This essentially says that what something is made of doesn’t affect how gravity influences its movements.</p>
<p>This prediction has been tested to extremely high accuracy for gravitational forces with a variety of matter particles, but never directly on the motion of antimatter. </p>
<p>Even with matter particles, gravity stands apart from other physical theories, in that is has yet to be unified with the theories that describe antimatter. Any observed difference with antimatter gravitation may help shed light on both issues.</p>
<p>To date, there have been no direct measurements on the gravitational motion of antimatter. It is quite challenging t study because gravity is the weakest force. </p>
<p>That means it is difficult to distinguish the effects of gravity from other external influences. It has only been with the <a href="https://www.nature.com/articles/s41467-017-00760-9">recent advances of techniques</a> to produce stable (long-lived), neutral and cold antimatter that measurements have become feasible.</p>
<h2>Trapped antimatter</h2>
<p>Our work took place at the <a href="https://alpha.web.cern.ch/">ALPHA-g experiment</a> at Cern, the world’s largest particle physics lab, based in Switzerland, which was designed to test the effects of gravity by containing antihydrogen in a vertical, two-metre tall trap. Antihydrogen is created in the trap by combining its antimatter constituents: the position and the antiproton. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="The ALPHA-g apparatus being installed in 2018." src="https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=1077&fit=crop&dpr=1 600w, https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=1077&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=1077&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1354&fit=crop&dpr=1 754w, https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1354&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/550546/original/file-20230927-25-atlnu1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1354&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The ALPHA-g apparatus being installed in 2018.</span>
<span class="attribution"><span class="source">William Bertsche / University of Manchester</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Positrons are readily produced by some radioactive materials – we used radioactive table salt. To create cold antiprotons, however, we had to use <a href="https://home.cern/science/physics/antimatter">immense particle accelerators</a> and a unique decelerating facility that operates at Cern. </p>
<p>Both ingredients are electrically charged and can be trapped and stored independently as antimatter in special devices called Penning traps, which consist of electric and magnetic fields.</p>
<p>Anti-atoms, however, are not confined by the Penning traps, and so we had an additional device called a “magnet bottle trap”, which confined the anti-atoms. This trap was created by magnetic fields generated by numerous superconducting magnets. </p>
<p>These were operated to control the relative strengths of the different sides of the bottle. Notably, if we weakened the top and bottom of the bottle, the atoms would be able to leave the trap under the influence of gravity.</p>
<p>We counted how many anti-atoms escaped upwards and downwards by detecting the antimatter annihilations created as the anti-atoms collided with surrounding matter particles in the trap. By comparing these results against detailed computer models of this process in normal hydrogen atoms, we were able to infer the effect of gravity on the anti-hydrogen atoms. </p>
<p>Our results are the first from the ALPHA-g experiment and the first direct measurement of antimatter’s motion in a gravitational field. They show that antihydrogen gravitation is the same as that of hydrogen, it falls downwards rather than rising, within the uncertainty limits of the experiment. </p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/zpQQ43nrCp4?wmode=transparent&start=190" frameborder="0" allowfullscreen=""></iframe>
</figure>
<p>This means our research has empirically ruled out a <a href="https://www.sciencedirect.com/science/article/pii/037015739190138C?via%3Dihub">number of historical theories</a> involving so-called “anti-gravity” suggesting that antimatter would gravitate in exactly the opposite direction as normal matter. </p>
<p>The current measurement is an important milestone on the experiment’s goal. Future work ALPHA-g experiment will improve its precision through better characterisation and control of important aspects of the experiment, such as the traps and the atom cooling systems. </p>
<p>There’s still ample room to find new results than can help explain matter-antimatter asymmetry. Physics is meant to describe observed reality, and there can always be surprises in the way the world works.</p><img src="https://counter.theconversation.com/content/214285/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>William Bertsche receives funding from EPSRC and STFC. </span></em></p>
It seems there isn’t a sci-fi part if the universe in which everything is made of antimatter.
William Bertsche, Reader in Particle Accelerators, University of Manchester
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/211280
2023-08-10T15:41:30Z
2023-08-10T15:41:30Z
Is there new physics beyond the Standard Model of particle physics? Our finding will help settle the question
<figure><img src="https://images.theconversation.com/files/542125/original/file-20230810-25-qmb702.jpg?ixlib=rb-1.1.0&rect=49%2C24%2C5472%2C3612&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Muon g2 experiment.</span> <span class="attribution"><span class="source">Fermilab</span></span></figcaption></figure><p>Despite its tremendous success in predicting the existence of new particles and forces, the <a href="https://home.cern/science/physics/standard-model">Standard Model of particle physics</a>, designed over 50 years ago to explain the smallest building blocks of nature, isn’t the complete “theory of everything” physicists have been longing for.</p>
<p>The theory <a href="https://theconversation.com/great-mysteries-of-physics-do-we-really-need-a-theory-of-everything-203534">has several problems</a>. It neither describes gravity nor the unknown components that make up most of the energy density in the universe: dark matter and dark energy. Particle physicists are therefore on a treasure hunt looking for any possible deviation from “expected” behaviour that could hint at new physics.</p>
<p>Now, our large international team of physicists working at the <a href="https://muon-g-2.fnal.gov/">Muon g-2 experiment</a> at Fermilab in the US, <a href="https://indico.fnal.gov/event/60738/">has made a measurement</a> of how a certain fundamental particle wobbles that could have massive impacts on the the status of the Standard Model.</p>
<p>Our result, which has not yet been peer reviewed but <a href="https://muon-g-2.fnal.gov/result2023.pdf">has been submitted</a> to Physical Review Letters, <a href="https://theconversation.com/how-we-found-hints-of-new-particles-or-forces-of-nature-and-why-it-could-change-physics-158564">backs up results from 2021</a> and sheds light on a massive puzzle in theoretical physics – for which one possible solution could be new particles or forces influencing the measurement.</p>
<p>One <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">fundamental building block</a> in the Standard Model is the muon, a particle similar to an electron but 200 times more massive. The muon has a long history of revolutionising particle physics – <a href="https://timeline.web.cern.ch/anderson-and-neddermeyer-discover-muon#:%7E:text=The%20muon%20was%20discovered%20as,Anderson%20and%20Seth%20Neddermeyer.">even its discovery was a shock</a>.</p>
<p>Our experiment studies how these particles interact with a 1.45 Tesla magnetic field. This causes the muons to wobble like spinning tops, with the rate of the wobble proportional to the strength of the field. </p>
<p>The experiment produces and stores billions of muons in a 14-metre diameter circular magnet called the storage ring. Eventually, muons decay to electrons, which are counted by detectors around the inside of the ring. </p>
<p>Another quirk of nature means that the number of detected electrons varies proportionately to the rate of the wobble. So counting electrons tells us the rate of the muons’ wobble. And the more electrons you count, the more precise the measurement gets.</p>
<p>The interaction between the muon’s wobble and the field is quantified by a dimensionless constant called “g”, the gyromagnetic ratio. The physicist Paul Dirac predicted its value to be g = 2. But according to quantum mechanics, the theory governing the subatomic world that the Standard Model relies on, is that empty space <a href="https://theconversation.com/what-is-nothing-martin-rees-qanda-101498">is filled with “virtual” particles</a>, which appear for a fleeting moment and then disappear again by annihilation. </p>
<p>These particles affect the muon’s interaction with the magnetic field, increasing g to slightly more than 2. This is why the experiment, which studies this difference, is named “g-2”. Any missing pieces in the Standard Model would modify the rate by an amount slightly higher or lower than predicted, making this a powerful search tool for new physics. </p>
<p>A measurement at Brookhaven National Laboratory in the US made waves in <a href="https://www.g-2.bnl.gov/">2004</a> after discovering the wobble was slightly faster than expected, potentially hinting at something new. The value was measured again at Fermilab in <a href="https://theconversation.com/how-we-found-hints-of-new-particles-or-forces-of-nature-and-why-it-could-change-physics-158564">April 2021</a>, confirming the original measurement and increasing the size of the gap between experiment and theory.</p>
<figure class="align-center ">
<img alt="Results chart, with error bars." src="https://images.theconversation.com/files/542127/original/file-20230810-23-4tmi6b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/542127/original/file-20230810-23-4tmi6b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/542127/original/file-20230810-23-4tmi6b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/542127/original/file-20230810-23-4tmi6b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/542127/original/file-20230810-23-4tmi6b.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/542127/original/file-20230810-23-4tmi6b.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/542127/original/file-20230810-23-4tmi6b.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Results chart, with error bars.</span>
<span class="attribution"><span class="source">Fermilab</span></span>
</figcaption>
</figure>
<p>Now, our new result from Fermilab, using data collected in 2019 and 2020, examines four times as many muons as the 2021 result, cutting the total uncertainty by a factor of two. This makes the measurement the most precise determination of the muon’s wobble ever made.</p>
<h2>Boosting accuracy</h2>
<p>In practice, the experiment is much more challenging than simply counting muons. While the statistical uncertainty has been reduced, other improvements were needed to make the measurement even more precise. The magnetic field direction determines the axis of the wobble, so keeping the temperature fluctuations of the magnet under control was crucial. </p>
<p>Differences in temperature also cause the magnet pieces to expand and contract, which changes the magnetic field slightly. At our level of accuracy, even a change one thousandth of a millimetre could have a huge effect on the wobble. For this reason, a thermal coat was installed around the ring and a cooling system in the experimental hall. </p>
<p>Another challenge is the fact that muons in the ring do not want to stay on a perfectly circular orbit – rather, they like to swim around and explore all regions of the ring. We therefore upgraded the high-voltage systems that push the beam into the right place.</p>
<p>Conventionally, particle physicists estimate how well two results (for example a theoretical and an experimental onne) agree by using a statistical measure called sigma. This can estimate the chances of any difference being a statistical fluke. However, that doesn’t make sense this time, because it is becoming increasingly unclear which Standard Model prediction we should compare the result with. </p>
<p>A collaboration of theorists, called the <a href="https://muon-gm2-theory.illinois.edu/">Muon g-2 Theory Initiative</a>, calculated their value in 2020. That’s what was used in 2021, giving a sigma of 4.2, which suggested the chance that the result was a fluke was one in 40,000. But since then, there have been developments yielding new predictions: one from a novel approach by another <a href="https://www.nature.com/articles/s41586-021-03418-1">group of theorists</a>. </p>
<p>There has also been an updated <a href="https://arxiv.org/pdf/2302.08834.pdf">experimental measurement</a> from the <a href="https://inspirehep.net/experiments/1108205">CMD-3 collaboration</a> in Russia that will feed into any new calculations. These could modify the 2020 value, potentially bringing them closer in line with the Standard Model. </p>
<p>It is apparent that there are huge challenges on both sides of the story, where theory doesn’t even agree with theory. Our collaboration is now working towards our final experimental result, expected in 2025, using the entire dataset – over twice as much data. But until the theory controversy is resolved, there will be a cloud of doubt hanging over any interpretation of the discrepancy. </p>
<p>There are two possible outcomes. The theory and experiment may eventually fail to agree, signifying that new particles or forces of nature have been hiding here all along. This could mean that the Standard Model ultimately fails – needing an update. Or, the updated predictions close the gap, which would be a massive boost for the Standard Model.</p>
<p>Either way, our ultra precise measurement sets the stage for the final showdown.</p><img src="https://counter.theconversation.com/content/211280/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Dominika Vasilkova receives funding from the Leverhulme Trust. </span></em></p><p class="fine-print"><em><span>Ce Zhang receives funding from the Leverhulme Trust. </span></em></p><p class="fine-print"><em><span>Elia Bottalico receives funding from the Leverhulme Trust. </span></em></p><p class="fine-print"><em><span>Saskia Charity receives funding from UKRI (STFC). </span></em></p>
New measurement of wobbling muons back up previous findings – potentially challenging the Standard Model of Particle Physics.
Dominika Vasilkova, Postdoctoral research associate, University of Liverpool
Ce Zhang, Postdoctoral research associate, University of Liverpool
Elia Bottalico, Postdoctoral Research Associate, University of Liverpool
Saskia Charity, Postdoctoral Research Associate, Particle Physics, University of Liverpool
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/211075
2023-08-09T12:31:45Z
2023-08-09T12:31:45Z
Researchers dig deep underground in hopes of finally observing dark matter
<figure><img src="https://images.theconversation.com/files/541255/original/file-20230804-21123-c0m9ny.jpeg?ixlib=rb-1.1.0&rect=11%2C0%2C1280%2C831&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The inside of the LZ outer detector. The LZ is a super sensitive machine that may one day detect a dark matter particle. </span> <span class="attribution"><span class="source">Matt Kapust, SURF</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>Physicists like me don’t fully understand what makes up about <a href="https://doi.org/10.1051/0004-6361/201833910">83% of the matter of the universe</a> — something we call “<a href="https://theconversation.com/dark-matter-the-mystery-substance-physics-still-cant-identify-that-makes-up-the-majority-of-our-universe-85808">dark matter</a>.” But with a <a href="https://sanfordlab.org/experiment/lux-zeplin">tank full of xenon</a> buried nearly a mile under South Dakota, we might one day be able to measure what dark matter really is.</p>
<p>In the typical model, dark matter accounts for most of the gravitational attraction in the universe, providing the glue that allows structures like galaxies, including our own Milky Way, <a href="https://www.esa.int/Science_Exploration/Space_Science/Planck/History_of_cosmic_structure_formation">to form</a>. As the solar system orbits around the center of the Milky Way, Earth moves through a <a href="https://theconversation.com/dark-matter-our-method-for-catching-ghostly-haloes-could-help-unveil-what-its-made-of-147953">dark matter halo</a>, which makes up most of the matter in our galaxy. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing the Milky Way galaxy, with a blurrred region or 'halo' around it indicating dark matter." src="https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=601&fit=crop&dpr=1 600w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=601&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=601&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=755&fit=crop&dpr=1 754w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=755&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/541254/original/file-20230804-21-fug5qn.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=755&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An artist’s rendition of the halo of dark matter surrounding the central spiral disk of the Milky Way.</span>
<span class="attribution"><span class="source">NASA/ESA/A Feild STSci</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>I’m a <a href="http://hep.ucsb.edu/people/hugh/">physicist</a> interested in understanding the nature of dark matter. One popular guess is that dark matter is a new type of particle, the <a href="https://www.britannica.com/science/weakly-interacting-massive-particle">Weakly Interacting Massive Particle</a>, or WIMP. “WIMP” captures the particle’s essence quite nicely – it has mass, meaning it interacts gravitationally, but it otherwise interacts very weakly – or rarely – with normal matter. WIMPs in the Milky Way theoretically fly through us on Earth all the time, but because they interact weakly, they just don’t hit anything.</p>
<h2>Searching for WIMPs</h2>
<p>Over the past 30 years, scientists have developed <a href="https://doi.org/10.48550/arXiv.2209.07426">an experimental program</a> to try to detect the rare interactions between WIMPs and regular atoms. On Earth, however, we are constantly surrounded by low, nondangerous levels of radioactivity coming from trace elements – mainly uranium and thorium – in the environment, as well as cosmic rays from space. The goal in hunting for dark matter is to build as sensitive a detector as possible, so it can see the dark matter, and to put it in as quiet a place as possible, so the dark matter signal can be seen over the background radioactivity. </p>
<p>With <a href="https://doi.org/10.1103/PhysRevLett.131.041002">results published in July 2023</a>, the <a href="https://sanfordlab.org/experiment/lux-zeplin">LUX-ZEPLIN</a>, or LZ, collaboration has done just that, building the largest dark matter detector to date and operating it 4,850 feet (1,478 meters) underground in the <a href="https://sanfordlab.org/">Sanford Underground Research Facility</a> in Lead, South Dakota. </p>
<p>At the center of LZ rests <a href="https://sanfordlab.org/feature/searching-dark-matter">10 metric tons (10,000 kilograms) of liquid xenon</a>. When particles pass through the detector, they may collide with xenon atoms, leading to a flash of light and the release of electrons.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing a particle interacting and releasing an electron, which registers in the detector" src="https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=627&fit=crop&dpr=1 600w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=627&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=627&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=788&fit=crop&dpr=1 754w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=788&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/541256/original/file-20230804-29-ohjyrk.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=788&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Particles interact with xenon in the LZ, releasing light that is detected by two light-sensing arrays at top and bottom.</span>
<span class="attribution"><span class="source">SLAC/LZ</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In LZ, two massive electrical grids apply an electric field across the volume of liquid, which pushes these released electrons to the liquid’s surface. When they breach the surface, they are pulled into the space above the liquid, which is filled with xenon gas, and accelerated by another electric field to create a second flash of light. Two large arrays of light sensors collect these two flashes of light, and together they allow researchers to reconstruct the position, energy and type of interaction that took place. </p>
<h2>Reducing radioactivity</h2>
<p>All materials on Earth, including those used in WIMP detector construction, <a href="https://www.world-nuclear.org/information-library/safety-and-security/radiation-and-health/naturally-occurring-radioactive-materials-norm.aspx">emit some radiation</a> that could potentially mask dark matter interactions. Scientists therefore build dark matter detectors using the most “radiopure” materials – that is, free of radioactive contaminants – they can find, both inside and outside the detector. </p>
<p>For example, by working with metal foundries, LZ was able to use the <a href="https://doi.org/10.1016/j.astropartphys.2017.09.002">cleanest titanium on Earth</a> to build the central cylinder – or cryostat – that holds the liquid xenon. Using this special titanium reduces the radioactivity in LZ, creating a clear space to see any dark matter interactions. Furthermore, liquid xenon is so dense that it actually acts as a radiation shield, and it is easy to purify the xenon of radioactive contaminants that might sneak in. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="An LZ worker wearing a white hazard suit stands by a tall white cylinder." src="https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/541259/original/file-20230804-21-3lr4h7.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">In the inner detector of LZ, two light-sensing arrays at top and bottom view a central cylinder that will be filled with liquid xenon.</span>
<span class="attribution"><span class="source">Matt Kapust, SURF</span>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>In LZ, the central xenon detector lives inside two other detectors, called the xenon skin and the outer detector. These supporting layers catch radioactivity on the way in or out of the central xenon chamber. Because dark matter interactions are so rare, a dark matter particle will only ever interact one time in the entire apparatus. Thus, if we observe an event with multiple interactions in the xenon or the outer detector, we can assume it’s not being caused by a WIMP. </p>
<p>All of these objects, including the central detector, the cryostat and the outer detector, live in a large water tank nearly a mile underground. The water tank shields the detectors from the cavern, and the underground environment shields the water tank from cosmic rays, or charged particles that are constantly hitting the Earth’s atmosphere.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/dwoFeiqiNe0?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">The LZ lives underground to block out cosmic radiation. But in order to get it down there, SURF engineers had to figure out a way to transport all the machinery and equipment.</span></figcaption>
</figure>
<h2>The hunt continues</h2>
<p>In the result <a href="https://doi.org/10.1103/PhysRevLett.131.041002">just published</a>, using 60 days of data, LZ recorded about five events per day in the detector. That’s about a trillion fewer events than a typical particle detector on the surface would record in a day. By looking at the characteristics of these events, researchers can safely say that no interaction so far has been caused by dark matter. The result is, alas, not a discovery of new physics – but we can set limits on exactly how weakly dark matter must interact, as it remains unseen by LZ.</p>
<p>These limits help to tell physicists what dark matter is not – and LZ does that better than any experiment in the world. Meanwhile, there’s hope for what comes next in the search for dark matter. LZ is collecting more data now, and we expect to take more than 15 times more data over the next few years. A WIMP interaction may already be in that data set, just waiting to be revealed in the next round of analysis.</p><img src="https://counter.theconversation.com/content/211075/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Hugh Lippincott receives funding from the US Department of Energy Office of Science. </span></em></p>
To detect dark matter, you need to build an ultra-sensitive detector and put it somewhere ultra-quiet. For one physics collaboration, that place is almost a mile under Lead, S.D.
Hugh Lippincott, Associate Professor of Physics, University of California, Santa Barbara
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/208622
2023-06-29T20:01:42Z
2023-06-29T20:01:42Z
A neutrino portrait of our galaxy reveals high-energy particles from within the Milky Way
<figure><img src="https://images.theconversation.com/files/534726/original/file-20230629-23-u6xkg.jpg?ixlib=rb-1.1.0&rect=643%2C0%2C1211%2C850&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">IceCube Collaboration/Science Communication Lab for CRC 1491</span></span></figcaption></figure><p>Our Milky Way galaxy is an awe-inspiring feature of the night sky, viewable with the naked eye as a hazy band of stars stretching from horizon to horizon.</p>
<p>For the first time, the IceCube Neutrino Observatory in Antarctica has produced an image of the Milky Way using neutrinos – tiny, ghost-like astronomical messengers. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo of the band of the Milky Way with extra shading in blue." src="https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534734/original/file-20230629-25-v10rmi.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A portrait of the Milky Way combining visible light and neutrino emissions (in blue).</span>
<span class="attribution"><span class="source">IceCube Collaboration/US National Science Foundation (Lily Le & Shawn Johnson)/ESO (S. Brunier)</span></span>
</figcaption>
</figure>
<p>In <a href="http://dx.doi.org/10.1126/science.adc9818">research published today</a> in the journal Science, the IceCube Collaboration – an international group of more than 350 scientists – presents evidence of high-energy neutrino emission coming from the Milky Way.</p>
<p>We have not yet figured out exactly where in our galaxy these particles are coming from. But today’s result brings us closer to finding some of the galaxy’s most extreme environments.</p>
<h2>Neutrino astronomy</h2>
<p>Neutrinos offer a unique view of the cosmos as they can travel directly from places no other radiation or particles can escape from. This makes them very interesting to astronomers, because neutrinos offer a window into the extreme cosmic environments that create another kind of particle called cosmic rays.</p>
<p>Cosmic rays are high-energy particles that permeate our Universe, but their origins are difficult to pin down. Cosmic rays are electrically charged, which means their path through space is scrambled by magnetic fields, and by the time one arrives at Earth there is no way to tell where it came from. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/spotting-astrophysical-neutrinos-is-just-the-tip-of-the-icecube-20499">Spotting astrophysical neutrinos is just the tip of the IceCube</a>
</strong>
</em>
</p>
<hr>
<p>However, the environments that accelerate cosmic rays to extraordinary energies also produce neutrinos – and neutrinos have no electric charge, so they travel in nice straight lines. So if we can detect the path of neutrinos arriving at Earth, this will point back to where the neutrinos were created. </p>
<p>But detecting those neutrinos is not so easy. </p>
<h2>How to hunt neutrinos</h2>
<p>The IceCube Neutrino Observatory is not far from the South Pole. It uses more than 5,000 light sensors arrayed throughout a cubic kilometre of pristine Antarctic ice to search for signs of high-energy neutrinos from our galaxy and beyond. </p>
<p>Vast numbers of neutrinos are streaming through Earth all the time, but only a tiny fraction of them bump into anything on their way through.</p>
<p>Each neutrino interaction makes a tiny flash of light – and those tiny flashes are what the IceCube sensors look out for. The direction and energy of the neutrino can be determined from the amount and pattern of light detected.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534733/original/file-20230629-23-b8qav.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
<span class="attribution"><span class="source">IceCube Collaboration</span></span>
</figcaption>
</figure>
<p>IceCube has previously detected high-energy neutrinos coming from outside the Milky Way. However, it has been more challenging to isolate the lower-energy neutrinos coming from within our galaxy.</p>
<p>This is because some flashes IceCube detected can be traced to cosmic rays hitting Earth’s atmosphere, which create neutrinos and other particles called muons. To filter out these flashes, IceCube researchers have developed ways to distinguish particles created in the atmosphere and those from further afield by the shape of the light patterns they create in the ice. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/an-antarctic-neutrino-telescope-has-detected-a-signal-from-the-heart-of-a-nearby-active-galaxy-193845">An Antarctic neutrino telescope has detected a signal from the heart of a nearby active galaxy</a>
</strong>
</em>
</p>
<hr>
<p>Filtering out the unwanted detections has made IceCube more sensitive to astrophysical neutrinos. The final breakthrough that allowed the creation of a neutrino image of the Milky Way came from machine-learning methods that improve the identification of cascades of light produced by neutrinos, as well as the determination of the neutrino’s direction and energy.</p>
<h2>Closing in on cosmic rays</h2>
<p>The new neutrino lens on our galaxy will help reveal where the most powerful accelerators of galactic cosmic rays are located. We hope to learn how energetic these particles can get, and the inner workings of these high-energy galactic engines.</p>
<p>However, we are yet to pinpoint these accelerators within the Milky Way. The new IceCube analysis found evidence for neutrinos coming from broad regions of the galaxy, but was not able to discern individual sources.</p>
<p>Our team, at the University of Canterbury in New Zealand and the University of Adelaide in Australia, has a plan to realise that next step.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=342&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=342&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=342&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=430&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=430&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534735/original/file-20230629-17-4f6jrd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=430&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Five views of the Milky Way: the top two bands show visible light and gamma rays, while the lower three show expected and real neutrino results, plus a measure of the significance of neutrino events detected by IceCube.</span>
<span class="attribution"><span class="source">IceCube Collaboration</span></span>
</figcaption>
</figure>
<p>We are making models to predict the neutrino signal close to likely particle accelerators so we can target our searches for neutrinos. </p>
<p>Undergraduate student Rhia Hewett and PhD student Ryan Burley are examining pairs of accelerator candidates and molecular dust clouds. They plan to estimate the flux of neutrinos produced by cosmic rays interacting in the clouds, after the neutrinos travel from the accelerators. </p>
<p>They will use their results to enable a focused search of IceCube data for the sources of neutrino emissions. We believe this will provide the key to using IceCube to unlock the secrets of the most energetic processes in the Milky Way.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=2067&fit=crop&dpr=1 600w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=2067&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=2067&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=2597&fit=crop&dpr=1 754w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=2597&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/534731/original/file-20230629-22-fmkvpi.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=2597&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A timeline of neutrino astronomy.</span>
<span class="attribution"><span class="source">IceCube Collaboration</span></span>
</figcaption>
</figure><img src="https://counter.theconversation.com/content/208622/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jenni Adams has received funding from the Marsden Fund Council from New Zealand Government funding, managed by the Royal Society Te Apārangi. </span></em></p>
Neutrinos are some of nature’s most elusive particles, but new research has used them to create an image of our own galaxy.
Jenni Adams, Professor, Physics and Astronomy, University of Canterbury
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/204109
2023-04-20T20:01:55Z
2023-04-20T20:01:55Z
New look at ‘Einstein rings’ around distant galaxies just got us closer to solving the dark matter debate
<figure><img src="https://images.theconversation.com/files/521998/original/file-20230420-3121-axsfat.jpeg?ixlib=rb-1.1.0&rect=0%2C0%2C2000%2C1320&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">ESA / Hubble & NASA</span></span></figcaption></figure><p>Physicists believe most of the matter in the universe is made up of an invisible substance that we only know about by its indirect effects on the stars and galaxies we can see.</p>
<p>We’re not crazy! Without this “dark matter”, the universe as we see it would make no sense. </p>
<p>But the nature of dark matter is a longstanding puzzle. However, <a href="https://www.nature.com/articles/s41550-023-01943-9">a new study</a> by Alfred Amruth at the University of Hong Kong and colleagues, published in Nature Astronomy, uses the gravitational bending of light to bring us a step closer to understanding. </p>
<h2>Invisible but omnipresent</h2>
<p>The reason we think dark matter exists is that we can see the effects of its gravity in the behaviour of galaxies. Specifically, dark matter seems to make up about 85% of the universe’s mass, and most of the distant galaxies we can see appear to be surrounded by a halo of the mystery substance.</p>
<p>But it’s called dark matter because it doesn’t give off light, or absorb or reflect it, which makes it incredibly difficult to detect. </p>
<p>So what is this stuff? We think it must be some kind of unknown fundamental particle, but beyond that we’re not sure. All attempts to detect dark matter particles in laboratory experiments so far have failed, and physicists have been debating its nature for decades.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">Why do astronomers believe in dark matter?</a>
</strong>
</em>
</p>
<hr>
<p>Scientists have proposed two leading hypothetical candidates for dark matter: relatively heavy characters called weakly interacting massive particles (or WIMPs), and extremely lightweight particles called axions. In theory, WIMPs would behave like discrete particles, while axions would behave a lot more like waves due to quantum interference. </p>
<p>It has been difficult to distinguish between these two possibilities – but now light bent around distant galaxies has offered a clue.</p>
<h2>Gravitational lensing and Einstein rings</h2>
<p>When light travelling through the universe passes a massive object like a galaxy, its path is bent because – according to Albert Einstein’s theory of general relativity – the gravity of the massive object distorts space and time around itself.</p>
<p>As a result, sometimes when we look at a distant galaxy we can see distorted images of other galaxies behind it. And if things line up perfectly, the light from the background galaxy will be smeared out into a circle around the closer galaxy. </p>
<p>This distortion of light is called “gravitational lensing”, and the circles it can create are called “Einstein rings”.</p>
<p>By studying how the rings or other lensed images are distorted, astronomers can learn about the properties of the dark matter halo surrounding the closer galaxy. </p>
<h2>Axions vs WIMPs</h2>
<p>And that’s exactly what Amruth and his team have done in their new study. They looked at several systems where multiple copies of the same background object were visible around the foreground lensing galaxy, with a special focus on one called HS 0810+2554.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=589&fit=crop&dpr=1 600w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=589&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=589&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=740&fit=crop&dpr=1 754w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=740&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/522004/original/file-20230420-2407-ijctq6.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=740&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Multiple images of a background image created by gravitational lensing can be seen in the system HS 0810+2554.</span>
<span class="attribution"><a class="source" href="https://hubblesite.org/contents/media/images/2020/05/4613-Image?news=true">Hubble Space Telescope / NASA / ESA</a></span>
</figcaption>
</figure>
<p>Using detailed modelling, they worked out how the images would be distorted if dark matter were made of WIMPs vs how they would if dark matter were made of axions. The WIMP model didn’t look much like the real thing, but the axion model accurately reproduced all features of the system.</p>
<p>The result suggests axions are a more probable candidate for dark matter, and their ability to explain lensing anomalies and other astrophysical observations has scientists buzzing with excitement. </p>
<h2>Particles and galaxies</h2>
<p>The new research builds on previous studies that have also pointed towards axions as the more likely form of dark matter. For example, <a href="https://doi.org/10.1093/mnras/sty271">one study</a> looked at the effects of axion dark matter on the cosmic microwave background, while <a href="https://doi.org/10.1093/mnras/stx1941">another</a> examined the behaviour of dark matter in dwarf galaxies. </p>
<p>Although this research won’t yet end the scientific debate over the nature of dark matter, it does open new avenues for testing and experiment. For example, future gravitational lensing observations could be used to probe the wave-like nature of axions and potentially measure their mass.</p>
<p>A better understanding of dark matter will have implications for what we know about particle physics and the early universe. It could also help us to understand better how galaxies form and change over time. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Explainer: Standard Model of Particle Physics</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/204109/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Rossana Ruggeri does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.</span></em></p>
For decades physicists have argued over the nature of the elusive dark matter that pervades the Universe. A clever new study uses gravitational lensing to bring new evidence to the debate.
Rossana Ruggeri, Research Fellow in Cosmology, The University of Queensland
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/185754
2022-11-14T22:12:12Z
2022-11-14T22:12:12Z
Powerful linear accelerator begins smashing atoms – 2 scientists on the team explain how it could reveal rare forms of matter
<figure><img src="https://images.theconversation.com/files/484140/original/file-20220912-16-rp5qhi.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C3000%2C1199&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A new particle accelerator at Michigan State University is set to discover thousands of never-before-seen isotopes. </span> <span class="attribution"><span class="source">Facility for Rare Isotope Beams</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>Just a few hundred feet from where we are sitting is a large metal chamber devoid of air and draped with the wires needed to control the instruments inside. A beam of particles passes through the interior of the chamber silently at around half the speed of light until it smashes into a solid piece of material, resulting in a burst of rare isotopes.</p>
<p>This is all taking place in the <a href="https://frib.msu.edu/">Facility for Rare Isotope Beams</a>, or FRIB, which is operated by Michigan State University for the U.S. Department of Energy Office of Science. Starting in May 2022, national and international teams of scientists converged at Michigan State University and began running scientific experiments at FRIB with the goal of creating, isolating and studying new isotopes. The experiments promised to provide new insights into the fundamental nature of the universe.</p>
<p>We are two professors in <a href="https://www.chemistry.msu.edu/faculty-research/faculty-members/liddick-sean.aspx">nuclear chemistry</a> and <a href="https://scholar.google.com/citations?user=vlmJRrsAAAAJ&hl=en&oi=sra">nuclear physics</a> who study rare isotopes. Isotopes are, in a sense, different flavors of an element with the same number of protons in their nucleus but different numbers of neutrons. </p>
<p>The accelerator at FRIB started working at low power, but when it finishes ramping up to full strength, it will be the most powerful heavy-ion accelerator on Earth. By accelerating heavy ions – electrically charged atoms of elements – FRIB will allow scientists like us to create and study thousands of never-before-seen isotopes. A community of roughly <a href="https://fribusers.org/">1,600 nuclear scientists from all over the world</a> has been waiting for a decade to begin doing science enabled by the new particle accelerator.</p>
<p>The <a href="https://newscenter.lbl.gov/2022/11/14/frib-experiment-pushes-elements-to-the-limit/">first experiments at FRIB</a> were completed over the summer of 2022. Even though the facility is currently running at only a fraction of its full power, multiple scientific collaborations working at FRIB have already produced and <a href="https://doi.org/10.1103/PhysRevLett.129.212501">detected about 100 rare isotopes</a>. These early results are helping researchers learn about some of the rarest physics in the universe.</p>
<figure>
<iframe width="440" height="260" src="https://www.youtube.com/embed/yGHuZnfnUtI?wmode=transparent&start=0" frameborder="0" allowfullscreen=""></iframe>
<figcaption><span class="caption">Rare isotopes are radioactive and decay over time as they emit radiation – visible here as the streaks coming from the small piece of uranium in the center.</span></figcaption>
</figure>
<h2>What is a rare isotope?</h2>
<p>It takes incredibly high amounts of energy to produce most isotopes. In nature, heavy rare isotopes are produced during the cataclysmic deaths of massive stars called <a href="https://physicstoday.scitation.org/doi/10.1063/1.1825268">supernovas</a> or during the <a href="https://doi.org/10.1038/s41586-019-1676-3">merging of two neutron stars</a>.</p>
<p>To the naked eye, two isotopes of any element look and behave the same way – all isotopes of the element mercury would look just like the liquid metal used in old thermometers. However, because the nuclei of isotopes of the same element have different numbers of neutrons, they differ in how long they live, what type of radioactivity they emit and in many other ways.</p>
<p>For example, some isotopes are stable and do not decay or emit radiation, so they are common in the universe. Other isotopes of the very same element can be radioactive so they inevitably decay away as they turn into other elements. Since radioactive isotopes disappear over time, they are relatively rarer. </p>
<p>Not all decay happens at the same rate though. Some radioactive elements – like potassium-40 – emit particles through decay at such a low rate that a small amount of the isotope can <a href="https://www.nndc.bnl.gov/nudat3/">last for billions of years</a>. Other, more highly radioactive isotopes like magnesium-38 exist for only a fraction of a second before decaying away into other elements. Short-lived isotopes, by definition, do not survive long and are rare in the universe. So if you want to study them, you have to make them yourself.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram of a large facility." src="https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=463&fit=crop&dpr=1 600w, https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=463&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=463&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=582&fit=crop&dpr=1 754w, https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=582&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/480464/original/file-20220822-77906-xtxwle.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=582&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Facility for Rare Isotope Beams was designed to allow researchers to create rare isotopes and measure them before they decay.</span>
<span class="attribution"><a class="source" href="https://frib.zenfolio.com/p798584095">Facility for Rare Isotope Beams</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Creating isotopes in a lab</h2>
<p>While only about <a href="https://doi.org/10.1038/nature11188">250 isotopes naturally occur on Earth</a>, theoretical models predict that about <a href="https://doi.org/10.1038/nature11188">7,000 isotopes should exist in nature</a>. Scientists have used particle accelerators to produce around <a href="http://www.nndc.bnl.gov/ensdf/">3,000 of these rare isotopes</a>.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A hallway with dozens of large chambers on either side extending into the distance." src="https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=403&fit=crop&dpr=1 600w, https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=403&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=403&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=507&fit=crop&dpr=1 754w, https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=507&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/480463/original/file-20220822-76734-8linop.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=507&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The green-colored chambers use electromagnetic waves to accelerate charged ions to nearly half the speed of light.</span>
<span class="attribution"><a class="source" href="https://frib.zenfolio.com/p798584095">Facility for Rare Isotope Beams</a>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The FRIB accelerator is 1,600 feet long and made of three segments folded in roughly the shape of a paperclip. Within these segments are numerous, extremely cold vacuum chambers that alternatively pull and push the ions using powerful electromagnetic pulses. FRIB can accelerate any naturally occurring isotope – whether it is as light as oxygen or as heavy as uranium – to approximately <a href="https://frib.msu.edu/science/nuclearphysics/index.html">half the speed of light</a>.</p>
<p>To create radioactive isotopes, you only need to smash this beam of ions into a solid target like a piece of beryllium metal or a rotating disk of carbon.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A complicated machine in a large tube." src="https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/480466/original/file-20220822-38135-yurmqa.JPG?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">There are many different instruments designed to measure specific attributes of the particles created during experiments at FRIB – like this instrument called FDSi, which is built to measure charged particles, neutrons and photons.</span>
<span class="attribution"><span class="source">Facility for Rare Isotope Beams</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The impact of the ion beam on the fragmentation target <a href="https://doi.org/10.1098/rsta.1998.0260">breaks the nucleus of the stable isotope apart</a> and produces many hundreds of rare isotopes simultaneously. To isolate the interesting or new isotopes from the rest, a separator sits between the target and the sensors. Particles with the right momentum and electrical charge will be passed through the separator while the rest are absorbed. Only a <a href="https://frib.msu.edu/users/instruments/operation.html">subset of the desired isotopes will reach the many instruments</a> built to observe the nature of the particles. </p>
<p>The probability of creating any specific isotope during a single collision can be very small. The odds of creating some of the rarer exotic isotopes can be on the order of <a href="https://doi.org/10.1088/0031-8949/91/5/053003">1 in a quadrillion</a> – roughly the same odds as winning back-to-back Mega Millions jackpots. But the powerful beams of ions used by FRIB contain so many ions and produce so many collisions in a single experiment that the team can reasonably expect to <a href="https://groups.nscl.msu.edu/frib/rates/fribrates.html">find even the rarest of isotopes</a>. According to calculations, FRIB’s accelerator should be able to <a href="https://msu.edu/discoverfrib">produce approximately 80% of all theorized isotopes</a>.</p>
<h2>The first two FRIB scientific experiments</h2>
<p>A multi-institution team led by researchers at Lawrence Berkeley National Laboratory, Oak Ridge National Laboratory (ORNL), University of Tennessee, Knoxville (UTK), Mississippi State University and Florida State University, together with researchers at MSU, began running the first experiment at FRIB on May 9, 2022. The group directed a beam of calcium-48 – a calcium nucleus with 28 neutrons instead of the usual 20 – into a beryllium target at 1 kW of power. Even at one quarter of a percent of the facility’s 400-kW maximum power, approximately 40 different isotopes passed through the separator to the <a href="https://fds.ornl.gov/initiator/">instruments</a>.</p>
<p>The FDSi device recorded the time each ion arrived, what isotope it was and when it decayed away. Using this information, the collaboration deduced the half-lives of the isotopes; the team has already <a href="https://doi.org/10.1103/PhysRevLett.129.212501">reported on five previously unknown half-lives</a>.</p>
<p>The second FRIB experiment began on June 15, 2022, led by a collaboration of researchers from Lawrence Livermore National Laboratory, ORNL, UTK and MSU. The facility accelerated a beam of selenium-82 and used it to produce rare isotopes of the elements scandium, calcium and potassium. These isotopes are commonly found in neutron stars, and the goal of the experiment was to better understand what type of radioactivity these isotopes emit as they decay. Understanding this process could shed light on <a href="https://doi.org/10.1038/nature12757">how neutron stars lose energy</a>.</p>
<p>The first two FRIB experiments were just the tip of the iceberg of this new facility’s capabilities. Over the coming years, FRIB is set to explore four big questions in nuclear physics: First, what are the properties of atomic nuclei with a large difference between the numbers of protons and neutrons? Second, how are elements formed in the cosmos? Third, do physicists understand the fundamental symmetries of the universe, like why there is more matter than antimatter in the universe? Finally, how can the information from rare isotopes be applied in medicine, industry and national security? </p>
<p><em>This story was updated to correctly represent the number of neutrons in the nucleus of calcium-48.</em></p><img src="https://counter.theconversation.com/content/185754/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sean Liddick receives funding from the Department of Energy . </span></em></p><p class="fine-print"><em><span>Artemis Spyrou receives funding from the National Science Foundation in the U.S.</span></em></p>
A new particle accelerator has just begun operation. It is the most powerful accelerator of its kind on Earth and will allow physicists to study some of the rarest matter in the universe.
Sean Liddick, Associate Professor of Chemistry, Michigan State University
Artemis Spyrou, Professor of Nuclear Physics, Michigan State University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/191927
2022-10-06T17:51:10Z
2022-10-06T17:51:10Z
What is quantum entanglement? A physicist explains the science of Einstein’s ‘spooky action at a distance’
<figure><img src="https://images.theconversation.com/files/488150/original/file-20221004-12421-klkh40.jpg?ixlib=rb-1.1.0&rect=0%2C0%2C5064%2C3294&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">When two particles are entangled, the state of one is tied to the state of the other. </span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/quantum-entanglement-conceptual-artwork-royalty-free-illustration/1333715460">Victor de Schwanberg/Science Photo Library via Getty Images</a></span></figcaption></figure><p>The <a href="https://theconversation.com/nobel-prize-physicists-share-prize-for-insights-into-the-spooky-world-of-quantum-mechanics-191884">2022 Nobel Prize in physics</a> recognized three scientists who made groundbreaking contributions in understanding one of the most mysterious of all natural phenomena: quantum entanglement.</p>
<p>In the simplest terms, quantum entanglement means that aspects of one particle of an entangled pair depend on aspects of the other particle, no matter how far apart they are or what lies between them. These particles could be, for example, electrons or photons, and an aspect could be the state it is in, such as whether it is “spinning” in one direction or another.</p>
<p>The strange part of quantum entanglement is that when you measure something about one particle in an entangled pair, you immediately know something about the other particle, even if they are millions of light years apart. This odd connection between the two particles is instantaneous, <a href="https://doi.org/10.1103/PhysRev.47.777">seemingly breaking a fundamental law of the universe</a>. Albert Einstein famously called the phenomenon “spooky action at a distance.”</p>
<p>Having spent the better part of <a href="https://scholar.google.com/citations?user=r8sBeycAAAAJ&hl=en&oi=ao">two decades conducting experiments rooted in quantum mechanics</a>, I have come to accept its strangeness. Thanks to ever more precise and reliable instruments and the work of this year’s Nobel winners, <a href="https://scholar.google.com/citations?user=-6d6dV4AAAAJ&hl=en&oi=sra">Alain Aspect</a>, <a href="https://scholar.google.com/citations?user=BDm2SGcAAAAJ&hl=en&oi=ao">John Clauser</a> and <a href="https://scholar.google.com/citations?user=cuqIY0oAAAAJ&hl=en&oi=ao">Anton Zeilinger</a>, physicists now integrate quantum phenomena into their knowledge of the world with an exceptional degree of certainty.</p>
<p>However, even until the 1970s, researchers were still divided over whether quantum entanglement was a real phenomenon. And for good reasons – who would dare contradict the great Einstein, who himself doubted it? It took the development of new experimental technology and bold researchers to finally put this mystery to rest.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A cat sitting in a box." src="https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488154/original/file-20221004-18-6uzgqx.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">According to quantum mechanics, particles are simultaneously in two or more states until observed – an effect vividly captured by Schrödinger’s famous thought experiment of a cat that is both dead and alive simultaneously.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Cat_in_a_box_2.jpg#/media/File:Cat_in_a_box_2.jpg">Michael Holloway/Wikimedia Commons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Existing in multiple states at once</h2>
<p>To truly understand the spookiness of quantum entanglement, it is important to first understand <a href="https://doi.org/10.1103/RevModPhys.71.S288">quantum superposition</a>. Quantum superposition is the idea that particles exist in multiple states at once. When a measurement is performed, it is as if the particle selects one of the states in the superposition.</p>
<p>For example, many particles have an attribute called spin that is measured either as “up” or “down” for a given orientation of the analyzer. But until you measure the spin of a particle, it simultaneously exists in a superposition of spin up and spin down.</p>
<p>There is a probability attached to each state, and it is possible to predict the average outcome from many measurements. The likelihood of a single measurement being up or down depends on these probabilities, <a href="https://theconversation.com/could-schrodingers-cat-exist-in-real-life-our-research-may-provide-the-answer-147752">but is itself unpredictable</a>.</p>
<p>Though very weird, the mathematics and a vast number of experiments have shown that quantum mechanics correctly describes physical reality.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo of Albert Einstein" src="https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=774&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=774&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=774&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=973&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=973&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488577/original/file-20221006-18-bitlqh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=973&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Albert Einstein, Boris Podolsky and Nathan Rosen pointed out an apparent problem with quantum entanglement in 1935 that prompted Einstein to describe quantum entanglement as ‘spooky action at a distance.’</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Einstein-formal_portrait-35.jpg#/media/File:Einstein-formal_portrait-35.jpg">Sophie Dela/Wikimedia Commons</a></span>
</figcaption>
</figure>
<h2>Two entangled particles</h2>
<p>The <a href="https://doi.org/10.1103/PhysRev.48.696">spookiness of quantum entanglement</a> emerges from the reality of quantum superposition, and was clear to the founding fathers of quantum mechanics who developed the theory in the 1920s and 1930s.</p>
<p>To create entangled particles you essentially break a system into two, where the sum of the parts is known. For example, you can split a particle with spin of zero into two particles that necessarily will have opposite spins so that their sum is zero. </p>
<p>In 1935, Albert Einstein, Boris Podolsky and Nathan Rosen <a href="https://doi.org/10.1103/PhysRev.47.777">published a paper</a> that describes a thought experiment designed to illustrate a <a href="https://doi.org/10.1103/PhysRev.47.777">seeming absurdity of quantum entanglement</a> that challenged a foundational law of the universe.</p>
<p>A <a href="https://doi.org/10.1103/PhysRev.48.696">simplified version of this thought experiment</a>, attributed to David Bohm, considers the decay of a particle called the pi meson. When this particle decays, it produces an electron and a positron that have opposite spin and are moving away from each other. Therefore, if the electron spin is measured to be up, then the measured spin of the positron could only be down, and vice versa. This is true even if the particles are billions of miles apart.</p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Two blue circles with an arrow pointing up and an arrow pointing down." src="https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=405&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=405&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=405&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=509&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=509&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488317/original/file-20221005-23-t916hd.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=509&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Entanglement can be created between a pair of particles with one measured as spin up and the other as spin down.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/spin-quantum-physics-and-computing-concept-royalty-free-image/1346594645?phrase=particle%20spin%20physics&adppopup=true">atdigit/iStock via Getty Images</a></span>
</figcaption>
</figure>
<p>This would be fine if the measurement of the electron spin were always up and the measured spin of the positron were always down. But because of quantum mechanics, the spin of each particle is both part up and part down until it is measured. Only when the measurement occurs does the quantum state of the spin “collapse” into either up or down – instantaneously collapsing the other particle into the opposite spin. This seems to suggest that the particles communicate with each other through some means that moves faster than the speed of light. But according to the laws of physics, nothing can travel faster than the speed of light. Surely the measured state of one particle cannot instantaneously determine the state of another particle at the far end of the universe?</p>
<p>Physicists, including Einstein, proposed a number of alternative interpretations of quantum entanglement in the 1930s. They theorized there was some unknown property – dubbed hidden variables – <a href="https://doi.org/10.1103/PhysRev.47.777">that determined the state of a particle before measurement</a>. But at the time, physicists did not have the technology nor a definition of a clear measurement that could test whether quantum theory needed to be modified to include hidden variables.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A photo of John Stuart Bell in front of a chalkboard." src="https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=609&fit=crop&dpr=1 600w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=609&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=609&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=766&fit=crop&dpr=1 754w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=766&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/488613/original/file-20221006-18-cedhj0.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=766&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">John Bell, an Irish physicist, came up with the means to test the reality of whether quantum entanglement relied on hidden variables.</span>
<span class="attribution"><a class="source" href="https://cds.cern.ch/record/1823937">CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Disproving a theory</h2>
<p>It took until the 1960s before there were any clues to an answer. John Bell, a brilliant Irish physicist who did not live to receive the Nobel Prize, devised a scheme to test whether the notion of hidden variables made sense.</p>
<p><a href="https://doi.org/10.1103/PhysicsPhysiqueFizika.1.195">Bell produced</a> an equation now known as Bell’s inequality that is always correct – and only correct – for hidden variable theories, and not always for quantum mechanics. Thus, if Bell’s equation was found not to be satisfied in a real-world experiment, local hidden variable theories can be ruled out as an explanation for quantum entanglement.</p>
<p>The experiments of the 2022 Nobel laureates, particularly those of <a href="https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.49.91">Alain Aspect</a>, were the first <a href="https://doi.org/10.1038/18296">tests of the Bell inequality</a>. The experiments used entangled photons, rather than pairs of an electron and a positron, as in many thought experiments. The results conclusively ruled out the existence of hidden variables, a mysterious attribute that would predetermine the states of entangled particles. Collectively, these and <a href="https://www.nature.com/articles/nature15759">many</a> <a href="https://doi.org/10.1038/35057215">follow-up</a> <a href="https://doi.org/10.1103/PhysRevD.14.2543">experiments</a> have vindicated quantum mechanics. Objects can be correlated over large distances in ways that physics before quantum mechanics can not explain.</p>
<p>Importantly, there is also no conflict with <a href="https://www.ams.org/journals/bull/1935-41-04/S0002-9904-1935-06046-X/S0002-9904-1935-06046-X.pdf">special relativity, which forbids faster-than-light communication</a>. The fact that measurements over vast distances are correlated does not imply that information is transmitted between the particles. Two parties far apart performing measurements on entangled particles <a href="https://www.forbes.com/sites/startswithabang/2020/01/02/no-we-still-cant-use-quantum-entanglement-to-communicate-faster-than-light/?sh=730ad18c4d5d">cannot use the phenomenon to pass along information</a> faster than the speed of light.</p>
<p>Today, physicists <a href="https://doi.org/0.1103/PhysRevLett.103.217402">continue to research quantum entanglement</a> and <a href="https://theconversation.com/a-quantum-computing-future-is-unlikely-due-to-random-hardware-errors-126503">investigate potential</a> <a href="https://theconversation.com/the-search-for-dark-matter-gets-a-speed-boost-from-quantum-technology-153604">practical applications</a>. Although quantum mechanics can predict the probability of a measurement with incredible accuracy, many researchers remain skeptical that it provides a complete description of reality. One thing is certain, though. Much remains to be said about the mysterious world of quantum mechanics.</p><img src="https://counter.theconversation.com/content/191927/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Andreas Muller receives funding from the National Science Foundation. </span></em></p>
A multitude of experiments have shown the mysterious phenomena of quantum mechanics to be how the universe functions. The scientists behind these experiments won the 2022 Nobel Prize in physics.
Andreas Muller, Associate Professor of Physics, University of South Florida
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/187463
2022-08-03T16:13:19Z
2022-08-03T16:13:19Z
Even scientists can’t keep up with all the newly discovered particles – our new naming scheme could help
<figure><img src="https://images.theconversation.com/files/476488/original/file-20220728-20412-9dc9yp.jpg?ixlib=rb-1.1.0&rect=147%2C57%2C5316%2C3571&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/man-think-how-solve-mathematical-problem-766012597">fran_kie/Shutterstock</a></span></figcaption></figure><p>Physicists at Cern have discovered a plethora of new exotic particles being created in the collisions produced by the Large Hadron Collider over the past few years. So many have been found in fact, that our collaboration (LHCb), which has discovered 59 out of 66 recent particles, has <a href="https://arxiv.org/pdf/2206.15233.pdf">come up with a new naming scheme</a> to help us impose some order on the growing particle zoo</p>
<p>Particle physicists have a rather chequered history when it comes to naming things. As more and more particles were discovered over the course of the 20th century, the nomenclature became increasingly befuddling. For instance, in the group of leptons we have electrons, muons and then taus, but not tauons. </p>
<p>And when two rival teams in the 1970s were unable to agree whether a <a href="https://arxiv.org/abs/hep-ph/9910468">new particle</a> consisting of two quarks (the smallest building blocks of matter) they had just discovered should be called J or ψ (psi), they ended up awkwardly smooshing the two names together to get J/ψ. </p>
<p>Even today, physicists are unable to agree whether to call the fifth heaviest quark “bottom” or “beauty” – and therefore use the two interchangeably. And let’s not even get started on the appallingly named bestiary of particles predicted by the theory known as “supersymmetry”, which suggests every particle we know also has a (yet undiscovered) super partner: sstrange [sic], squark, smuon or gluino anyone? Frankly, it’s just as well they don’t seem to exist.</p>
<h2>Complex hadrons</h2>
<p>The LHC has been a treasure trove for new types of particles called hadrons. These are subatomic particles made from two or more quarks. Conventionally, these come in two types. Baryons, such as the protons and neutrons which make up the atomic nucleus, are made of three quarks. Mesons, on the other hand, are made of a quark paired with an antiquark (each fundamental particle has an antiparticle with the same mass but opposite charge).</p>
<p>Although there are only six different types of quarks, and only five of these form hadrons, there are a huge number of possible combinations. In the 1980s, particle physicists <a href="https://pdg.lbl.gov/2021/web/viewer.html?file=../reviews/rpp2021-rev-naming-scheme-hadrons.pdf">devised a naming scheme</a> for the hadron zoo, with a symbol for each particle that made it easy to discern its quark content, such as the Greek letter Π (pi) to denote pions, the lightest mesons.</p>
<p>Until recent years, all newly discovered particles fitted nicely into that scheme as either baryons or mesons. But scientists eventually realised that more complicated hadrons with more than three quarks could also be possible: so-called tetraquarks, composed of two quarks and two antiquarks; and pentaquarks, composed of four quarks and one antiquark (or the other way around). </p>
<p>The first clear tetraquark candidates were discovered <a href="https://doi.org/10.1103/PhysRevLett.100.142001">by the Belle</a> and <a href="https://doi.org/10.1103/PhysRevLett.110.252001">BESIII</a> collaborations, and labelled Z<sub>c</sub> states (this was a random choice, X and Y had already been used to label other states). This was followed by the <a href="https://theconversation.com/exotic-particles-containing-five-quarks-discovered-at-the-large-hadron-collider-114211">spectacular discovery of pentaquark states</a>, labelled P<sub>c</sub>, by the LHCb collaboration. Since around 2019, the rate of discovery has accelerated, with names such as X, Z<sub>cs</sub>, P<sub>cs</sub> and T<sub>cc</sub> being assigned in a more-or-less ad-hoc fashion, leading to an alphabet soup of particles.</p>
<p>The absence of logic underlying the names given to the new particles led, perhaps inevitably, to some confusion. A particular problem was that the subscript “c” in the Z<sub>c</sub> and P<sub>c</sub> symbols was meant to imply that these hadrons contain both charm and anticharm quarks (sometimes called “hidden charm”). By contrast, the subscript “s” in the Z<sub>cs</sub> and P<sub>cs</sub> symbols implies that these hadrons also contain a strange quark (“open strangeness”). So then what should states that contain both open charm (a charm quark alone) and strangeness, as found recently by the LHCb collaboration, be named? </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/475868/original/file-20220725-15-9f3r2h.png?ixlib=rb-1.1.0&rect=14%2C11%2C1874%2C1028&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475868/original/file-20220725-15-9f3r2h.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475868/original/file-20220725-15-9f3r2h.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475868/original/file-20220725-15-9f3r2h.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475868/original/file-20220725-15-9f3r2h.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475868/original/file-20220725-15-9f3r2h.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475868/original/file-20220725-15-9f3r2h.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Recently discovered particles known as pentaquark.</span>
<span class="attribution"><span class="source">Dominguez, Daniel/Cern</span>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<p>As the range of new states and their assigned names risked becoming further perplexing, we and colleagues in the LHCb collaboration decided it was time to try to restore some semblance of order – at least for the newly discovered particles. Our new naming scheme, follows some guiding principles. Firstly, the basic idea should be simple enough for non-experts to follow, achieved with a base symbol of T for tetraquarks and P for pentaquarks. </p>
<p>The scheme should also allow for all possible combinations to be distinguished; this is done by addition of superscripts and subscripts to the base to denote which quarks each particle is made from and other quantum information. But these should be consistent with the existing scheme for conventional mesons and baryons – achieved by reusing existing symbols.</p>
<p>Current names for exotic hadrons would need to be changed, however. For example, the Z<sub>cs</sub> and P<sub>cs</sub> states mentioned above will become known as T<sub>ψs</sub> and P<sub>ψs</sub>, respectively (the J/ψ particle contains hidden charm), solving the problem of distinguishing hidden from open charm by reusing ψ for the former and c for the latter. </p>
<p>The final guiding principle behind the scheme is that it should be accepted by the wider particle physics community. Although the LHCb collaboration has discovered most of the new particles, which traditionally gives us some naming rights, there are other current and planned experiments in this area, and their contributions are essential for the progress of the field. There are also, of course, many theorists across the world working hard to interpret the measurements that are being made.</p>
<p>Both the general principles and the details of the new naming scheme have been discussed with these different groups, with positive and constructive feedback incorporated into our final version.</p>
<p>A naming scheme is an important part of the language used to communicate between people working in particle physics. We hope that this new scheme will help in the ongoing quest to understand how the so-called strong force confines quarks inside hadrons, for example – a feature that defies deep mathematical understanding. </p>
<p>New experimental results including the discoveries of new hadrons are fuelling improvements in theoretical understanding. Further discoveries could one day lead to a breakthrough. Ultimately, though, the success of the new scheme will be judged by how often conversations include the phrase: “Remind me, which one is that again?”</p><img src="https://counter.theconversation.com/content/187463/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Harry Cliff receives funding from the Science and Technology Facilities Council (UK). </span></em></p><p class="fine-print"><em><span><a href="mailto:t.j.gershon@warwick.ac.uk">t.j.gershon@warwick.ac.uk</a> receives funding from the Science and Technology Facilities Council (UK).</span></em></p>
Recent discoveries have led to an alphabet soup of particles.
Harry Cliff, Particle physicist, University of Cambridge
Tim Gershon, Professor of Particle Physics, University of Warwick
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/182675
2022-07-26T20:04:32Z
2022-07-26T20:04:32Z
A new book about 12 experiments that changed the world sidelines the role of beautiful theory in physics
<figure><img src="https://images.theconversation.com/files/475022/original/file-20220720-14-qoiduh.jpg?ixlib=rb-1.1.0&rect=0%2C4%2C1467%2C930&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Simulation of lead ion collisions within the ALICE experiment at the Large Hadron Collider -- one of eight detector experiments.</span> <span class="attribution"><span class="source">CERN</span></span></figcaption></figure><p><a href="https://www.goodreads.com/book/show/59999316-the-matter-of-everything">The Matter of Everything</a> tells the history of physics through experiments. Any book about the history of science for a general audience will, of necessity, be something of a distortion. The question is whether the distortion is useful: does it offer a new perspective on the history of physics? While there is much to like about the book, I found it to be largely polemic and unhelpful.</p>
<hr>
<p><em>Review: The Matter of Everything: 12 experiments that changed the world – Suzie Sheehy (Bloomsbury)</em></p>
<hr>
<p>Here’s what I liked about the book: it is extremely detailed. It takes us through 12 important experiments within physics from roughly the last century and a half. </p>
<p>The experiments range from the study of X-rays and the nature of light in the early 20th century, to the early development of particle accelerators to detect and study subatomic particles throughout the 20th century, culminating in the modern era of Big Science and the use of the Large Hadron Collider to find the <a href="https://home.cern/science/physics/higgs-boson">Higgs boson</a>. They are described in a manner that is rigorous and accessible. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475012/original/file-20220720-20-qoiduh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">A technician works in the LHC (Large Hadron Collider) tunnel of the European Organization for Nuclear Research, CERN, in 2016.</span>
<span class="attribution"><span class="source">Laurent Gillieron/AP</span></span>
</figcaption>
</figure>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/higgs-boson-ten-years-after-its-discovery-why-this-particle-could-unlock-new-physics-beyond-the-standard-model-186076">Higgs boson: ten years after its discovery, why this particle could unlock new physics beyond the standard model</a>
</strong>
</em>
</p>
<hr>
<p>Rigour and accessibility clearly trade off, at least for a non-technical audience.
The book manages this trade off beautifully. Complex experiments are described in a manner that is easily understood.</p>
<p>The role that those experiments play in pushing forward the frontiers of particle physics – the study of an increasingly large array of very small pieces of reality, including those that constitute matter such as electrons, along with the forces that bind them – is also explained well. </p>
<p>It is done so without needing to take the reader through the details of some imposing theories, most notably: the various quantum field theories within the standard model of particle physics. </p>
<p>Author Suzie Sheehy, an Australian physicist with academic roles at Oxford and Melbourne universities, also does an incredible job of explaining the wider implications of the experiments considered. Sheehy is an expert in accelerator physics: the design and implementation of particle accelerators to conduct experiments.</p>
<p>Careful attention is paid to spin-off technologies developed in the course of building particle accelerators, including the development of Magnetic Resonance Imaging (MRIs) as well as the production of radio isotopes for use in medical imaging more generally. </p>
<figure class="align-left zoomable">
<a href="https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=922&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=922&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=922&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1159&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1159&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475020/original/file-20220720-25-zobmk7.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1159&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption"></span>
</figcaption>
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<p>The point is well-made that developing these technologies was not an aim of scientific investigation but an unpredictable by-product. A word of caution underlies much of the discussion of these technologies: industry should be in the service of science, and not the other way around. </p>
<p>I also loved the book’s relish for the ingenuity of the inventor. For each of the 12 experiments described a common story unfolds: there is something we want to test but we just don’t know how to do it.</p>
<p>Scientists must invent new ways of managing electricity, magnetism, and more just so they can carry out their experiments. The world of experimental particle physics feels suddenly familiar: scientists are tinkerers, hammering out new pieces of equipment in much the same way one might invent a new kitchen utensil on the fly with some duct tape and a healthy dose of optimism.</p>
<h2>A distorted history</h2>
<p>As noted, The Matter of Everything is an inevitable distortion of the history of physics. One of the main distortions lies with the central premise of the book. The 12 experiments chosen are from the realm of particle physics. Whether by design or by accident, the history of 20th century physics is recast as the history of particle physics. </p>
<p>To say that this leaves a lot out, is an understatement. The standard model of particle physics is rivalled, in rigour and experimental confirmation, only by the general theory of relativity. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-einsteins-theory-of-general-relativity-3481">Explainer: Einstein's Theory of General Relativity</a>
</strong>
</em>
</p>
<hr>
<p>Whereas the standard model describes the world of particles and particle interactions, general relativity describes the large-scale structure of the universe and gravity.</p>
<p>In the 20th century, general relativity was both motivated and ultimately confirmed by a fascinating array of experiments, starting from the ingenious <a href="https://scienceworld.wolfram.com/physics/Michelson-MorleyExperiment.html">interferometer experiments</a> in the early 20th century to the detection of gravity waves in 2015. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/gravitational-waves-discovered-scientists-explain-why-it-is-such-a-big-deal-54521">Gravitational waves discovered: scientists explain why it is such a big deal</a>
</strong>
</em>
</p>
<hr>
<p>The focus on experiments relating to particle physics not only paints a strange picture of 20th century physics, but it also tends to cast the standard model in a rosy light. For we now know that the standard model is, in some sense, incomplete. The standard model “conflicts” with general relativity. The two theories are in need of replacement.</p>
<p>A more balanced telling of the history of 20th century physics might have included a wider array of experiments. Of course, a single book cannot cover everything. But some remarks on what is being left out should be offered. Otherwise, an idiosyncratic take on the history of 20th century physics quickly turns into a polemic retelling of where the “real” physics lies.</p>
<h2>Experiment and theory</h2>
<p>Why experiments? This is a question I kept asking myself throughout the book. Ultimately, the answer appears to be a political one. The book works hard to impress upon the reader the importance of experimental physics. Experiments are where the action is in science. Progress can only be made through gathering empirical data.</p>
<p>This focus on the experimenter as the pioneer, forging a path into new scientific terrain, is at best, a half truth. Companion to the experimenter is the theoretician. Theoretical work and experimental work generally go hand-in-hand. Theoretical physics, however, seems to be downplayed throughout the book.
This is perplexing, given that theories are essential to experimental work twice-over. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=545&fit=crop&dpr=1 600w, https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=545&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=545&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=685&fit=crop&dpr=1 754w, https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=685&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/474483/original/file-20220718-4540-tlj9xo.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=685&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Trajectories in a Cloud Chamber.</span>
<span class="attribution"><a class="source" href="http://cerncourier.com/cws/article/cern/28742">Image from Gordon Fraser/CERN, http://cerncourier.com/cws/article/cern/28742)</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>First, theories are typically needed to generate hypotheses for experimental testing. Much experimental work tests the predictions of known theories in order to confirm them. There are, of course, cases in which an experiment is conducted and produces results that challenge all known theories. But even then, it is the interplay between theory and experiment that drives science forward. </p>
<p>Second, theories are needed to make sense of empirical data. A theory of some kind is typically needed to understand how a given experiment works. </p>
<p>The Large Hadron Collider – a massive ring of electromagnets used to accelerate particles to high velocities before smashing them together, to see what they’re made of – is a case in point. The experiment is so complex that understanding it requires grasping an array of theories from different areas of science. Experimental data in a vacuum is virtually meaningless. Theories provide context for experimental data.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/new-physics-at-the-large-hadron-collider-scientists-are-excited-but-its-too-soon-to-be-sure-157871">New physics at the Large Hadron Collider? Scientists are excited, but it's too soon to be sure</a>
</strong>
</em>
</p>
<hr>
<p>The suppression of theoretical work in physics is part of the book’s gimmick. But, again, the picture this conveys of 20th century physics is unrealistic. The story of 20th century physics is as much one of beautiful theory, as it is of ingenious experiment. Again, it is hard not to see the focus on experiment as something of a normative statement on how science ought to be done.</p>
<h2>Lost voices</h2>
<p>People play a large role in the Matter of Everything. Glorious experimental machinery is set against the backdrop of scientist-inventors who tinker and toil. This focus on people is welcome. It helps to humanise the story of 20th century physics, and give the reader a sense that they too could contribute to science, if only they mucked around in the shed long enough. </p>
<p>That being said, the book might have said more about scientists who are widely acknowledged to have been unjustly neglected in the history of their field. As the book itself acknowledges, there is, for example, a need to tell the story of women scientists.</p>
<p>Given this, I found the omission of Marie Curie, and her daughter Irene, striking. Marie and Irene pass in and out of the book at various places, but their story is never properly told. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=646&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=646&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=646&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=811&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=811&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475019/original/file-20220720-12-t9q7r1.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=811&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Marie and Irene Curie.</span>
<span class="attribution"><span class="source">Wikimedia Commons</span></span>
</figcaption>
</figure>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/radioactive-new-marie-curie-biopic-inspires-but-resonates-uneasily-for-women-in-science-148986">Radioactive: new Marie Curie biopic inspires, but resonates uneasily for women in science</a>
</strong>
</em>
</p>
<hr>
<p>This is particularly odd given that both were involved in experimental work in particle physics, and one was a Nobel laureate. Ultimately, the book doesn’t fully heed its own warning, and what we are left with is a history of physics with notable gaps. This is a shame, since it was an opportunity to set the record straight.</p>
<h2>Limitations</h2>
<p>Overall, The Matter of Everything suffers from some serious limitations. It claims to be a history of 20th century physics but, at best, tells the story of experimental particle physics. </p>
<p>Theoretical work is missing, as are some of the experiments that relate to gravitational work in physics. The book also has significant gaps when it comes to the scientists themselves. </p>
<p>I thus don’t recommend the book as a complete history of 20th century physics. But read it if you’re interested in particle accelerators, and if you’re keen to know why they matter so much to everyday life, and not just big science.</p><img src="https://counter.theconversation.com/content/182675/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sam Baron receives funding from the Australian Research Council.</span></em></p>
The Matter of Everything is a partial account of the history of physics, which leaves out a lot, including the story of some key women scientists.
Sam Baron, Associate professor, Australian Catholic University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/187014
2022-07-25T20:01:55Z
2022-07-25T20:01:55Z
This Australian experiment is on the hunt for an elusive particle that could help unlock the mystery of dark matter
<figure><img src="https://images.theconversation.com/files/475787/original/file-20220725-22-e2kyjx.jpeg?ixlib=rb-1.1.0&rect=150%2C25%2C5440%2C4166&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>Australian scientists are making strides towards solving one of the greatest mysteries of the universe: the nature of invisible “dark matter”.</p>
<p>The ORGAN Experiment, Australia’s first major dark matter detector, recently completed a search for a hypothetical particle called an axion – a popular candidate among theories that try to explain dark matter.</p>
<p>ORGAN has placed new limits on the possible characteristics of axions and thus helped narrow the search for them. But before we get ahead of ourselves …</p>
<h2>Let’s start with a story</h2>
<p>About 14 billion years ago, all the little pieces of matter – the fundamental particles that would later become you, the planet and the galaxy – were compressed into one very dense, hot region.</p>
<p>Then the Big Bang happened and everything flew apart. The particles combined into atoms, which eventually clumped together to make stars, which exploded and created all kinds of exotic matter. </p>
<p>After a few billion years came Earth, which was eventually crawling with little things called humans. Cool story, right? Turns out it’s not the whole story; it’s not even half.</p>
<p>People, planets, stars and galaxies are all made of “regular matter”. But we know regular matter makes up just one-sixth of all the matter in the universe. </p>
<p>The rest is made of what we call “dark matter”. Its name tells you almost everything we know about it. It doesn’t emit light (so we call it “dark”) and it has mass (so we call it “matter”).</p>
<h2>If it’s invisible, how do we know it’s there?</h2>
<p>When we observe the way things move in space, we find time and again that we can’t explain our observations if we consider only what we can see. </p>
<p>Spinning galaxies are a great example. Most galaxies spin at speeds that can’t be explained by the gravitational pull from visible matter alone. </p>
<p>So there must be dark matter in these galaxies, providing extra gravity and allowing them to spin faster – without parts being flung off into space. We think dark matter literally holds galaxies together.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Cluster of galaxies displayed in hues of pink and purple against a black cosmic background." src="https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=434&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=434&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=434&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=545&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=545&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475806/original/file-20220725-19-1mwcwz.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=545&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The ‘Bullet Cluster’ is a massive cluster of galaxies which has been interpreted as being strong evidence for the existence of dark matter.</span>
<span class="attribution"><a class="source" href="https://science.nasa.gov/matter-bullet-cluster">NASA</a></span>
</figcaption>
</figure>
<p>So there must be an enormous amount of dark matter in the universe, pulling on all the things we can see. It’s passing through you, too, like some kind of cosmic ghost. You just can’t feel it.</p>
<h2>How could we detect it?</h2>
<p>Many scientists believe dark matter could be composed of hypothetical particles called axions. Axions were originally proposed as part of a solution to another major problem in particle physics called the “strong CP problem” (which we could write a whole article about). </p>
<p>Anyway, after the axion was proposed, scientists realised the particle could also make up dark matter under certain conditions. That’s because axions are expected to have very weak interactions with regular matter, but still have some mass: the two conditions needed for dark matter.</p>
<p>So how do you go about searching for axions? </p>
<p>Well, since dark matter is thought to be all around us, we can build detectors right here on Earth. And, luckily, the theory that predicts axions also predicts that axions can convert into photons (particles of light) under the right conditions.</p>
<p>This is good news, because we’re great at detecting photons. And this is exactly what ORGAN does. It engineers the correct conditions for axion–photon conversion and looks for weak photon signals – little flashes of light generated by dark matter passing through the detector. </p>
<p>This kind of experiment is called an axion haloscope and was first proposed in the <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.51.1415">1980s</a>. There are a few in the world today, each one slightly different in important ways.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/475807/original/file-20220725-24-v7qdc9.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The ORGAN Experiment’s main detector. A small copper cylinder called a ‘resonant cavity’ traps photons generated during dark matter conversion. The cylinder is bolted to a ‘dilution refrigerator’ which cools the experiment to very low temperatures.</span>
<span class="attribution"><span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Shining a light on dark matter</h2>
<p>An axion is believed to convert into a photon in the presence of a strong magnetic field. In a typical haloscope, we generate this magnetic field using a big electromagnet called a “superconducting solenoid”.</p>
<p>Inside the magnetic field we place one or several hollow chambers of metal, which are meant to trap the photons and cause them to bounce around inside, making them easier to detect.</p>
<p>However, there is one hiccup. Everything that has a temperature constantly emits small random flashes of light (which is why thermal imaging cameras work). These random emissions, or “noise”, make it harder to detect the faint dark matter signals we’re looking for. </p>
<p>To work around this, we’ve placed our resonator in a “dilution refrigerator”. This fancy fridge cools the experiment to cryogenic temperatures, about −273°C, which greatly reduces the noise. </p>
<p>The colder the experiment is, the better we can “listen” for faint photons produced during dark matter conversion.</p>
<h2>Targeting mass regions</h2>
<p>An axion of a certain mass will convert into a photon of a certain frequency, or colour. But since the mass of axions is unknown, experiments must target their search to different regions, focusing on those where dark matter is considered more likely to exist.</p>
<p>If no dark matter signal is found, then either the experiment is not sensitive enough to hear the signal above the noise, or there’s no dark matter in the corresponding axion mass region. </p>
<p>When this happens, we set an “exclusion limit” – which is just a way of saying “we didn’t find any dark matter in this mass range, to this level of sensitivity”. This tells the rest of the dark matter research community to direct their searches elsewhere.</p>
<p>ORGAN is the most sensitive experiment in its targeted frequency range. Its recent run detected no dark matter signals. This result has set an important exclusion limit on the possible characteristics <a href="https://www.science.org/doi/10.1126/sciadv.abq3765">of axions</a>.</p>
<p>This is the first phase of a multi-year plan to search for axions. We’re currently preparing the next experiment, which will be more sensitive and target a new, as-yet-unexplored mass range. </p>
<h2>But why does dark matter matter?</h2>
<p>Well, for one, we know from history that when we invest in fundamental physics, we end up developing important technologies. For instance, all modern computing relies on our understanding of quantum mechanics.</p>
<p>We never would have discovered electricity, or radio waves, if we didn’t pursue things that, at the time, appeared to be strange physical phenomena beyond our understanding. Dark matter is the same.</p>
<p>Consider everything humans have accomplished by understanding just one-sixth of the matter in the universe – and imagine what we could do if we unlocked the rest.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/the-search-for-dark-matter-gets-a-speed-boost-from-quantum-technology-153604">The search for dark matter gets a speed boost from quantum technology</a>
</strong>
</em>
</p>
<hr>
<img src="https://counter.theconversation.com/content/187014/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ben McAllister works for The University of Western Australia. The work referenced in this article is funded by the Australian Research Council.</span></em></p>
Regular matter makes up just one-sixth of all the matter in the universe. What would it mean to finally understand what makes up the rest?
Ben McAllister, Research Fellow, Department of Physics, The University of Western Australia
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/186076
2022-07-01T14:46:48Z
2022-07-01T14:46:48Z
Higgs boson: ten years after its discovery, why this particle could unlock new physics beyond the standard model
<figure><img src="https://images.theconversation.com/files/472056/original/file-20220701-13-xm3fo4.jpeg?ixlib=rb-1.1.0&rect=24%2C26%2C1556%2C1046&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Press conference for the announcement of the Higgs boson discovery.</span> <span class="attribution"><span class="source">Cern</span></span></figcaption></figure><p>Ten years ago, scientists announced <a href="https://theconversation.com/nobel-prize-in-physics-goes-to-discovery-of-the-higgs-boson-19014">the discovery of the Higgs boson</a>, which helps explain why elementary particles (the smallest building blocks of nature) have mass. For particle physicists, this was the end of a decades-long and hugely difficult journey – and arguably the most important result in the history of the field. But this end also marked the beginning of a new era of experimental physics.</p>
<p>In the past decade, measurements of the properties of the Higgs boson have confirmed the predictions of the <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">standard model of particle physics</a> (our best theory for particles). But it has also raised questions about the limitations of this model, such as whether there’s a more fundamental theory of nature. </p>
<figure class="align-right ">
<img alt="Image of Peter Higgs." src="https://images.theconversation.com/files/472033/original/file-20220701-18-d0lxh5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/472033/original/file-20220701-18-d0lxh5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/472033/original/file-20220701-18-d0lxh5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/472033/original/file-20220701-18-d0lxh5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/472033/original/file-20220701-18-d0lxh5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/472033/original/file-20220701-18-d0lxh5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/472033/original/file-20220701-18-d0lxh5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=754&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Physicist Peter Higgs.</span>
<span class="attribution"><span class="source">wikipedia</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Physicist <a href="https://www.nobelprize.org/prizes/physics/2013/higgs/facts/">Peter Higgs</a> predicted the Higgs boson in a series of papers between 1964 and 1966, as an inevitable consequence of the mechanism responsible for giving elementary particles mass. This theory suggests particle masses are a consequence of elementary particles interacting with a field, dubbed the Higgs field. And according to the same model, such a field should also give rise to a Higgs particle – meaning if the Higgs boson wasn’t there, this would ultimately falsify the entire theory.</p>
<p>But it soon became clear that discovering this particle would be challenging. When three theoretical physicists calculated the properties of a Higgs boson, <a href="https://www.sciencedirect.com/science/article/pii/0550321376903825?via%3Dihub">they concluded with an apology</a>. “We apologize to experimentalists for having no idea what is the mass of the Higgs boson … and for not being sure of its couplings to other particles … For these reasons, we do not want to encourage big experimental searches for the Higgs boson.”</p>
<p>It took until 1989 for the first experiment with a serious chance of discovering the Higgs boson to begin its search. The idea was to smash particles together with such high energy that a Higgs particle could be created in a 27km long tunnel at Cern in Geneva, Switzerland – the largest electron-positron (a positron is almost identical to an electron but has opposite charge) collider ever built. It ran for 11 years, but its maximum energy turned out to be just 5% too low to produce the Higgs boson.</p>
<p>Meanwhile, the most ambitious American collider in history, the <a href="https://www.fnal.gov/pub/tevatron/tevatron-accelerator.html#:%7E:text=The%20Tevatron%20was%20the%20second,around%20a%20four%2Dmile%20circumference.">Tevatron</a>, had started taking data at Fermilab, close to Chicago. The Tevatron collided protons (which, along with neutrons, make up the atomic nucleus) and antiprotons (nearly identical to protons but with opposite charge) with an energy five times higher than what was achieved in Geneva – surely, enough to make the Higgs. But proton-antiproton collisions produce a lot of debris, making it much harder to extract the signal from the data. In 2011, the Tevatron ceased operations – the Higgs boson escaped detection again. </p>
<p>In 2010, the <a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">Large Hadron Collider</a> (LHC) began colliding protons with seven times more energy than the Tevatron. Finally, on July 4 2012, two independent experiments at Cern had each collected enough data to declare the discovery of the Higgs boson. In the following year, Higgs and his collaborator François Englert <a href="https://www.nobelprize.org/prizes/physics/2013/summary/">won the Nobel prize</a> “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles”.</p>
<p>This almost sells it short. Without the Higgs boson, the whole theoretical framework describing particle physics at its smallest scales breaks apart. Elementary particles would be massless, there would be no atoms, no humans, no solar systems and no structure in the universe. </p>
<h2>Trouble on the horizon</h2>
<p>Yet the discovery has raised new, fundamental questions. Experiments at Cern have continued to probe the Higgs boson. Its properties not only determine the masses of elementary particles, but also how stable they are. As it stands, the results indicate that <a href="https://link.springer.com/article/10.1007/JHEP08(2012)098">our universe isn’t in a perfectly stable state</a>. Instead, similar to ice at the melting point, the universe could suddenly undergo a rapid “phase transition”. But rather than going from a solid to a liquid, like ice transitioning to water, this would involve crucially changing the masses – and the laws of nature in the universe.</p>
<p>The fact that the universe nevertheless seems stable suggests something might be missing in the calculations – something we have not discovered yet. </p>
<p>After a three-year hiatus for maintenance and upgrades, collisions at the LHC are now about to resume at an unprecedented energy, nearly double that used to detect the Higgs boson. This could help find missing particles that move our universe away from the apparent knife-edge between being stable and rapidly undergoing a phase transition.</p>
<p>The experiment could help answer other questions, too. Could the unique properties of the Higgs boson make it a portal to discovering dark matter, the invisible substance making up most of the matter in the universe? Dark matter is not charged. And the Higgs boson <a href="https://www.sciencedirect.com/science/article/pii/S0146641021000351?via%3Dihub">has a unique way of interacting</a> with uncharged matter.</p>
<p>The same unique properties have made physicists question whether the Higgs boson might not be a fundamental particle after all. Could there be a new, unknown force beyond the other forces of nature – gravity, electromagnetism and the weak and strong nuclear forces? Perhaps a force that binds so far unknown particles into a composite object we call the Higgs boson? </p>
<figure class="align-center ">
<img alt="Image of the LHC experiment at Cern." src="https://images.theconversation.com/files/472031/original/file-20220701-16-v4td7n.jpg?ixlib=rb-1.1.0&rect=89%2C50%2C4096%2C1911&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/472031/original/file-20220701-16-v4td7n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=277&fit=crop&dpr=1 600w, https://images.theconversation.com/files/472031/original/file-20220701-16-v4td7n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=277&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/472031/original/file-20220701-16-v4td7n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=277&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/472031/original/file-20220701-16-v4td7n.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=349&fit=crop&dpr=1 754w, https://images.theconversation.com/files/472031/original/file-20220701-16-v4td7n.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=349&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/472031/original/file-20220701-16-v4td7n.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=349&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">It’s been 10 years since the Higgs was discovered.</span>
<span class="attribution"><a class="source" href="https://www.shutterstock.com/image-photo/switzerland-april-2010-cern-european-organization-1287557629">D-VISIONS/Shutterstock</a></span>
</figcaption>
</figure>
<p>Such theories may help to address the controversial <a href="https://theconversation.com/the-standard-model-of-particle-physics-may-be-broken-an-expert-explains-182081">results of recent measurements</a> which suggest some particles do not behave exactly the way the standard model suggests they should. So studying the Higgs boson is vital to working out whether there is physics to be discovered beyond the standard model.</p>
<p>Eventually, the LHC will run into the same problem as the Tevatron did. Proton collisions are messy and the energy of its collisions will only reach so far. Even though we have the full arsenal of modern particle physics – including sophisticated detectors, advanced detection methods and machine learning – at our disposal, there is a limit to what the LHC can achieve. </p>
<p>A future high-energy collider, specifically designed to produce Higgs bosons, would enable us to precisely measure its most important properties, including how the Higgs boson interacts with other Higgs bosons. This in turn would determine how the Higgs boson interacts with its own field. Studying this interaction could therefore help us probe the underlying process which gives particles masses. Any disagreement between the theoretical prediction and a future measurement <a href="https://journals.aps.org/prd/abstract/10.1103/PhysRevD.78.063518">would be a crystal-clear sign</a> that we need to invent brand new physics.</p>
<p>These measurements will have a profound impact that reaches far beyond collider physics, guiding or constraining our understanding of the origin of dark matter, the birth of our universe – and, perhaps, its ultimate fate.</p><img src="https://counter.theconversation.com/content/186076/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martin Bauer is an associate professor at the Institute for Particle Physics Phenomenology (IPPP) at Durham University. He receives funding from UKRI through a Future Leaders fellowhip. The IPPP is funded by the Science and Technology and Facilities Council (STFC). Martin Bauer is a member of the STFC Science Board.</span></em></p><p class="fine-print"><em><span>Stephen Jones is an assistant professor at the Institute for Particle Physics Phenomenology (IPPP) at Durham University. He receives funding from the Royal Society through a University Research Fellowship. The IPPP is funded by the Science and Technology and Facilities Council (STFC).</span></em></p>
Studying the properties of the Higgs boson could throw up some shocking truths about the nature of reality.
Martin Bauer, Associate Professor of Physics, Durham University
Stephen Jones, Assistant Professor of Physics, Durham University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/182081
2022-05-06T15:43:30Z
2022-05-06T15:43:30Z
The standard model of particle physics may be broken – an expert explains
<figure><img src="https://images.theconversation.com/files/461804/original/file-20220506-16-hdf1s0.jpg?ixlib=rb-1.1.0&rect=220%2C110%2C7004%2C4724&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The storage-ring magnet for the Muon G-2 experiment at Fermilab.</span> <span class="attribution"><a class="source" href="https://vms.fnal.gov/asset/detail?recid=1950114">Reidar Hahn/wikipedia</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>As a physicist working at the Large Hadron Collider (LHC) at Cern, one of the most frequent questions I am asked is “When are you going to find something?”. Resisting the temptation to sarcastically reply “Aside from the Higgs boson, which won the Nobel Prize, and a whole slew of new composite particles?”, I realise that the reason the question is posed so often is down to how we have portrayed progress in particle physics to the wider world.</p>
<p>We often talk about progress in terms of discovering new particles, and it often is. Studying a new, very heavy particle helps us view underlying physical processes – often without annoying background noise. That makes it easy to explain the value of the discovery to the public and politicians.</p>
<p>Recently, however, a series of precise measurements of already known, bog-standard particles and processes have threatened to shake up physics. And with the LHC getting ready to run <a href="https://www.scientificamerican.com/article/large-hadron-collider-seeks-new-particles-after-major-upgrade/">at higher energy and intensity</a> than ever before, it is time to start discussing the implications widely.</p>
<p>In truth, particle physics has always proceeded in two ways, of which new particles is one. The other is by making very precise measurements that test the predictions of theories and look for deviations from what is expected. </p>
<p>The early evidence for Einstein’s theory of general relativity, for example, came from discovering small deviations in the apparent positions of stars and from the motion of Mercury in its orbit. </p>
<h2>Three key findings</h2>
<p>Particles obey a counter-intuitive but hugely successful theory called quantum mechanics. This theory shows that particles far too massive to be made directly in a lab collision can still influence what other particles do (through something called “quantum fluctuations”). Measurements of such effects are very complex, however, and much harder to explain to the public. </p>
<p>But recent results hinting at unexplained new physics beyond the standard model are of this second type. Detailed <a href="https://theconversation.com/new-physics-latest-results-from-cern-further-boost-tantalising-evidence-170133">studies from the LHCb experiment</a> found that a particle known as a beauty quark (quarks make up the protons and neutrons in the atomic nucleus) “decays” (falls apart) into an electron much more often than into a muon – the electron’s heavier, but otherwise identical, sibling. According to the standard model, this shouldn’t happen – hinting that new particles or even forces of nature may influence the process. </p>
<figure class="align-center ">
<img alt="Image of the LHCb experiment." src="https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">LHCb experiment.</span>
<span class="attribution"><span class="source">Cern</span></span>
</figcaption>
</figure>
<p>Intriguingly, though, measurements of similar processes involving “top quarks” from the ATLAS experiment at the LHC show this decay <a href="https://www.nature.com/articles/s41567-021-01236-w">does happen at equal rates</a> for electrons and muons.</p>
<p>Meanwhile, the Muon g-2 experiment at Fermilab in the US has recently made <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics/">very precise studies</a> of how muons “wobble” as their “spin” (a quantum property) interacts with surrounding magnetic fields. It found a small but significant deviation from some theoretical predictions – again suggesting that unknown forces or particles may be at work.</p>
<p>The <a href="https://www.nature.com/articles/d41586-022-01014-5">latest surprising result</a> is a measurement of the mass of a fundamental particle called the <a href="https://home.cern/science/physics/w-boson-sunshine-and-stardust">W boson</a>, which carries the weak nuclear force that governs radioactive decay. After many years of data taking and analysis, the experiment, also at Fermilab, suggests it is significantly heavier than theory predicts – deviating by an amount that would not happen by chance in more than a million million experiments. Again, it may be that yet undiscovered particles are adding to its mass.</p>
<p>Interestingly, however, this also disagrees with some lower-precision measurements from the LHC (presented in <a href="https://link.springer.com/article/10.1140/epjc/s10052-017-5475-4">this study</a> and <a href="https://link.springer.com/article/10.1140/epjc/s10052-018-6354-3">this one</a>).</p>
<h2>The verdict</h2>
<p>While we are not absolutely certain these effects require a novel explanation, the evidence seems to be growing that some new physics is needed.</p>
<p>Of course, there will be almost as many new mechanisms proposed to explain these observations as there are theorists. Many will look to various forms of “<a href="https://home.cern/science/physics/supersymmetry#:%7E:text=Supersymmetry%20is%20an%20extension%20of,mass%20of%20the%20Higgs%20boson.">supersymmetry</a>”. This is the idea that there are twice as many fundamental particles in the standard model than we thought, with each particle having a “super partner”. These may involve additional Higgs bosons (associated with the field that gives fundamental particles their mass).</p>
<p>Others will go beyond this, invoking less recently fashionable ideas such as “<a href="https://www.forbes.com/sites/brucedorminey/2014/11/19/cerns-higgs-discovery-as-portal-to-new-technicolor-physics/?sh=3deeed1925d8">technicolor</a>”, which would imply that there are additional forces of nature (in addition to gravity, electromagnetism and the weak and strong nuclear forces), and might mean that the Higgs boson is in fact a composite object made of other particles. Only experiments will reveal the truth of the matter - which is good news for experimentalists.</p>
<p>The experimental teams behind the new findings are all well respected and have worked on the problems for a long time. That said, it is no disrespect to them to note that these measurements are extremely difficult to make. What’s more, predictions of the standard model usually require calculations where approximations have to be made. This means different theorists can predict slightly different masses and rates of decay depending on the assumptions and level of approximation made. So, it may be that when we do more accurate calculations, some of the new findings will fit with the standard model. </p>
<p>Equally, it may be the researchers are using subtly different interpretations and so finding inconsistent results. Comparing two experimental results requires careful checking that the same level of approximation has been used in both cases. </p>
<p>These are both examples of sources of “systematic uncertainty”, and while all concerned do their best to quantify them, there can be unforeseen complications that under- or over-estimate them.</p>
<p>None of this makes the current results any less interesting or important. What the results illustrate is that there are multiple pathways to a deeper understanding of the new physics, and they all need to be explored. </p>
<p>With the restart of the LHC, there are still prospects of new particles being made through rarer processes or found hidden under backgrounds that we have yet to unearth.</p><img src="https://counter.theconversation.com/content/182081/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Roger Jones receives funding from STFC. I am a member of the ATLAS Collaboration </span></em></p>
A series of thrilling research means physicists may have to start inventing brand new physics.
Roger Jones, Professor of Physics, Head of Department, Lancaster University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/181028
2022-04-14T12:13:39Z
2022-04-14T12:13:39Z
A decade of science and trillions of collisions show the W boson is more massive than expected – a physicist on the team explains what it means for the Standard Model
<figure><img src="https://images.theconversation.com/files/458000/original/file-20220413-16-ptwkj1.jpg?ixlib=rb-1.1.0&rect=235%2C188%2C5006%2C3143&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Measuring the mass of W bosons took 10 years – and the result was not what physicists expected.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/balls-balancing-on-scale-royalty-free-image/1284113909">PM Images/Digital Vision via Getty Images</a></span></figcaption></figure><p>“You can do it quickly, you can do it cheaply, or you can do it right. We did it right.” These were some of the opening remarks from David Toback, leader of the Collider Detector at Fermilab, as he announced the results of a decadelong experiment to <a href="https://doi.org/10.1126/science.abk1781">measure the mass of a particle called the W boson</a>.</p>
<p>I am a <a href="https://physics.ucdavis.edu/directory/faculty/john-conway">high energy particle physicist</a>, and I am part of the team of hundreds of scientists that built and ran the Collider Detector at Fermilab in Illinois – known as CDF.</p>
<p>After trillions of collisions and years of data collection and number crunching, the CDF team found that the W boson has <a href="https://doi.org/10.1126/science.abk1781">slightly more mass than expected</a>. Though the discrepancy is tiny, the results, described in a paper published in Science on April 7, 2022, have <a href="https://doi.org/10.1038/d41586-022-01014-5">electrified the particle physics world</a>. If the measurement is correct, it is <a href="https://theconversation.com/2021-a-year-physicists-asked-what-lies-beyond-the-standard-model-173132">yet another strong signal</a> that there are missing pieces to the physics puzzle of how the universe works.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A chart showing many particles." src="https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=574&fit=crop&dpr=1 600w, https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=574&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=574&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=721&fit=crop&dpr=1 754w, https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=721&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/458003/original/file-20220413-17-m5k8mw.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=721&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Standard Model of particle physics describes the particles that make up the mass and forces of the universe.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg#/media/File:Standard_Model_of_Elementary_Particles.svg">MissMJ/WikimediaCommons</a></span>
</figcaption>
</figure>
<h2>A particle that carries the weak force</h2>
<p>The <a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">Standard Model of particle physics</a> is science’s current best framework for the basic laws of the universe and <a href="https://www.iop.org/explore-physics/physics-stepping-stones/standard-model">describes three basic forces</a>: the electromagnetic force, the weak force and the strong force. </p>
<p>The strong force holds atomic nuclei together. But some nuclei are unstable and undergo radioactive decay, slowly releasing energy by emitting particles. This process is driven by the weak force, and since the early 1900s, physicists sought an explanation for why and how atoms decay.</p>
<p>According to the Standard Model, forces are transmitted by particles. In the 1960s, a series of <a href="https://www.routledge.com/Weak-Neutral-Currents-The-Discovery-Of-The-Electro-weak-Force/Cline/p/book/9780367216139">theoretical and experimental breakthroughs</a> proposed that the weak force is transmitted by particles called W and Z bosons. It also postulated that a third particle, the Higgs boson, is what gives all other particles – including W and Z bosons – mass.</p>
<p>Since the advent of the Standard Model in the 1960s, scientists have been working their way down the list of predicted yet undiscovered particles and measuring their properties. In 1983, two experiments at CERN in Geneva, Switzerland, <a href="https://doi.org/10.1016/0370-2693(83)90860-2">captured the first evidence of the existence of the W boson</a>. It appeared to have the mass of roughly a medium-sized atom such as bromine. </p>
<p>By the 2000s, there was just one piece missing to complete the Standard Model and tie everything together: the Higgs boson. I helped search for the Higgs boson on three successive experiments, and at last we <a href="https://home.cern/science/physics/higgs-boson">discovered it in 2012</a> at the Large Hadron Collider at CERN. </p>
<p>The Standard Model was complete, and all the measurements we made hung together beautifully with the predictions.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A large yellow tube surrounded by electronics." src="https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/458007/original/file-20220413-14-a1ml2j.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Collider Detector at Fermilab collected data from trillions of collisions that produced millions of W bosons.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Collider_Detector_at_Fermilab.jpg#/media/File:Collider_Detector_at_Fermilab.jpg">Bodhita/WikimediaCommons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Measuring W bosons</h2>
<p>Testing the Standard Model is fun – you just smash particles together at very high energies. These collisions briefly produce heavier particles that then decay back into lighter ones. Physicists use huge and very sensitive detectors at places like Fermilab and CERN to measure the properties and interactions of the particles produced in these collisions. </p>
<p>In CDF, W bosons are produced about <a href="http://www.hep.ph.ic.ac.uk/%7Ewstirlin/plots/crosssections2012_v5.pdf">one out of every 10 million times</a> when a proton and an antiproton collide. Antiprotons are the antimatter version of protons, with exactly the same mass but opposite charge. Protons are made of smaller fundamental particles called quarks, and antiprotons are made of antiquarks. It is the collision between quarks and antiquarks that <a href="https://inspirehep.net/literature/261813">create W bosons</a>. W bosons decay so fast that they are impossible to measure directly. So physicists track the energy produced from their decay to measure the mass of W bosons.</p>
<p>In the 40 years since scientists first detected evidence of the W boson, successive experiments have attained ever more precise measurements of its mass. But it is only since the measurement of the Higgs boson – since it gives mass to all other particles – that researchers could check the measured mass of W bosons against the <a href="https://doi.org/10.1007/JHEP12(2013)084">mass predicted by the Standard Model</a>. The prediction and the experiments always matched up – until now.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A chart showing two circles near a line." src="https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=567&fit=crop&dpr=1 600w, https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=567&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=567&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=713&fit=crop&dpr=1 754w, https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=713&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/458024/original/file-20220413-23-edjk9x.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=713&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The new measurement of the W boson (red circle) is much farther from the mass predicted by the Standard Model (purple line) and also greater than the preliminary measurement from the experiment.</span>
<span class="attribution"><a class="source" href="https://www.science.org/doi/10.1126/science.abk1781">CDF Collaboration via Science Magazine</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>Unexpectedly heavy</h2>
<p>The CDF detector at Fermilab is excellent at accurately measuring W bosons. From 2001 to 2011, the accelerator collided protons with antiprotons trillions of times, producing millions of W bosons and recording as much data as possible from each collision.</p>
<p>The Fermilab team published <a href="https://inspirehep.net/literature/1097099">initial results</a> using a fraction of the data in 2012. We found the mass to be slightly off, but close to the prediction. The team then spent a decade painstakingly analyzing the full data set. The process included numerous internal cross-checks and required years of computer simulations. To avoid any bias creeping into the analysis, nobody could see any results until the full calculation was complete.</p>
<p>When the physics world finally saw the result on April 7, 2022, we were all surprised. Physicists measure elementary particle masses in units of millions of electron volts – shortened to MeV. The W boson’s mass <a href="https://doi.org/10.1126/science.abk1781">came out to be 80,433 MeV</a> – 70 MeV higher than what the Standard Model predicts it should be. This may seem like a tiny excess, but the measurement is accurate to within 9 MeV. This is a deviation of nearly eight times the margin of error. When my colleagues and I saw the result, our reaction was a resounding “wow!” </p>
<h2>What this means for the Standard Model</h2>
<p>The fact that the measured mass of the W boson doesn’t match the predicted mass within the Standard Model could mean three things. Either the math is wrong, the measurement is wrong or there is something missing from the Standard Model.</p>
<p>First, the math. In order to calculate the W boson’s mass, physicists use the mass of the Higgs boson. CERN experiments have allowed physicists to <a href="https://doi.org/10.1007/JHEP10(2012)140">measure the Higgs boson mass</a> to within a quarter-percent. Additionally, theoretical physicists have been <a href="https://doi.org/10.1103/PhysRevD.96.093005">working on the W boson mass calculations for decades</a>. While the math is sophisticated, the prediction is solid and not likely to change.</p>
<p>The next possibility is a flaw in the experiment or analysis. Physicists all over the world are already reviewing the result to try to poke holes in it. Additionally, future experiments at CERN may eventually achieve a more precise result that will either confirm or refute the Fermilab mass. But in my opinion, the experiment is as good a measurement as is currently possible.</p>
<p>That leaves the last option: There are unexplained particles or forces causing the upward shift in the W boson’s mass. Even before this measurement, some theorists had <a href="https://doi.org/10.1126/science.abk1781">proposed potential new particles or forces</a> that would result in the observed deviation. In the coming months and years, I expect a raft of new papers seeking to explain the puzzling mass of W bosons. </p>
<p>As a particle physicist, I am confident in saying that there must be more physics waiting to be discovered beyond the Standard Model. If this new result holds up, it will be the latest in a series of findings showing that the Standard Model and real-world measurements often <a href="https://theconversation.com/proof-of-new-physics-from-the-muons-magnetic-moment-maybe-not-according-to-a-new-theoretical-calculation-157829">don’t quite match</a>. It is these mysteries that give physicists new clues and new reasons to keep searching for fuller understanding of matter, energy, space and time.</p>
<p>[<em>Get fascinating science, health and technology news.</em> <a href="https://memberservices.theconversation.com/newsletters/?nl=science&source=inline-science-fascinating">Sign up for The Conversation’s weekly science newsletter</a>.\</p><img src="https://counter.theconversation.com/content/181028/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>John Conway receives funding from US Department of Energy and US National Science Foundation</span></em></p>
A decadelong experiment produced the most accurate measurement yet of the mass of W bosons. These particles are responsible for the weak force, and the result is more evidence for undiscovered physics.
John Conway, Professor of Physics, University of California, Davis
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/173132
2021-12-22T13:12:07Z
2021-12-22T13:12:07Z
2021: a year physicists asked, ‘What lies beyond the Standard Model?’
<figure><img src="https://images.theconversation.com/files/438717/original/file-20211221-48250-esf86c.jpg?ixlib=rb-1.1.0&rect=45%2C21%2C1566%2C1027&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Experiments at the Large Hadron Collider in Europe, like the ATLAS calorimeter seen here, are providing more accurate measurements of fundamental particles.</span> <span class="attribution"><a class="source" href="https://cds.cern.ch/record/910381">Maximilien Brice</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span></figcaption></figure><p>If you ask a physicist like me to explain how the world works, my lazy answer might be: “It follows the Standard Model.”</p>
<p><a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">The Standard Model</a> explains the fundamental physics of how the universe works. It has endured over 50 trips around the Sun despite experimental physicists constantly probing for cracks in the model’s foundations. </p>
<p>With few exceptions, it has stood up to this scrutiny, passing experimental test after experimental test with flying colors. But this wildly successful model has conceptual gaps that suggest there is a bit more to be learned about how the universe works.</p>
<p>I am a <a href="https://scholar.google.com/citations?user=N_cqAjYAAAAJ&hl=en&oi=sra">neutrino physicist</a>. <a href="https://neutrinos.fnal.gov/whats-a-neutrino/">Neutrinos</a> represent three of the <a href="https://www.iop.org/explore-physics/physics-stepping-stones/standard-model">17 fundamental particles in the Standard Model</a>. They zip through every person on Earth at all times of day. I study the properties of interactions between <a href="https://neutrinos.fnal.gov/whats-a-neutrino/">neutrinos</a> and normal matter particles.</p>
<p>In 2021, physicists around the world ran a number of experiments that probed the Standard Model. Teams measured basic parameters of the model more precisely than ever before. Others investigated the fringes of knowledge where the best experimental measurements don’t quite match the predictions made by the Standard Model. And finally, groups built more powerful technologies designed to push the model to its limits and potentially discover new particles and fields. If these efforts pan out, they could lead to a more complete theory of the universe in the future.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A chart showing the particles of the Standard Model." src="https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438763/original/file-20211222-17-y5l097.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=424&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Standard Model of physics allows scientists to make incredibly accurate predictions about how the world works, but it doesn’t explain everything.</span>
<span class="attribution"><a class="source" href="https://cds.cern.ch/images/OPEN-PHO-CHART-2015-001-1/">CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by-nc/4.0/">CC BY-NC</a></span>
</figcaption>
</figure>
<h2>Filling holes in Standard Model</h2>
<p>In 1897, J.J. Thomson discovered the first fundamental particle, the electron, using nothing more than <a href="https://www.britannica.com/science/atom/Discovery-of-electrons">glass vacuum tubes and wires</a>. More than 100 years later, physicists are still discovering new pieces of the Standard Model.</p>
<p><a href="https://www.energy.gov/science/doe-explainsthe-standard-model-particle-physics">The Standard Model</a> is a <a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">predictive framework</a> that does two things. First, it explains what the basic particles of matter are. These are things like electrons and the quarks that make up protons and neutrons. Second, it predicts how these matter particles interact with each other using “messenger particles.” These are called bosons – they include photons and the famous Higgs boson – and they communicate the basic forces of nature. The Higgs boson wasn’t <a href="https://atlas.cern/updates/feature/higgs-boson">discovered until 2012</a> after decades of work at CERN, the huge particle collider in Europe.</p>
<p>The Standard Model is incredibly good at predicting many aspects of how the world works, but it does have some holes.</p>
<p>Notably, it does not include any description of gravity. While Einstein’s theory of <a href="https://theconversation.com/why-does-gravity-pull-us-down-and-not-up-162141">General Relativity describes how gravity works</a>, physicists have not yet discovered a particle that conveys the force of gravity. A proper “Theory of Everything” would do everything the Standard Model can, but also include the messenger particles that communicate how gravity interacts with other particles.</p>
<p>Another thing the Standard Model can’t do is explain why any particle has a certain mass – physicists must measure the mass of particles directly using experiments. Only after experiments give physicists these exact masses can they be used for predictions. The better the measurements, the better the predictions that can be made.</p>
<p>Recently, physicists on a team at CERN measured <a href="https://atlas.cern/updates/briefing/twice-higgs-twice-challenge">how strongly the Higgs boson feels itself</a>. Another CERN team also measured the Higgs boson’s mass <a href="https://cms.cern/news/cms-precisely-measures-mass-higgs-boson">more precisely than ever before</a>. And finally, there was also progress on measuring the mass of neutrinos. Physicists know neutrinos have more than zero mass but less than the amount currently detectable. A team in Germany has continued to refine the techniques that could allow them to <a href="https://www.katrin.kit.edu/index.php">directly measure the mass of neutrinos</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A blue circular particle acellerator." src="https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438712/original/file-20211221-15-17289qh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Projects like the Muon g-2 experiment highlight discrepancies between experimental measurements and predictions of the Standard Model that point to problems somewhere in the physics.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Fermilab_g-2_(E989)_ring.jpg#/media/File:Fermilab_g-2_(E989)_ring.jpg">Reidar Hahn/WikimediaCommons</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>Hints of new forces or particles</h2>
<p>In April 2021, members of the <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics/">Muon g-2 experiment at Fermilab announced</a> their first <a href="https://theconversation.com/how-we-found-hints-of-new-particles-or-forces-of-nature-and-why-it-could-change-physics-158564">measurement of the magnetic moment of the muon</a>. The muon is one of the fundamental particles in the Standard Model, and this measurement of one of its properties is the most accurate to date. The reason this experiment was important was because the measurement didn’t perfectly match the Standard Model prediction of the magnetic moment. Basically, muons don’t behave as they should. This finding could point to <a href="https://news.uchicago.edu/story/what-muon-g-2-results-mean-how-we-understand-universe">undiscovered particles that interact with muons</a>.</p>
<p>But simultaneously, in April 2021, physicist Zoltan Fodor and his colleagues showed how they used a mathematical method called Lattice QCD to <a href="https://theconversation.com/proof-of-new-physics-from-the-muons-magnetic-moment-maybe-not-according-to-a-new-theoretical-calculation-157829">precisely calculate the muon’s magnetic moment</a>. Their theoretical prediction is different from old predictions, still works within the Standard Model and, importantly, matches experimental measurements of the muon.</p>
<p>The disagreement between the previously accepted predictions, this new result and the new prediction must be reconciled before physicists will know if the experimental result is truly beyond the Standard Model.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A spinning galaxy in space." src="https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=436&fit=crop&dpr=1 600w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=436&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=436&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=548&fit=crop&dpr=1 754w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=548&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/438713/original/file-20211221-129369-zkmpnf.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=548&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">New tools will help physicists search for dark matter and other things that could help explain mysteries of the universe.</span>
<span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/illustration/dark-matter-halo-surrounding-galaxy-royalty-free-illustration/932730112?adppopup=true">Mark Garlick/Science Photo Library via Getty Images</a></span>
</figcaption>
</figure>
<h2>Upgrading the tools of physics</h2>
<p>Physicists must swing between crafting the mind-bending ideas about reality that make up theories and advancing technologies to the point where new experiments can test those theories. 2021 was a big year for advancing the experimental tools of physics. </p>
<p>First, the world’s largest particle accelerator, the <a href="https://theconversation.com/ten-years-of-large-hadron-collider-discoveries-are-just-the-start-of-decoding-the-universe-102331">Large Hadron Collider at CERN</a>, was shut down and underwent some substantial upgrades. Physicists just restarted the facility in October, and they plan to begin the <a href="https://cerncourier.com/a/protons-back-with-a-splash/">next data collection run in May 2022</a>. The upgrades have boosted the power of the collider so that it can <a href="https://www.universetoday.com/140769/the-large-hadron-collider-has-been-shut-down-and-will-stay-down-for-two-years-while-they-perform-major-upgrades/">produce collisions at 14 TeV</a>, up from the previous limit of 13 TeV. This means the batches of tiny protons that travel in beams around the circular accelerator together carry the same amount of energy as an 800,000-pound (360,000-kilogram) passenger train traveling at 100 mph (160 kph). At these incredible energies, physicists may discover new particles that were too heavy to see at lower energies.</p>
<p>Some other technological advancements were made to help the search for dark matter. Many astrophysicists believe that dark matter particles, which don’t currently fit into the Standard Model, could answer some outstanding questions regarding the way gravity bends around stars – called <a href="https://www.nasa.gov/content/discoveries-highlights-shining-a-light-on-dark-matter">gravitational lensing</a> – as well as the <a href="https://phys.org/news/2019-10-dark-massive-spiral-galaxies-breakneck.html">speed at which stars rotate in spiral galaxies</a>. Projects like the Cryogenic Dark Matter Search have yet to find dark matter particles, but the teams are <a href="https://supercdms.slac.stanford.edu/overview">developing larger and more sensitive detectors</a> to be deployed in the near future. </p>
<p>[<em>Over 140,000 readers rely on The Conversation’s newsletters to understand the world.</em> <a href="https://memberservices.theconversation.com/newsletters/?source=inline-140ksignup">Sign up today</a>.]</p>
<p>Particularly relevant to my work with neutrinos is the development of immense new detectors like <a href="http://www.hyper-k.org/en/">Hyper-Kamiokande</a> and <a href="https://lbnf-dune.fnal.gov/">DUNE</a>. Using these detectors, scientists will hopefully be able to answer questions about a <a href="https://cerncourier.com/a/why-does-cp-violation-matter-to-the-universe/">fundamental asymmetry in how neutrinos oscillate</a>. They will also be used to watch for proton decay, a proposed phenomenon that certain theories predict should occur. </p>
<p>2021 highlighted some of the ways the Standard Model fails to explain every mystery of the universe. But new measurements and new technology are helping physicists move forward in the search for the Theory of Everything.</p><img src="https://counter.theconversation.com/content/173132/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Aaron McGowan has received funding in the past from the U.S. Department of Energy. </span></em></p>
Physicists know a lot about the most fundamental properties of the universe, but they certainly don’t know everything. 2021 was a big year for physics – what was learned and what’s coming next?
Aaron McGowan, Principal Lecturer in Physics and Astronomy, Rochester Institute of Technology
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/170369
2021-10-31T19:06:07Z
2021-10-31T19:06:07Z
The hunt for ‘sterile neutrinos’: a new experiment has dashed hopes of an undiscovered particle
<figure><img src="https://images.theconversation.com/files/429011/original/file-20211028-15-2e1m3q.jpg?ixlib=rb-1.1.0&rect=25%2C34%2C5695%2C5421&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">
</span> <span class="attribution"><a class="source" href="https://vms.fnal.gov/asset/detail?recid=1961196">Reidar Hahn / Fermilab</a></span></figcaption></figure><p>Physicists searching for evidence of a “light sterile neutrino”, a hypothetical particle that could give clues to cosmic puzzles such as the nature of dark matter and why the Universe is made of matter at all, have announced their hunt has <a href="https://news.fnal.gov/2021/10/microboone-experiments-first-results-show-no-hint-of-a-sterile-neutrino/">come back empty-handed</a>. </p>
<p>The MicroBooNE experiment at Fermilab was designed to follow up on earlier hints of neutrinos behaving oddly, but the negative result deals a blow to the idea of such a new elementary particle. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-the-elusive-neutrino-431">Explainer: the elusive neutrino</a>
</strong>
</em>
</p>
<hr>
<p>Neutrinos are lightweight, elusive subatomic particles, and current theories recognise three different types. In 1995, however, the Liquid Scintillator Neutrino Detector (LSND) experiment in Los Alamos detected more of one type than anyone expected. </p>
<p>Most attempts to explain the anomaly proposed the existence of a fourth kind of neutrino that barely interacts with normal matter at all: a so-called “sterile” neutrino. </p>
<p><a href="https://en.wikipedia.org/wiki/MiniBooNE">More</a> <a href="https://arxiv.org/abs/2005.05301">recent</a> <a href="https://en.wikipedia.org/wiki/Baksan_Neutrino_Observatory">experiments</a> have also reported results broadly consistent with the sterile neutrino hypothesis, but the MicroBooNE result casts the whole idea into doubt.</p>
<h2>What is a sterile neutrino?</h2>
<p>Neutrinos are subatomic particles postulated by Austrian physicist Wolfgang Pauli in 1930 to explain how some radioactive atoms fire out electrons. </p>
<p>Their existence wasn’t confirmed until 1956, when Americans Clyde Cowan and Frederick Reines observed tiny flashes of light made by neutrinos crashing into the atoms in a tank of water. </p>
<p>Today, neutrinos are an integral part of the “Standard Model of particle physics”. This is our best theory of the Universe’s particles, describing the 17 known elementary particles and how they interact via three fundamental forces (electromagnetism and the strong and weak forces).</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">Explainer: Standard Model of Particle Physics</a>
</strong>
</em>
</p>
<hr>
<p>The Standard Model divides the 17 particles into two basic groups: 12 fermions, which make up matter, and five bosons, which carry the forces.</p>
<p>Not all fermions interact with all the forces. For example, neutrinos are only affected by the weak force (and gravity, which doesn’t fit into the Standard Model).</p>
<p>The fermions are split into three families, each of which has a neutrino: the electron, muon, and tau neutrinos.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=574&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=574&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=574&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=722&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=722&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429013/original/file-20211028-22-1i69r0a.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=722&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The 17 particles of the Standard Model. The 12 fermions on the left make up matter, while the 5 bosons on the right carry forces. The three known neutrinos are on the bottom row.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg">MissMJ / Wikimedia</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>All of these neutrinos are “left-handed” with respect to the weak force. It’s hard to explain simply what that means, but suffice to say left- and right-handed particles are mirror images of one another, and they are affected differently by the weak force.</p>
<p>All other known fermions come in both left- and right-handed versions. This encourages us to think that right-handed neutrinos should also exist in nature. </p>
<p>Being right-handed, these hypothetical neutrinos are blind even to the weak force and are in this sense “sterile”. </p>
<p>But like all known particles, they should still feel gravity. Sterile neutrinos are also predicted by “grand unified theories” that try to combine the three forces into one.</p>
<h2>Hunting for sterile neutrinos</h2>
<p>If sterile neutrinos exist, how would we find them? One way is to use a process called “neutrino oscillation”, in which the three known kinds of neutrinos can transform into one another. </p>
<p>Experiments measuring these oscillations usually look at either how many of a given kind of neutrino appear in some situation, or how many disappear.</p>
<p>The LSND experiment which originally inspired the sterile neutrino hypothesis was an “appearance” experiment, as are MicroBooNE (which produced the new negative result) and its predecessor MiniBooNE.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=447&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=447&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=447&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=561&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=561&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429239/original/file-20211029-25-1yjs6bh.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=561&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Results from the MiniBooNE experiment hinted at the possible existence of a light sterile neutrino.</span>
<span class="attribution"><a class="source" href="https://www.hep.princeton.edu//~meyers/boone_pub/pss_install.html">Fred Ullrich / Fermilab</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>They fire a beam of muon-neutrinos over a relatively short distance (between 30 and 500 metres) and measure how many <em>electron</em>-neutrinos are detected at the other end.</p>
<p>At LSND and MiniBooNE, they saw more electron-neutrinos than expected. We know from other experiments that muon-neutrinos cannot oscillate directly into electron-neutrinos over these distances. </p>
<p>But if some of muon-neutrinos turn into very light sterile neutrinos and <em>then</em> into electron-neutrinos, it could explain how those extra electron-neutrinos appeared.</p>
<p>This is the sterile neutrino hypothesis.</p>
<h2>What if there are sterile neutrinos?</h2>
<p>If experiments did confirm the existence of a light sterile neutrino, there would be a good chance that heavier sterile neutrinos exist as well. </p>
<p>These heavier cousins could answer several major puzzles in particle physics, such as the nature of the “dark matter” that seems to make up most of the Universe, why neutrinos have any mass at all, and why the Universe contains so much more matter than antimatter.</p>
<p>There is but one problem. The light sterile neutrino we started with is a headache for cosmologists. </p>
<p>If it exists, we should be able to observe traces of sterile neutrinos formed just after the Big Bang.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=300&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=300&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=300&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=377&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=377&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429243/original/file-20211029-13-ejsnuj.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=377&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">If sterile neutrinos exist, they should have left traces in the cosmic microwave background, a faint afterglow of radiation from the dawn of the Universe that pervades the sky.</span>
<span class="attribution"><a class="source" href="https://www.esa.int/ESA_Multimedia/Images/2013/03/Planck_CMB">ESA / Planck Collaboration</a></span>
</figcaption>
</figure>
<p>However, no recent surveys of the cosmic microwave background radiation or the distribution of galaxies and light elements in between them show any sign these sterile neutrinos existed.</p>
<p>This could mean the sterile neutrino hypothesis is incorrect. But it is also possible that something else in our understanding of the Universe is amiss.</p>
<h2>MicroBooNE and the global picture</h2>
<p>MicroBooNE analysed its results in four different ways, and none of them turned up signs of extra electron-neutrinos. This is disappointing for the researchers behind the LSND and MiniBooNE collaborations, and for proponents of the sterile neutrino hypothesis.</p>
<p>It also raises the question of exactly what caused the results observed by the earlier experiments. Further analysis of the MicroBooNE data may help unravel this mystery.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=358&fit=crop&dpr=1 600w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=358&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=358&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=450&fit=crop&dpr=1 754w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=450&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/429017/original/file-20211028-15-2qr7dn.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=450&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The IceCube experiment in Antarctica has found no evidence in favour of sterile neutrinos.</span>
<span class="attribution"><a class="source" href="https://icecube.wisc.edu/gallery/landscapes-and-skyscapes/#modulagallery-6276-2088">Emanuel Jacobi / IceCube / NSF</a></span>
</figcaption>
</figure>
<p>Globally, however, MicroBooNE’s latest result is in line with the findings of two large “disappearance” experiments, MINOS+ and IceCube. Neither of these saw evidence of disappearing muon-neutrinos as the sterile neutrino hypothesis predicts.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/spotting-astrophysical-neutrinos-is-just-the-tip-of-the-icecube-20499">Spotting astrophysical neutrinos is just the tip of the IceCube</a>
</strong>
</em>
</p>
<hr>
<p>Elsewhere, there have been claims of disappearing neutrinos in nuclear reactor experiments. However, calculating how many neutrinos a nuclear reactor will emit is notoriously difficult, so these claims are best taken with a grain of salt.</p>
<h2>Future searches</h2>
<p>The MicroBooNE collaboration has so far analysed only half of its collected data. </p>
<p>Some have also questioned whether no excess of electron-neutrinos necessarily means no neutrino oscillations, given the measurement has been made at only one distance. Technically, we need measurements at two distances or more to definitively establish oscillations or otherwise.</p>
<p>These measurements are likely to come in the next few years, when Fermilab switches on two more detectors as part of the <a href="https://sbn.fnal.gov/">Fermilab Short Baseline Neutrino program</a>. The trio of detectors will test for disappearance of muon-neutrinos and appearance of electron-neutrinos using a single beam of source neutrinos.</p>
<p>The prospects for a final verdict on light sterile neutrinos in the next decade are therefore good.</p><img src="https://counter.theconversation.com/content/170369/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Yvonne Wong receives funding from the Australian Research Council. </span></em></p><p class="fine-print"><em><span>Michael Schmidt receives funding from the Australian Research Council.</span></em></p>
A new result from the MicroBooNE neutrino experiment has dashed hopes for a neat resolution to several puzzles for physicists.
Yvonne Wong, Associate professor of physics and ARC Future Fellow, UNSW Sydney
Michael Schmidt, Senior lecturer in physics, UNSW Sydney
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/170133
2021-10-19T11:04:14Z
2021-10-19T11:04:14Z
New physics: latest results from Cern further boost tantalising evidence
<figure><img src="https://images.theconversation.com/files/426951/original/file-20211018-57123-1i3a8rx.jpg?ixlib=rb-1.1.0&rect=0%2C26%2C1592%2C1159&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">There's more going on in the universe than we know.</span> <span class="attribution"><a class="source" href="https://flickr.com/photos/zoltlevay/17290564055/in/photolist-skUEin-2jpVGiL-SmMs6J-25HSziS-26CVPhm-2jGcDsw-ZUtHrQ-2hYELzE-2jrDPQ5-2hYTHbt-2hMCnmp-Zxttv9-ac29Ct-kX7TJF-2mgGPXm-KK7dFy-VXFewH-2m493mC-23yam7D-2htZuyS-PAuyY2-2kZk4Sv-ANHfHd-2gqkJuK-vpPXp2-J9FLaV-JDEPLy-2jpUttD-2m8vFgG-qDywSW-4hq6fG-2hXF5T9-28prfUU-2hYCngf-Mg6wGM-Nky8yz-2hYEXQz-j5pqja-VPCZMh-UUESai-zdR8je-2hnbV4q-2jt14fV-UXEgSc-CmHioc-2hnbUQ9-E3gqc3-KYm3GB-BpjFZt-2hAkpkX">Zolt Levay/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The Large Hadron Collider (LHC) sparked worldwide excitement in March as particle physicists reported <a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">tantalising evidence</a> for new physics - potentially a new force of nature. Now, <a href="https://arxiv.org/abs/2110.09501">our new result</a>, yet to be peer reviewed, from Cern’s gargantuan particle collider seems to be adding further support to the idea.</p>
<p>Our current best theory of particles and forces is known as the <a href="https://home.cern/science/physics/standard-model">standard model</a>, which describes everything we know about the physical stuff that makes up the world around us with unerring accuracy. The standard model is without doubt the most successful scientific theory ever written down and yet at the same time we know it must be incomplete.</p>
<p>Famously, it describes only three of the <a href="https://www.space.com/four-fundamental-forces.html">four fundamental forces</a> – the electromagnetic force and strong and weak forces, leaving out gravity. It has no explanation for the <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">dark matter</a> that astronomy tells us dominates the universe, and cannot explain <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">how matter survived</a> during the big bang. Most physicists are therefore confident that there must be more cosmic ingredients yet to be discovered, and studying a variety of fundamental particles known as beauty quarks is a particularly promising way to get hints of what else might be out there.</p>
<p>Beauty quarks, sometimes called bottom quarks, are <a href="https://home.cern/science/physics/standard-model">fundamental particles</a>, which in turn make up bigger particles. There are six flavours of quarks that are dubbed up, down, strange, charm, beauty/bottom and truth/top. Up and down quarks, for example, make up the protons and neutrons in the atomic nucleus.</p>
<p>Beauty quarks are unstable, living on average just for about 1.5 trillionths of a second before decaying into other particles. The way beauty quarks decay can be strongly influenced by the existence of other fundamental particles or forces. When a beauty quark decays, it transforms into a set of lighter particles, such as electrons, through the influence of the weak force. One of the ways a new force of nature might make itself known to us is by subtly changing how often beauty quarks decay into different types of particles. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/a-new-force-of-nature-the-inside-story-of-fresh-evidence-from-cern-thats-exciting-physicists-podcast-158198">A new force of nature? The inside story of fresh evidence from Cern that's exciting physicists – podcast</a>
</strong>
</em>
</p>
<hr>
<p>The March paper was based on data from the LHCb experiment, one of four giant particle detectors that record the outcome of the ultra high-energy collisions produced by the LHC. (The “b” in LHCb stands for “beauty”.) It found that beauty quarks were decaying into electrons and their heavier cousins called muons at different rates. This was truly surprising because, according to the standard model, <a href="https://www.isis.stfc.ac.uk/Pages/What-is-a-muon.aspx">the muon</a> is basically a carbon copy of the electron – identical in every way except for being around 200 times heavier. This means that all the forces should pull on electrons and muons with equal strength – when a beauty quark decays into electrons or muons via the weak force, it ought to do so equally often. </p>
<p>Instead, my colleagues found that the muon decay was only happening about 85% as often as the electron decay. Assuming the result is correct, the only way to explain such an effect would be if some new force of nature that pulls on electrons and muons differently is interfering with how beauty quarks decay. </p>
<p>The result caused huge excitement among particle physicists. We’ve been searching for signs of something beyond the standard model for decades, and despite ten years of work at the LHC, nothing conclusive has been found so far. So discovering a new force of nature would be a huge deal and could finally open the door to answering some of the deepest mysteries facing modern science. </p>
<h2>New results</h2>
<p>While the result was tantalising, it wasn’t conclusive. All measurements come with a certain degree of uncertainty or “error”. In this case there was only around a one in 1,000 chance that the result was down to a random statistical wobble – or “three sigma” as we say in particle physics parlance. </p>
<p>One in 1,000 may not sound like a lot, but we make a very large number of measurements in particle physics and so you might expect a small handful to throw up outliers just by random chance. To be really sure that the effect is real, we’d need to get to five sigma – corresponding to less than a one in a million chance of the effect being down to a cruel statistical fluke.</p>
<p>To get there, we need to reduce the size of the error, and to do this we need more data. One way to achieve this is simply to run the experiment for longer and record more decays. The LHCb experiment is currently <a href="https://gtr.ukri.org/projects?ref=ST%2FV003127%2F1">being upgraded</a> to be able to record collisions at a much higher rate in future, which will allow us to make much more precise measurements. But we can also get useful information out of the data we’ve already recorded by looking for similar types of decays that are harder to spot.</p>
<figure class="align-center ">
<img alt="Image of the LHCb experiment." src="https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/390889/original/file-20210322-19-nytkho.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">LHCb experiment.</span>
<span class="attribution"><span class="source">Cern</span></span>
</figcaption>
</figure>
<p>This is what my colleagues and I have done. Strictly speaking, we never actually study beauty quark decays directly, since all quarks are always bound together with other quarks to make larger particles. The March study looked at beauty quarks that were paired up with “up” quarks. Our result studied two decays: one where the beauty quarks that were paired with “down” quarks and another where they were also paired with up quarks. That the pairing is different shouldn’t matter, though – the decay that’s going on deep down is the same and so we’d expect to see the same effect, if there really is a new force out there.</p>
<p>And that is exactly what we’ve seen. This time, muon decays were only happening around 70% as often as the electron decays but with a larger error, meaning that the result is about “two sigma” from the standard model (around a two in a hundred chance of being a statistical anomaly). This means that while the result isn’t precise enough on its own to claim firm evidence for a new force, it does line up very closely with the previous result and adds further support to the idea that we might be on the brink of a major breakthrough.</p>
<p>Of course, we should be cautious. There is some way to go still before we can claim with a degree of certainty that we really are seeing the influence of a fifth force of nature. My colleagues are currently working hard to squeeze as much information as possible out of the existing data, while busily preparing for the first run of the upgraded LHCb experiment. Meanwhile, other experiments at the LHC, as well at the <a href="https://www.belle2.org">Belle 2 experiment in Japan</a>, are closing in on the same measurements. It’s exciting to think that in the next few months or years a new window could be opened on the most fundamental ingredients of our universe.</p><img src="https://counter.theconversation.com/content/170133/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Harry Cliff is employed by the University of Cambridge, is a member of the LHCb collaboration and receives funding from STFC. </span></em></p>
Particle physicists might be on the brink of a major breakthrough.
Harry Cliff, Particle physicist, University of Cambridge
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/159307
2021-04-22T15:51:44Z
2021-04-22T15:51:44Z
Antimatter: scientists find way to trap elusive material by blasting it with lasers
<figure><img src="https://images.theconversation.com/files/396533/original/file-20210422-20-ujnwp3.jpeg?ixlib=rb-1.1.0&rect=105%2C2%2C1211%2C752&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Cern scientists have successfully cooled antimatter with a laser for the first time.</span> <span class="attribution"><span class="source">Chukman So</span>, <span class="license">Author provided</span></span></figcaption></figure><p>Antimatter is believed to play a huge part in the story of our universe. It’s the counterpart to matter: identical in every way – with protons, neutrons and electrons – but with an opposite electric charge. According to our best understanding of the <a href="https://www.newscientist.com/article/dn17111-how-dirac-predicted-antimatter/">laws of physics</a>, the universe of today should be equally populated by both matter and antimatter.</p>
<p>Yet, as far as we can tell, <a href="https://home.cern/science/physics/matter-antimatter-asymmetry-problem">it’s not</a>. Antimatter is elusive, and one of the major conundrums in modern physics is how we can explain a “<a href="https://www.symmetrymagazine.org/article/october-2005/explain-it-in-60-seconds">symmetrical</a>” universe of equal parts matter and antimatter when, after decades of searching, the universe appears to be almost entirely void of antimatter.</p>
<p>To try to unravel this cosmic mystery, physicists are studying <a href="https://theconversation.com/cern-discovery-sheds-light-on-the-great-mystery-of-why-the-universe-has-less-antimatter-than-matter-147226">various features</a> of antimatter. In particular, we’re interested in small differences between matter and antimatter that could explain the asymmetry we observe – in turn validating existing laws of physics.</p>
<p>But studying antimatter is incredibly difficult. It takes huge amounts of energy to create it, and even then it’s liable to disappear: annihilating itself when it comes into contact with the matter that surrounds us. </p>
<p><a href="https://www.nature.com/articles/s41586-021-03289-6">Research by</a> my colleagues at Cern and I has produced a way to create, trap and laser-cool antimatter for long enough for us to target a whole new set of more accurate measurements. Our experiments could be a significant step in solving the mystery of the missing antimatter in our universe.</p>
<h2>Making antimatter</h2>
<p>Just as matter is made up of atoms, antimatter is made up of antiatoms. The easiest antiatom to make is antihydrogen, <a href="https://home.cern/news/press-release/cern/first-atoms-antimatter-produced-cern">first created</a> by Cern in 1995 and <a href="https://theconversation.com/antimatter-measured-for-the-first-time-5782">first measured</a> in 2012. Consisting of just one antielectron (called a positron) orbiting around a one antiproton nucleus, antihydrogen (and hydrogen, its counterpart in matter) has the simplest atomic structure in the universe.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/explainer-what-is-antimatter-53414">Explainer: what is antimatter?</a>
</strong>
</em>
</p>
<hr>
<p>But making antihydrogen isn’t easy. The classical high-energy physics approach to the problem uses a particle collider – like the <a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">LHC at Cern</a> – to convert enormous amounts of kinetic energy into a plethora of sub-atomic shrapnel for us to study.</p>
<p>Particle accelerators can be used to create antiprotons. To make a single usable antiproton, though, we need 1 million protons and at least 26 million times the energy that’s eventually “stored” in an antiproton. This makes each antiproton we make incredibly precious.</p>
<p>Once we’d created enough antiprotons, we needed antielectrons (positrons) in order to build our antiatoms. Happily, positrons can be quite easily gathered from a <a href="https://www.sciencedirect.com/topics/engineering/positron-emitting-radionuclides">radioactive source</a>. With our core ingredients collected, we just needed to combine them.</p>
<p>This we achieved by forcing the antiprotons and positrons into contact within an electromagnetic trap. Crucially, this had to happen in a vacuum, because if the antiparticles were to make contact with any parts of the apparatus – which was of course made of matter – they’d simply annihilate on contact, disappearing altogether. Only after all of these steps could we form usable antihydrogen atoms, pinned in a vacuum by a combination of magnetic fields. </p>
<figure class="align-center ">
<img alt="Four electrodes around a laser" src="https://images.theconversation.com/files/396572/original/file-20210422-13-va7eme.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/396572/original/file-20210422-13-va7eme.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=348&fit=crop&dpr=1 600w, https://images.theconversation.com/files/396572/original/file-20210422-13-va7eme.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=348&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/396572/original/file-20210422-13-va7eme.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=348&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/396572/original/file-20210422-13-va7eme.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=438&fit=crop&dpr=1 754w, https://images.theconversation.com/files/396572/original/file-20210422-13-va7eme.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=438&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/396572/original/file-20210422-13-va7eme.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=438&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">This demonstration of our electromagnetic trap shows how the forces it creates can hold charged particles in space.</span>
<span class="attribution"><a class="source" href="https://alpha.web.cern.ch/gallery-images/paul-trap-action">Niels Madsen</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Measuring antihydrogen</h2>
<p>In this state, it’s possible to take measurements of the antihydrogen. What we’re looking to measure here is a key atomic transition between two energy states of the antihydrogen atom. This transition is particularly suitable for precise measurements, and the equivalent one in hydrogen has been measured with a staggering 15 decimal places of precision.</p>
<p>We took our antihydrogen measurement to 12 decimal places of precision. This is worse than the most precise measurement of ordinary hydrogen by a factor of 1,000, but it’s currently the best measure of antihydrogen anyone has done.</p>
<p>But one key limitation of our measurement is the movement of the antiatoms in the trap itself, due to their kinetic energy. By reducing this movement further, our measurements would be far more accurate. Our experiment achieved this, for the first time, by blasting the antiatoms with laser light.</p>
<figure class="align-center ">
<img alt="A man inserts a rod into a container of liquid hydrogen in a lab" src="https://images.theconversation.com/files/396574/original/file-20210422-20-1nlyyzd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/396574/original/file-20210422-20-1nlyyzd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/396574/original/file-20210422-20-1nlyyzd.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/396574/original/file-20210422-20-1nlyyzd.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/396574/original/file-20210422-20-1nlyyzd.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/396574/original/file-20210422-20-1nlyyzd.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/396574/original/file-20210422-20-1nlyyzd.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Liquid helium helps cool antihydrogen in our trap – but lasers help reduce the temperature further.</span>
<span class="attribution"><a class="source" href="https://alpha.web.cern.ch/gallery-images/alpha-uk-work">Niels Madsen</a>, <span class="license">Author provided</span></span>
</figcaption>
</figure>
<h2>Laser cooling</h2>
<p>The light in a laser is made up of photons, which <a href="https://www.britannica.com/science/photon">carry a momentum</a> of their own. When an atom absorbs a photon, the atom’s velocity changes slightly. By following this basic principle, we knew we could use the momentum contained in our laser beam to reduce the kinetic energy of the trapped antiatoms – cooling them closer to absolute zero.</p>
<p>That required us to only hit the antiatoms with photons when they were moving towards the laser, as this would in effect cancel out some of the velocity of the antiatom: a bit like how you’d apply force to slow a child on a swing.</p>
<p>By using this targeted <a href="https://www.sciencedirect.com/topics/chemical-engineering/laser-cooling">laser-cooling</a>, we managed to reduce the temperature of our stored antihydrogen by a factor of ten, which has the potential to improve future measurement precision by a factor of four. </p>
<p>We’ve not yet made enough measurements to publish new, more precise data on antihydrogen – but that’s coming very soon. Beyond that, our laser-cooling technique has put us on a firm path towards higher precision in many measurements of both matter and antimatter, and takes us a step closer to making an even more precise measurement of hydrogen itself.</p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/cern-discovery-sheds-light-on-the-great-mystery-of-why-the-universe-has-less-antimatter-than-matter-147226">CERN: discovery sheds light on the great mystery of why the universe has less 'antimatter' than matter</a>
</strong>
</em>
</p>
<hr>
<p>Laser-cooling opens up exciting possibilities for measuring antihydrogen. Combined with existing techniques that allow us to accumulate relatively large amounts of antihydrogen (thousands of antiatoms per day) we will soon know even more about the nature of antihydrogen – and that extra knowledge could help us understand why matter is everywhere in our universe, while antimatter is so elusive.</p><img src="https://counter.theconversation.com/content/159307/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Niels Madsen receives (or has received) funding from the EPSRC, The Royal Society and the Leverhulm Trust. He is professor of experimental physics at Swansea University. </span></em></p>
Laser-cooling enables new measurements that could explain why antimatter is so scarce in our universe.
Niels Madsen, Professor of Physics, Swansea University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/157829
2021-04-09T15:31:39Z
2021-04-09T15:31:39Z
Proof of new physics from the muon’s magnetic moment? Maybe not, according to a new theoretical calculation
<figure><img src="https://images.theconversation.com/files/394121/original/file-20210408-13-1j4t129.jpg?ixlib=rb-1.1.0&rect=342%2C125%2C4017%2C2815&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Two new papers shed light on a longstanding mystery in particle physics.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/particle-movement-in-a-bubble-chamber-royalty-free-image/157506583?adppopup=true">Zmeel/E+ via Getty Images</a></span></figcaption></figure><p>When the results of an experiment don’t match predictions made by the best theory of the day, something is off.</p>
<p>Fifteen years ago, physicists at <a href="https://www.bnl.gov/world/">Brookhaven National Laboratory</a> discovered something perplexing. Muons – a type of subatomic particle – were moving in unexpected ways that didn’t match theoretical predictions. Was the theory wrong? Was the experiment off? Or, tantalizingly, was this evidence of new physics? </p>
<p>Physicists have been trying to solve this mystery every since. </p>
<p>One group from <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics">Fermilab</a> tackled the experimental side and on April 7, 2021, released results <a href="https://www.bnl.gov/newsroom/news.php?a=118760">confirming the original measurement</a>. But my colleagues and I took a different approach. </p>
<p><a href="https://scholar.google.com/citations?user=EDOpw0YAAAAJ&hl=en&oi=ao">I am a theoretical physicist</a> and the spokesperson and one of two coordinators of the <a href="http://www.bmw.uni-wuppertal.de/Home.html">Budapest-Marseille-Wuppertal collaboration</a>. This is a large–scale collaboration of physicists who have been trying to see if the older theoretical prediction was incorrect. We used a <a href="https://doi.org/10.1038/s41586-021-03418-1">new method</a> to calculate how muons interact with magnetic fields. </p>
<p>My team’s theoretical prediction is different from the original theory and matches both the old experimental evidence and the new Fermilab data. If our calculation is correct, it resolves the discrepancy between theory and experiment and would suggest that there is not an undiscovered force of nature.</p>
<p><a href="https://doi.org/10.1038/s41586-021-03418-1">Our result was published in the journal Nature</a> on April 7, 2021, the same day as the new experimental results.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="All of the particles and forces of the Standard Model of physics." src="https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=373&fit=crop&dpr=1 600w, https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=373&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=373&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=469&fit=crop&dpr=1 754w, https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=469&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/394122/original/file-20210408-21-1mlac6i.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=469&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Standard Model of physics is the most accurate theory of the universe to date.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_of_Elementary_Particles_Anti.svg#/media/File:Standard_Model_of_Elementary_Particles_Anti.svg">Cush/Wikimedia Commons</a></span>
</figcaption>
</figure>
<h2>The muon and the Standard Model</h2>
<p>The muon is a heavier, unstable sister of the electron. Muons are all around us and are, for example, created when <a href="https://www.radioactivity.eu.com/site/pages/Cosmic_Muons.htm">cosmic rays collide with particles in the Earth’s atmosphere</a>. They are able to pass through matter, and researchers have used them to probe the inaccessible interiors of structures from <a href="https://doi.org/10.1063/PT.3.1829">giant volcanoes</a> to the <a href="https://www.nature.com/news/cosmic-ray-particles-reveal-secret-chamber-in-egypt-s-great-pyramid-1.22939">Egyptian pyramids</a>. </p>
<p>Muons, like electrons, have an electric charge and generate tiny magnetic fields. The strength and orientation of this magnetic field is called the magnetic moment. </p>
<p>Almost everything in the universe – from how atoms are built to how your cellphone works to how galaxies move – can be described by four interactions. You are probably familiar with the first two: gravity and electromagnetism. The third is the <a href="https://www.livescience.com/49254-weak-force.html">weak interaction</a>, which is responsible for radioactive decay. Last is the <a href="https://en.wikipedia.org/wiki/Strong_interaction">strong interaction</a>, the force that holds the protons and neutrons in an atom’s nucleus together. Physicists call this framework – minus gravity – the Standard Model of particle physics.</p>
<p>All interactions of the Standard Model contribute to the muon’s magnetic moment and each do so in multiple different ways. Physicists very precisely know how <a href="https://doi.org/10.1103/PhysRevLett.109.111808">electromagnetism</a> and the <a href="https://doi.org/10.1103/PhysRevLett.76.3267">weak interaction</a> do so, but determining how the strong interaction contributes to the muon’s magnetic field has proven to be incredibly hard to do. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Iron filings showing the magnetic field lines of a magnet." src="https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=403&fit=crop&dpr=1 600w, https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=403&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=403&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=506&fit=crop&dpr=1 754w, https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=506&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/394123/original/file-20210408-15-3khioz.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=506&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The magnetic field of the muon has proven incredibly hard to predict.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Magnet0873.png#/media/File:Magnet0873.png">Newton Henry Black/Wikimedia Commons</a></span>
</figcaption>
</figure>
<h2>A magnetic mystery</h2>
<p>Of all of the effects that the strong interaction has on the muon’s magnetic moment, the largest and also hardest to calculate with the necessary precision is called the Leading Order Hadronic Vacuum Polarization.</p>
<p>In the past, to calculate this effect, physicists used a mixed theoretical–experimental approach. They would collect data from collisions between electrons and positrons – the opposite of electrons – and use it to calculate the strong interaction’s contribution to the muon’s magnetic moment. Physicists have been using this approach to <a href="https://doi.org/10.1103/PhysRev.168.1620">further refine the estimate for decades</a>. The latest results are from 2020 and produced a <a href="https://doi.org/10.1140/epjc/s10052-020-7792-2">very precise estimate</a>.</p>
<p>This calculation of the magnetic moment is what experimental physicists have been testing for decades. Until April 7, 2021, the most precise experimental result was 15 years old. For this measurement, at Brookhaven National Laboratory, researchers created muons in a particle accelerator and then watched how they moved through a magnetic field using a giant, 50-foot-wide (15-meter) electromagnet. By measuring how muons moved and decayed, they were able to directly measure the muon’s magnetic moment. It came as quite the surprise when Broohaven’s 2006 <a href="https://doi.org/10.1103/PhysRevD.73.072003">direct measurement of the muon’s magnetic moment</a> was larger than it should have been according to theory.</p>
<p>Faced with this discrepancy, there were three options: Either the theoretical prediction was incorrect, the experiment was incorrect or, as many physicists believed, this was a sign of an unknown force of nature. </p>
<p>So which was it?</p>
<h2>New theories</h2>
<p>My colleagues and I chose to pursue the first option: The theory might be off in some way. So, we decided to try to find a better way to calculate the prediction. Our team of physicists took the most basic underlying equations of the strong interaction, put the equations on an space-time grid and solved as many of them as possible at once.</p>
<p>The technique is kind of like making a weather forecast. As commercial aircrafts fly their routes, they measure pressure, temperature and the speed of wind at given points on Earth. Similarly, we placed the strong interaction equation on a space-time grid. The weather data at individual points are then put into a supercomputer that combines all of the data to predict the evolution of the weather. Our team put the strong interaction forces on a grid and looked for the evolution of these fields. The more planes collecting data, the better the prediction. In this metaphor, we used billions of airplanes to calculate the most precise magnetic moment we could using millions of computer processing hours at multiple supercomputer centers in Europe.</p>
<p>Our new approach produces an estimate of the strength of the muon’s magnetic field that closely matches the experimental value measured by the Brookhaven scientists. It essentially closes the gap between theory and experimental measurements and, if true, confirms the Standard Model that has guided particle physics for decades. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A large blue doughnut–shaped magnet used to measure muons." src="https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/394125/original/file-20210408-13-eebv45.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Fermilab experiment, using the same magnet from Brookhaven, measured an almost identical magnetic moment for the muon.</span>
<span class="attribution"><a class="source" href="https://vms.fnal.gov/asset/detail?recid=1950114">Reidar Hahn/Fermilab</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<h2>New experiments</h2>
<p>But my colleagues and I have not been the only ones pursuing this mystery. Other scientists, <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics">like the ones at Fermilab</a>, a particle accelerator close to Chicago, have chosen to test the second option: that the experiment was off.</p>
<p>At Fermilab, physicists have been continuing the experiment that was done at Brookhaven to get a more precise experimental measurement of the muon’s magnetic moment. They used a more intense muon source that gave them a more precise result. It matched the <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics">old measurement almost perfectly</a>.</p>
<p>The Fermilab results strongly suggest that the experimental measurements are correct. The new theoretical prediction made by my colleagues and me matches with these experimental results. While it may have been exciting to discover hints of new physics, our new theory seems to say that this time, the Standard Model is holding up. </p>
<p>One mystery remains though: the gap between the original prediction and our new theoretical result. My team and I believe that ours is correct, but our result is the very first of its sort. As always in science, other calculations need to be done to confirm or refute it.</p>
<p>[<em>Deep knowledge, daily.</em> <a href="https://theconversation.com/us/newsletters/the-daily-3?utm_source=TCUS&utm_medium=inline-link&utm_campaign=newsletter-text&utm_content=deepknowledge">Sign up for The Conversation’s newsletter</a>.]</p><img src="https://counter.theconversation.com/content/157829/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Zoltan Fodor receives funding from DFG, BMBF.</span></em></p>
For 15 years, there has been a mismatch in physics. A particle called the muon wasn’t behaving the way theory predicted it should. A new theory and new experiment might solve this problem.
Zoltan Fodor, Professor of Physics/ICDS Fellow, Penn State
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/158564
2021-04-08T16:35:08Z
2021-04-08T16:35:08Z
How we found hints of new particles or forces of nature – and why it could change physics
<figure><img src="https://images.theconversation.com/files/394026/original/file-20210408-17-1ngm55l.jpg?ixlib=rb-1.1.0&rect=154%2C132%2C7172%2C4726&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The muon experiment.</span> <span class="attribution"><a class="source" href="https://vms.fnal.gov/asset/detail?recid=1950114">Reidar Hahn/Fermilab</a></span></figcaption></figure><p>Seven years ago, a huge magnet was transported over 3,200 miles (5,150km) across land and sea, in the hope of studying a subatomic particle called a muon.</p>
<p>Muons are closely related to electrons, which orbit every atom and form the building blocks of matter. The electron and muon both have properties precisely predicted by our current best scientific theory describing the subatomic, quantum world, the <a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">standard model of particle physics</a>. </p>
<p>A whole generation of scientists have dedicated themselves to measuring these properties in exquisite detail. In 2001, an experiment hinted that one property of the muon was not exactly as the standard model predicted, but new studies were needed to confirm. Physicists moved part of the experiment to a new accelerator, at Fermilab, and started taking more data.</p>
<p>A <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.141801">new measurement</a> has now confirmed the initial result. This means new particles or forces may exist that aren’t accounted for in the standard model. If this is the case, the laws of physics will have to be revised and no one knows where that may lead.</p>
<iframe src="https://embed.acast.com/60087127b9687759d637bade/606490a356dcab18893447f3?cover=true&ga=false" frameborder="0" allow="autoplay" width="100%" height="110"></iframe>
<p>This latest result comes from an international collaboration, of which we are both a part. Our team has been using particle accelerators to measure a property called the magnetic moment of the muon.</p>
<p>Each muon behaves like a tiny bar magnet when exposed to a magnetic field, an effect called the magnetic moment. Muons also have an intrinsic property called “spin”, and the relation between the spin and the magnetic moment of the muon is known as the g-factor. The “g” of the electron and muon is predicted to be two, so g minus two (g-2) should be measured to be zero. This is what’s we’re testing at Fermilab.</p>
<p>For these tests, scientists have used accelerators, the same kind of technology Cern uses at the LHC. The Fermilab accelerator produces muons in very large quantities and measures, very precisely, how they interact with a magnetic field. </p>
<hr>
<p>
<em>
<strong>
Read more:
<a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">Evidence of brand new physics at Cern? Why we're cautiously optimistic about our new findings</a>
</strong>
</em>
</p>
<hr>
<p>The muon’s behaviour is influenced by “virtual particles” that pop in and out of existence from the vacuum. These exist fleetingly, but for long enough to affect how the muon interacts with the magnetic field and change the measured magnetic moment, albeit by a tiny amount. </p>
<p>The standard model predicts very precisely, to better than one part in a million, what this effect is. As long as we know what particles are bubbling in and out of the vacuum, experiment and theory should match. But, if experiment and theory don’t match, our understanding of the soup of virtual particles may be incomplete.</p>
<h2>New particles</h2>
<p>The possibility of new particles existing is not idle speculation. Such particles might help in explaining several of the big problems in physics. Why, for example, does the universe have <a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">so much dark matter</a> – causing the galaxies to rotate faster than we’d expect – and why has nearly all the anti-matter created in the Big Bang disappeared? </p>
<p>The problem to date has been that nobody has seen any of these proposed new particles. It was hoped the LHC at Cern would produce them in collisions between high energy protons, but they’ve not yet been observed. </p>
<figure class="align-center ">
<img alt="A truck carrying a much wider cargo down a road." src="https://images.theconversation.com/files/393806/original/file-20210407-19-1b2ee57.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/393806/original/file-20210407-19-1b2ee57.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=401&fit=crop&dpr=1 600w, https://images.theconversation.com/files/393806/original/file-20210407-19-1b2ee57.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=401&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/393806/original/file-20210407-19-1b2ee57.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=401&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/393806/original/file-20210407-19-1b2ee57.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/393806/original/file-20210407-19-1b2ee57.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/393806/original/file-20210407-19-1b2ee57.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=503&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Moving the muon ring.</span>
<span class="attribution"><a class="source" href="https://vms.fnal.gov/asset/detail?recid=1819456">Reidar Hahn/Fermilab</a></span>
</figcaption>
</figure>
<p>The new measurement used the same technique as an experiment at “Brookhaven National Laboratory in New York, at the beginning of the century, which itself followed a series of measurements at Cern.</p>
<p>The Brookhaven experiment measured a discrepancy with the standard model that had a one in 5,000 chance of being a statistical fluke. This is approximately the same probability as throwing a coin 12 times in a row, all heads up. </p>
<p>This was tantalising, but way below the threshold for discovery, which is generally required to be better than one in 1.7 million – or 21 coin throws in a row. To determine whether new physics was in play, scientists would have to increase the sensitivity of the experiment by a factor of four.</p>
<p>To make the improved measurement, the magnet at the heart of the experiment had to be moved in 2013 3,200 miles from Long Island along sea and road, to Fermilab, outside Chicago, whose accelerators could produce a copious source of muons. </p>
<p>Once in place, a new experiment was built around the magnet with state of the art detectors and equipment. The muon g-2 experiment began taking data in 2017, with a collaboration of veterans from the Brookhaven experiment and a new generation of physicists.</p>
<p>The new results, from the first year of data at Fermilab, are in line with the measurement from the Brookhaven experiment. Combining results reinforces the case for a disagreement between experimental measurement and the standard model. The chances now lie at about one in 40,000 of the discrepancy being a fluke – still shy of the gold standard discovery threshold.</p>
<figure class="align-center ">
<img alt="A graph showing the prediction for the muon magnetic moment and the experimental results." src="https://images.theconversation.com/files/393970/original/file-20210408-23-17mxxvq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/393970/original/file-20210408-23-17mxxvq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/393970/original/file-20210408-23-17mxxvq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/393970/original/file-20210408-23-17mxxvq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/393970/original/file-20210408-23-17mxxvq.jpeg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/393970/original/file-20210408-23-17mxxvq.jpeg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/393970/original/file-20210408-23-17mxxvq.jpeg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=566&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The prediction and the results.</span>
<span class="attribution"><a class="source" href="https://news.fnal.gov/wp-content/uploads/2021/04/Muon-g-2-results-plot.jpg">Ryan Postel, Fermilab/Muon g-2 collaboration</a></span>
</figcaption>
</figure>
<h2>The LHC</h2>
<p>Intriguingly, a <a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">recent observation by the LHCb experiment</a> at Cern also found possible deviations from the standard model. What’s exciting is that this also refers to the properties of muons. This time it’s a difference in how muons and electrons are produced from heavier particles. The two rates are expected to be the same in the standard model, but the experimental measurement found them to be different. </p>
<p>Taken together, the LHCb and Fermilab results strengthen the case that we’ve observed the first evidence of the standard model prediction failing, and that there are new particles or forces in nature out there to be discovered. </p>
<p>For the ultimate confirmation, this needs more data both from the Fermilab muon experiment and from Cern’s LHCb experiment. Results will be forthcoming in the next few years. Fermilab already has four times more data than was used in this recent result, currently being analysed, Cern has started taking more data and a new generation of muon experiments is being built. This is a thrilling era for physics.</p><img src="https://counter.theconversation.com/content/158564/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Themis Bowcock receives funding from UKRI. </span></em></p><p class="fine-print"><em><span>Mark Lancaster receives funding from UKRI (STFC), Horizon 2020.</span></em></p>
New particles or forces may exist that aren’t accounted for in the standard model.
Themis Bowcock, Professor of Particle Physics, University of Liverpool
Mark Lancaster, Professor of Physics, University of Manchester
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/158244
2021-04-06T10:38:19Z
2021-04-06T10:38:19Z
What a possible new breakthrough at Cern could reveal about the structure of the universe
<figure><img src="https://images.theconversation.com/files/393099/original/file-20210401-15-1hof5g7.jpg?ixlib=rb-1.1.0&rect=40%2C15%2C1396%2C942&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Cern's LHCb experiment has spotted more evidence of an anomaly in the standard model of physics. </span> <span class="attribution"><a class="source" href="http://cds.cern.ch/record/2302374?ln=fr#24">© 2018-2021 CERN</a></span></figcaption></figure><p><em>This is a transcript of Episode 9 of The Conversation Weekly podcast, <a href="https://theconversation.com/a-new-force-of-nature-the-inside-story-of-fresh-evidence-from-cern-thats-exciting-physicists-podcast-158198">A new force of nature? The inside story of fresh evidence from Cern that’s exciting physicists</a>. In this episode, listen to how scientists working at Cern’s Large Hadron Collider found tantalising new evidence which could mean we have to rethink what we know about the universe. And an update on the situation for Rohingya refugees from Myanmar living in Bangladesh after a deadly fire swept through a refugee camp there.</em></p>
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<p><em>NOTE: Transcripts may contain errors. Please check the corresponding audio before quoting in print.</em></p>
<p>Gemma Ware: Hello and welcome to The Conversation Weekly. </p>
<p>Dan Merino: This week, we talk to experts about new evidence from particle physics that could mean we have to rethink what we know about the universe. </p>
<p>Harry Cliff: So the only way you can explain this effect is if there’s some new force. </p>
<p>Gemma: And we talk to an expert on Myanmar’s Rohingya minority about the situation after a devastating fire ripped through a refugee camp in Bangladesh where many have been living.</p>
<p>Rubayat Jesmin: There were losses of lives and more than 10,000 houses were burnt. </p>
<p>Dan: I’m Dan Merino in San Francisco.</p>
<p>Dan: And I’m Gemma Ware in London and you’re listening to The Conversation Weekly, the world explained by experts. </p>
<p>Dan: OK Gemma, so the first story today is about, well, just the substance of the universe. </p>
<p>Gemma: Nothing big then. </p>
<p>Dan: Well actually quite small, what we’re talking about here is tiny, at the subatomic level. Basically what’s going on inside the centre of an atom, the stuff which makes up, you, me and everything around us. </p>
<p>Gemma: Essentially it’s invisible to you and me then. </p>
<p>Dan: Yep, totally invisible to the naked eye and you couldn’t even see it on a microscope. Physicists know a lot about this suabtomic world, but there are so many unanswered questions. And in late March, physicists working at the Large Hadron Collider, a massive particle accelerator at Cern in Geneva, announced, tentatively I should add, that they’d had a bit of a breakthrough. </p>
<p>Dan: If what they think they’ve seen is proven correct, it could mean entirely new physics – essentially the model that we use to understand the universe needs a bit of a tweak. </p>
<p>Gemma: Now, people who study particle physicists can be broken down into two broad types: those who do experiments at places like Cern’s Large Hadron Collider, these are the experimentalists; and those who try to generate theories using super complex mathematics, these are the theorists. </p>
<p>Dan: And we’re going to hear from one of each of them in this story. Harry Cliff is an experimental particle physicist at the University of Cambridge. He works at Cern and was part of the team that <a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">recently announced this new tantalising discovery</a>. Celine Boehm is a theorist who studies particle physics and cosmology. She’s a professor and head of physics at the University of Sydney. And I’m going to let Celine, who also worked at Cern for a while, set the scene for us.</p>
<p>Celine Boehm: When you go down there, it’s absolutely amazing because you, you can take a lift and you go down 100 metres. When the doors open you’re in a cavity and suddenly there is this massive experiment. And I used to do the visits there and I used to say it’s like a cathedral for physics. It’s like a major building in front of you but it’s just dedicated to understand the most fundamental aspects of the world we live in.</p>
<p>The name Large Hadron Collider means there are three terms, large, hadron and collider. Collider means it’s basically a facility where you take particles, you accelerate them and then you smash them against each other. And the reason why we do this is that we, we hope that by smashing them, we discover even more fundamental particles. Now we call it a hadron because it’s a certain type of particles, which are called the protons. </p>
<p>The reason why we called it large is that it is actually the largest in the world that has ever been built. So you have a tunnel which is circular, which is 27 kilometre diameter, and it’s passing through France and Switzerland. And in those tunnels basically we have pipes in which we have those proton which are accelerated and smashed. And we need to observe how it works, so we have four locations in which we have what we call a detector. So we have four experiments those experiments are basically detecting the result of the collisions and they’re essentially detecting energy and light, which is emitted through the collision. </p>
<p>Gemma: It really makes you want to go there, this enormous underground tunnel with all these protons smashing into each other at really high speed. But what is Cern actually looking for?</p>
<p>Dan: Well, you probably heard of this one. The first, and probably most famous discovery was the the Higgs Boson. </p>
<p>Dan: The Higgs Boson was detected at Cern in 2012. It’s a particle that has to do with why things have mass, and when one is created it only exists for a split second before decaying and disappearing. </p>
<p>Gemma: And they were expecting to find this Higgs Boson, weren’t they?</p>
<p>Dan: Yeah, it had actually been predicted by theoretical physicists back in the 70s. And in fact, it would have been kind of worrying if the Large Hadron Collider hadn’t found evidence of the Higgs Boson. That would have meant something was off in what’s called the standard model of particle physics. This time, physicists at Cern have found evidence that something might be missing from this model. But what, you might be wondering is the standard model. Well, let’s have Harry Cliff explain it.</p>
<p>Harry: The standard model is probably the most successful scientific theory that we’ve ever developed. So it’s a theory that describes the basic building blocks of the universe. So fundamental particles, the things that make up atoms, the forces that bind them together. It’s been around more or less in the current form since the mid 1970s, when it was sort of put together theoretically.</p>
<p>And since then, every single prediction that it’s made has been verified in experiment. And that, I guess the most recent version of that was when the Higgs Boson was discovered almost a decade ago at Cern. </p>
<p>So it’s this like incredibly successful framework for understanding the microscopic, subatomic world. But we know it’s seriously incomplete. So from astronomy, particularly we know that there’s way, way more stuff in the universe than we can see with our telescopes. There’s huge amounts of two mysterious substances called dark matter and dark energy, which are really just words for, we don’t know what they are, they’re kind of mysterious things. The standard model doesn’t explain what they are. There are no particles in the standard model that could answer those questions. There’s loads of other things like, you know, the standard model predicts, for example, that there shouldn’t be any matter in the universe, which is a bit of a problem given that, you know, a theory that predicts that his own authors don’t exist is probably in trouble. So that there are lots of reasons for thinking there’s new stuff out there.</p>
<p>Dan: So we’ve got this predictive model, the standard model, but what does the standard model predict and what are those fundamental particles?</p>
<p>Harry: Right. OK, well, let’s start from the beginning. I guess everyone’s familiar with the idea of an atom. And if you zoom in on an atom, you see that it’s essentially like a little solar system. </p>
<p>In the centre, you’ve got a nucleus which contains most of the mass of the atom, and then you’ve got electrons that go around the outside. So the electron was the first fundamental particle to be discovered more than 100 years ago. That’s part of the standard model. If you go into the nucleus, you find quarks, which are the up and down quark, which make up the material in the nucleus.</p>
<p>Dan: OK, so to clarify real quick, at the centre of an atom, in the nucleus, you have protons and neutrons. Protons and neutrons are made of teeny, tiny particles called quarks. They come in two flavours, an up quark and a down quark, and that has to do with which direction they spin. Put a few quarks together, and you get a proton or a neutron. Put a few protons, neutrons and electrons together and you get an atom. </p>
<p>Harry: You’ve actually got 12 matter particles in total. So you’ve got the electron the up quark, the down quark. Something called a neutrino. And then for reasons we do not understand those four matter particles come in three families, which have very similar properties but different masses. So there’s like a heavy version of the electron called the muon and an even heavier version of that called the tau and the quarks all have heavy versions as well. We don’t really know why there are like 12 of these things. It’s a bit of a mystery. </p>
<p>The particles interact through four forces in nature. So we have gravity, which is probably the most familiar force to all of us. But one of the defects of the standard model is that it doesn’t say anything about gravity. So that’s sort of a big problem for, not for now, really. And then you’ve got these other three forces.
So there’s electromagnetic force, which is responsible for electricity, magnetism, light. And it has a particle called the photon. </p>
<p>Dan: A photon is essentially a particle of light. It’s what warms you skin when you stand in the sun and its how music is transmitted over the radio. </p>
<p>Harry: Then you’ve got two nuclear forces, the strong and the weak nuclear force, which are probably a bit less familiar, but the strong force essentially glues the quarks together inside the nucleus. That comes with a bunch of force particles called gluons cause they literally glue stuff together. And then the weak nuclear force, which is a sort of rather weird force, which basically allows particles to transform into each other so that it can, you know, one kind of quark can turn into another type of quark. So these forces are like kind of messengers that allow the matter particles, the electrons and the quarks and the muons and all these things to sort of interact and change between each other.</p>
<p>Dan: Explain to me where your recent discovery fits into this standard model.</p>
<p>Harry: OK, so that there’s broadly speaking two different ways you can look for new particles. There’s a direct method, which is kind of how the Higgs Boson was discovered, where you use your big particle accelerator. You smash particles into each other. The energy of those collisions is turned into new matter and you get new particles coming out and you detect them with your detector. </p>
<p>There’s another method, which is known as an indirect search and like a sort of analogy that I often use to describe this is if you’re like say looking for a very rare animal in a thick jungle, there are two ways you might go about that. One would be to like wander around in the jungle, trying to catch the animal itself, like maybe in a clearing somewhere. But if it’s a really big jungle and you don’t know what you’re looking for, you don’t really know where you’re supposed to be looking you might be wondering round for days and you won’t find anything.</p>
<p>So a slightly cannier thing to do would be, well, let’s look at the ground and let’s see if we can see footprints of anything, you know, kind of moving around in the jungle. So we might see a footprint and go, “OK, there’s clearly something out here, but we don’t necessarily know exactly what kind of animal it is, just from its footprint, but it gives us a clue about where to then look next.” So, that’s sort of what we do at LHCb, the experiment that this results come out from this week, where we do indirect measurements. </p>
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Read more:
<a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">Evidence of brand new physics at Cern? Why we're cautiously optimistic about our new findings</a>
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</p>
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<p>What we’re looking at basically are particles we already know about – so standard model particles. And you’re looking at how they behave and you’re measuring the way they behave very accurately. And you try to see them behaving in ways that you can’t explain with your current theory, so sort of deviations from your expectations. And the way this works is basically, the specific thing that we’ve seen is a particle called a beauty quark. </p>
<p>Dan: Beauty quark’s a fantastic name. What actually is a beauty quark?</p>
<p>Harry: So it’s basically like a heavy version of the down quark that you find inside every atom, but these quarks don’t exist in the universe normally they’re very short lived. You make loads of these at LHCb and they decay. So they’re, they’re unstable. They’re produce in the collisions that the LHC then decay into other particles. And, and like so these parties, they’re not breaking apart. It’s not like a, you know, a car that’s falling to pieces with bits coming off it. It’s literally changing into something else. So it transforms. The beauty quark is a fundamental particle, but it changes its nature and it becomes three other particles.</p>
<p>Dan: OK Beauty quarks dont last very long and decay into other particles. And your new finding was about beauty quarks decaying, right? So what exactly were you looking at with the Large Hadron Collider?</p>
<p>Harry: What we’re looking at is how often they decay into electrons versus how often they decay into muons, and according to the standard model and the forces in the standard model these two decays should happen equally as often. So, if you have like 1,000 beauty quark decays, 500 of them should decay to electrons, 500 should decay to muons. But what we’re actually seeing is the moun decays are happening less often and they’re happening about 85% as often as the electron decays. </p>
<p>And this is really, really weird because the forces in the standard model treat electrons and muons as like identical copies of each other, more or less. So any process involving electrons and muons for the most part should happen just as often. So, the only way you can explain this effect is if there’s some new force, basically that interacts with electrons and muons differently, that’s interfering with the process, changing how the decay happens. </p>
<p>And this is a bit like a footprint. So we’re sort of seeing the imprint of some heavy particle of a new force that we’ve never detected before that’s changing how these ordinary standard model particles behave. </p>
<p>Dan: Well that sounds like some huge news! Did this anomaly appear out of nowhere or had there been clues before?</p>
<p>Harry: This anomaly first appeared about seven years ago in 2014. At that point, we had quite a big uncertainty, there wasn’t that much data at that time. And over the last few years, this anomaly has got more and more precise. And the reason that it’s sort of big news this week is actually the anomaly is still there, and the error, the uncertainty on our measurement has shrunk to a sort of slightly arbitrary but nonetheless important statistical threshold, which in particle physics terms is known as three sigma. Basically if you did 1,000 experiments like this, you would expect one of them to land randomly this far away from your data. So there’s sort of a one in a thousand chance this is a random wobble in statistics. Like, you know, rolling lots of sixes in a row with a dice or something. </p>
<p>So really we won’t know for sure whether this is a real effect until we get to a much higher statistical threshold known as five sigma, and at that point there’s like a one in three and a half million chance of it being a random fluke. And that’s the sort of gold standard for saying, “OK, we’re really seeing something for sure here.” </p>
<p>Dan: OK, so you’ve been measuring and measuring all these decaying beauty quarks, and now we’ve got some pretty good evidence that something is off – a force seems to be missing from the standard model. Do you have any idea what this new force could be?</p>
<p>Harry: Theorists have come up with loads of ideas over the last few years, and there are broadly two candidates, so they both have quite, like, weird sounding names. One is called a Z prime, which is essentially, a sort of super weak force.
So there’s this particle in the standard model called the Z, which is the carrier of the weak force. So this would be like an even heavier, even weaker version of that called the Z prime. But unlike the Z, which decays to electrons and muons equally as often, this Z prime would treat electrons and muons differently. So it would have a preference for one or the other. So essentially that would be a new force of nature along the lines of the weak force that we already know about.</p>
<p>Dan: You mentioned there are two options for what might be causing this anomaly. What’s the other one?</p>
<p>Harry: There’s another thing called a leptoquark, which is a whole different object entirely. So this is sort of a, again, it’s kind of a force particle, but it’s a unique force particle in the sense that it can decay into a quark and what we call a lepton at the same time.</p>
<p>Dan: So, if you remember, atoms are made of protons, neutrons and electrons. Lepton is an umbrella term that refers to basically the electron family. There are a few particles, including electrons, muons and taus, that are identical in every way except some of them are heavier than the others. These are the leptons. What Harry told me is that all forces physicists know of can only turn into one thing at a time. Leptoquarks, if they exist, would be able to change into two things: leptons and quarks. So, leptoquarks. </p>
<p>Harry: The leptoquark might be part of a bigger picture that explains how these objects are related to each other and whether there’s like a deeper structure that explains why we have these 12 matter particles, which at the moment look a bit arbitrary and we don’t really know why they’re there. If either of these turns out to be true, it would be a really major discovery, and it would probably be telling us something about the structure of the standard model itself. So, why we have these particular particles in the universe, which is a question that we’ve not been able to answer so far.</p>
<p>Dan: What do you expect it might show about the universe?</p>
<p>Harry: So if this is real, and we find out what this particle is, it’s unlikely to be on its own. So in general, when we’ve found new particles in the past, they’re part of a bigger pattern. So it would be kind of weird if there was just one of these things. It’s probably the start of a collection of objects. </p>
<p>The standard model in a way is this, is like a part of a puzzle. It’s like maybe the top corner of a puzzle, which we filled in really nicely. And there’s the edges and we think there’s more pieces to go at the edge, but we don’t really know what they are. It’s unlikely to be one extra piece. It’s probably going to be a whole new set of pieces that enlarge the whole picture. </p>
<p>Dan: Harry was very, very careful to say that the team at Cern is “cautiously optimistic” about the finding, that’s kind of the official line. They’re claiming to have found evidence of an entirely new force of nature after all. That proof is coming though as physicists analyse more data and run more experiments at Cern and other particle accelerators around the world. </p>
<p>But even with the careful statement of caution, there’s real excitement about this paper. It’s also been a very exciting time for the theoretical physicists, like Celine Boehm. Part of what Celine studies is dark matter. Now, our regular listeners might remember what that is from our episode a few weeks ago, but here’s a quick recap. Dark matter is this mysterious stuff that makes up about 85% of the matter in the universe. It’s called dark matter, because, well, we can’t actually detect it in any direct way. The only way we know dark matter exists is becuase of its gravitational effect. Theoretical physicists have been trying for decades to find dark matter, and when Celine heard the recent news out of Cern, she immediately thought it could shed some light on the dark side of the universe. </p>
<p>Celine: It’s almost immediate to say, well there may be a link to the dark matter because it’s a new type of particle, so you could envisage, let’s say, a new type of force which these experiments have potentially discovered. That new type of force would basically be a mediator between the dark particles, the dark matter particles and the visible sector that we know of.</p>
<p>Dan: Remember, there are two theoretical particles that could explain the anomaly Harry and his colleagues found at Cern: one is the Z prime, and the other is the leptoquark. So, I asked Celine – have people been looking for these particles? </p>
<p>Celine: Yeah. So the leptoquarks have been looked for a long time. And in fact, just before my PhD, so as a master’s student, there was a claim already at that time, just as I was passing the exam actually, we heard some people shouting in the corridor because there was a claim for evidence of leptoquark. And that claim disappeared, essentially because it was not robust enough. </p>
<p>I don’t think it’s an unreasonable to think that they’re there, but they need to show up. And so far wherever we test them, we didn’t see them. So again, it’s a question of finding them and, it’s not clear whether they exist or not. So I think it’s going to take, a long time before we can actually validate whether it’s leptoquarks or Z prime or maybe some people will propose a new type of particles.</p>
<p>Dan: You’re a theoretician, now that you’ve got this little clue, are you going to go back to the chalkboard and start scribbling formulas to try and figure out maybe there’s something else? Or do you think it’s most likely the leptoquark or the Z prime?</p>
<p>Celine: I’ve already done that to be honest. I’ve already contacted my colleagues and said, “Hey guys, it’s time to revisit some of the ideas.” Yeah, so far for me, leptoquarks I can see how it works. Z prime, definitely I can see how it works. I don’t think that’s going to be easy to propose something else, but there are many people with a lot of imagination.</p>
<p>Dan: So, were you expecting this finding to happen eventually?</p>
<p>Celine: For this one, no, it’s unexpected for me because I’ve been working with Z prime boson for a long time and I kind of I basically decided, OK, they don’t exist and I wasted my time. </p>
<p>Dan: Oh no!</p>
<p>Celine: So it was, it was very interesting to see, oh actually, maybe not. </p>
<p>Dan: That must’ve felt very good. Did you pop a bottle of champagne or something?</p>
<p>Celine: No, I would, I would if they discover it at five sigma then, I would actually be very happy because they’re extremely cute particles. </p>
<p>Dan: Did you say cute?</p>
<p>Celine: Yeah, you know you get attached to what you’re doing basically. You know, every particle physicist have their own preference. Some prefer quarks, some prefer leptons. I’m addicted to leptons. They’re the most fundamental particles in the world. Most particles that we know of you can decompose them, you can break them. The leptons we try hard and we didn’t succeed so far. So they seem to be just the most fundamental particle you can think about. It’s like saying, you know, you start from a Russian doll, you remove a first layer and then the second layer you peel off that, then eventually you get this smallest one. And that’s it – and you can’t do more than that.</p>
<p>And in that respect leptons are exactly that last Russian doll. They’re an absolutely stable particle that never decays, they don’t disappear. So their lifetime is basically the same as the age of the universe and they’re extremely powerful articles because we can use them as a tool, basically, to probe the early universe. And we probe objects like clusters of galaxies. So knowing that something is wrong in that sector, that maybe there’s more physics there to explore that is very exciting to me.</p>
<p>Dan: I personally enjoy physics and dark matter and all these heady things, but for someone walking down the street or going to lunch or going to work, why does it matter? Why should they care? </p>
<p>Celine: So there are two things I can say, the very first one is fundamental knowledge. And, you know, a few centuries ago you would say, “Well, you think you’re at the centre of a universe, why would you care knowing that you’re not?” And yet basically the Copernicus revolution is a revolution in knowledge. </p>
<p>Dan: And by Copernicus revolution, you’re talking about Copernicus, the 16th century astronomer who first proposed that the Earth revolves around the Sun, not the other way around, right?</p>
<p>Celine: Yeah. Here, we’re talking about the same, the moment we were discovering dark matter particles, we would be saying that actually we are not the main form of matter in the universe. We’re actually negligible and most of the universe is made of something else. So it’s very similar to that problem of we’re not the centre of universe, we’re actually almost irrelevant. But we have a great privilege to be able to know that and observe it. </p>
<p>Now, the second thing I can say, there’s always consequences of fundamental discoveries and it’s very hard to anticipate. So for example, general relativity is a theory that I would say a very tiny amount of people can understand it. The rest of the world certainly doesn’t understand general relativity, yet we all use it because we all use GPS and it’s embedded in every mobile phone you have on Earth, basically. </p>
<p>So it’s one of those things where it’s hard to know what people will be using it for, but surely if it turns out it’s a fourth force, I’m sure we’re going to be able to harness it. It may take time. It may take centuries maybe more, but we will be using it eventually. </p>
<p>Dan: Celine, thank you so much. It’s been a pleasure. </p>
<p>Celine: You’re welcome. </p>
<p>Gemma: What’s so mind blowing about this kind of physics is just how much we don’t know. </p>
<p>Dan: If you want to read more about what the physicists at Cern think they may have found, you can find a link to <a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">a story by Harry Cliff and his colleagues</a> in the show notes. </p>
<p>Gemma: Coming up, why a bad situation just got even worse for many Rohingya refugees living in Bangladesh. But first, we’ve got a few recommendations from our colleague Nehal El-Hadi in Canada. </p>
<p>Nehal: Hiya, this is Nehal El-Hadi, science and technology editor at The Conversation in Toronto, Canada. My first recommendation for you this week is a story I worked on by Samantha Lawler at the University of Regina. I’m a science fiction fan and a book I very much enjoyed reading was the Three Body Problem by Chinese science fiction writer Liu Cixin. </p>
<p>Samantha’s article reminded me of the book because she writes about <a href="https://theconversation.com/previously-thought-to-be-science-fiction-a-planet-in-a-triple-star-system-has-been-discovered-153524">how a group of astronomers discovered an exoplanet with three stars</a>. Two stars A and B orbit each other while a third C orbits both A and B. The planet orbits star A. This three-star system was found using information from publicly available databases and scientists were able to study changes in brightness and the intervals between these changes to figure out the stars’ sizes and orbits.</p>
<p>My second story for you is by researchers at Simon Fraser University, who look at <a href="https://theconversation.com/bursting-social-bubbles-after-covid-19-will-make-cities-happier-and-healthier-again-155654">how the quality of our social interactions can make us happier</a> and lead to healthier cities. Because of the pandemic we’ve had to change the ways we interact with each other, things like physical distancing, stay at home measures and the introduction of social bubbles have changed the way we deal with other people. But when it comes time to rebuild after the pandemic, these researchers say that it’s important to consider the social connections and chance encounters that vibrant city life is built on. That’s it for me. Happy reading. </p>
<p>Dan: Nehal El-Hadi there, science editor at The Conversation in Canada. </p>
<p>Dan: Now, we’re turning to the situation for Rohingya refugees in Bangladesh.</p>
<p>Gemma: In 2017, an estimated 750,000 people from this minority, mainly Muslim, ethnic group, fled their homes in Myanmar’s Rakhine state after violent pogroms. </p>
<p>Gemma: The majority of these Rohingya refugees ended up in a city called Cox’s Bazar in neighbouring Bangladesh, now home to two giant refugee camps. In late March, a fire ripped through one of the camps there, leaving many people homeless. </p>
<p>Gemma: I’ve been speaking to Rubayat Jesmin, a PhD candidate at Binghamton University in New York. She’s researching the economic situation for Rohingya women in these refugee camps. I first spoke to Rubayat in February, just a few days after a recent coup in Myanmar and I called her up again just after the fire. I asked her to give me a bit of background about the situation .</p>
<p>Rubayat: The military junta took power in 1962 and the army government enacted the 1982 Citizenship Act, which totally stripped these Rohingyas of their nationality. This law recognises many ethnicities, except Rohingya Muslims, and that started this flow of Rohingyas into Bangladesh and other neighbouring countries. From time to time, there were other military crackdowns on the Rohingyas in Myanmar. So, some of these Rohingyas moved to Bangladesh. After 1978, some where repatriated to Myanmar, but after the influx of 1991-92, there was hardly any repatriation. </p>
<p>The situation really worsened in 2017. There was some big military crackdown in Rakhine state by the Myanmar army. So more than 750,000 Rohingyas crossed the border and take refuge in Bangladesh, mainly in two camps in Bangladesh, Kutapalong and Nayapara. After this influx, Kutapalong became the world’s largest refugee camp, and one of the most densely populated one. You can imagine more than 1 million Rohingya refugees living in a very small place. So it is overcrowded. Gradually, the government with the help of national and international NGOs, they made these camps sort of livable for these Rohingya refugees, but you can understand it’s not at all up to the mark. </p>
<p>Gemma: And you’ve obviously spent time there as part of your research. </p>
<p>Rubayat: Right. </p>
<p>Gemma: Can you give us a picture of what life is like there? </p>
<p>Rubayat: Almost 80% of these Rohingya refugees are women and children. These women especially are very much tormented by sexual abuse, rape and other kinds of traumas. So most of the efforts for the humanitarian workers are diverted toward the mental health issues of these women and children.</p>
<p>Gemma: What’s it been like during the pandemic in the camps?</p>
<p>Rubayat: The camps were kind of sealed from April. That means only the very essential humanitarian workers for food, health, these kind of facilities are allowed. None other activities are taking place. All the learning centres are closed. The security situation has deteriorated in recent times. Especially there have been some cases of trafficking, smuggling and even killing. </p>
<p>Rubayat: So since December 2020, the government has started to relocate some Rohingya refugees, batch by batch to this remote island. </p>
<p>Rubayat: Despite UNHCR and some other international community having strong reservation about this relocation because this island is flood-prone and cyclone prone.</p>
<p>Gemma: What’s the island called?</p>
<p>Rubayat: Pashanjatw. Pashanjatw in Bangla actually means a floating island. </p>
<p>Gemma: Floating island. </p>
<p>Rubayat: Yeah, the name itself says a lot. </p>
<p>Gemma: It’s a very impermanent place. And have they built facilities for the refugees there? </p>
<p>Rubayat: Yes. That, the government of Bangladesh did. They have built facilities to accommodate at least 100,000 Rohingyas. They have built schools, mosques and healthcare centres. So these facilities are there, but those are kind of very basic.</p>
<p>Gemma: You’ve got that happening, but also there’s been a long process talking about repatriation of sending or helping Rohingya to go back to Myanmar. Can you explain a bit about what’s been happening in terms of repatriation?</p>
<p>Rubayat: There were two failed attempts, because the Rohingyas didn’t want to go back to their country. So since this enactment of 1982 citizenship law, the Rohingya Muslims became the largest stateless population in the world. That’s why Rohingya who have taken refugee in different countries, they are afraid to go back because they don’t have the nationality or the access to better health, education, even marriage or civil and political rights.</p>
<p>So when this negotiation for repatriation started in 2018, these Rohingyas refused to go back because the situation that led them, across the border in the first days, wasn’t corrected. </p>
<p>Finally, after many months, China, took a lead and at the end of January this year, there was a tripartite meeting between China, Bangladesh and Myanmar, where the parties agreed for peaceful repatriation. The repatriation was supposed to start from June this year, June 2021. </p>
<p>Gemma: And were the Rohingya involved in this decision?</p>
<p>Rubayat: There’s another issue actually. During tripartite meeting, Rohingyas were not part of the conversation. These Rohingyas will now fear to go back when the military is again in power. At the same time, the military may want to continue their plans of making Rakhine state free of Rohingya Muslims.</p>
<p>Gemma: Hello? Hello?</p>
<p>Rubayat: Hello?</p>
<p>Gemma: Hi Rubayat. </p>
<p>Rubayat: Hi Gemma. </p>
<p>Gemma: Hi, good to speak to you again. </p>
<p>Rubayat: Same here. </p>
<p>Gemma: When we first spoke to you back in February, it was just a few days actually, after the coup in Myanmar. Two months on the situation has actually dramatically deteriorated for Rohingya refugees living in Cox’s Bazar in Bangladesh. There was a huge fire that ripped through one of the camps there on March 22. Can you tell us what we know about what happened so far? </p>
<p>Rubayat: Actually, the Bangladesh authorities are still investigating about the fire, but there were several fires in the past in the camps. You have to understand that these are very densely populated, overcrowded camps. So the houses are like adjacent to each other. Those are made of plastics and bamboos. So those are very flammable, right? But this time there were losses of lives and properties. Official estimate is saying more than 10,000 houses were burned and there was a big marketplace that was also burned.</p>
<p>So the initial suspect is that there is a cooking cylinder burst and that started the fire. And it very soon spread throughout these houses. But there is an apprehension that it could be done intentionally also. But Bangladesh authority has taken it seriously and they’re investigating it right now. </p>
<p>Gemma: And when we spoke back in early February, you had said that there had been this agreement between Myanmar, Bangladesh and China to start repatriation. Has there been any, any movement since then or any sign of what might happen? </p>
<p>Rubayat: I haven’t heard any such developments yet. If these people are repatriated to Myanmar, they cannot just go back to a place where the condition is not improved since they left. To my opinion, the root cause still is there – this citizenship law. Unless it is reformed, these people continue to remain stateless and deprived of all kinds of rights, and they don’t have their land or home anymore. Several villages have been demolished and in some places, the previous government in Myanmar had developed other infrastructure. So where these people are going to go back? </p>
<p>Gemma: I actually some reports that suggested that the brutal crackdown of protesters by the junta had actually kind of improved the cause within Myanmar for the Rohingya, because there are people who perhaps had been slightly ambivalent about what had been happening to them, who now realised what they’d been going through. Is that something you’ve also been hearing? </p>
<p>Rubayat: Yes. It’s not only the Rohingya Muslims that have been persecuted. There were other ethnic groups also persecuted by the junta government, earlier, right. Kachin and Christian Myanmar people. So now, what this coup kind of brought in is a solidarity among all the ethnic groups. They’re now together and fighting against this coup. So I see a very positive sign because now everybody understands what this persecuted minorities have been going through all these years. </p>
<p>Gemma: It’s sad it’s taken a coup for people to realise that, I guess. But I wanted to ask you finally, if you had a message for the international community who could do something, what would you say? What needs to be done?</p>
<p>Rubayat: Immediate need is definitely the humanitarian assistance like food, shelter, healthcare, education, right? In the longer term, definitely the international community need to strengthen their pressure on Myanmar government to repatriate them, create the favourable conditions for these people in their homeland and to do really something about this citizenship act. They cannot be stripped of their citizenship forever.</p>
<p>Gemma: OK, well, thank you Rubayat for speaking with us again, I appreciate your time. </p>
<p>Rubayat: Thank you, Gemma. Thank you once again.</p>
<p>Gemma: In Myanmar itself, the clampdown by the military against protesters has got more deadly in recent days. You can read about the <a href="https://theconversation.com/ca/topics/myanmar-coup-99739">ongoing protest movement on The Conversation,</a> or to hear more about the events which led up to the coup, you can listen to a story we did in early February in our <a href="https://theconversation.com/why-myanmar-is-rising-up-in-collective-fury-after-a-military-coup-the-conversation-weekly-podcast-154991">second episode of this podcast</a>. </p>
<p>Dan: We’ve got links to all of the expert analysis we mentioned in the shownotes. You can also find a link to sign up to The Conversation’s free daily email. If you want to reach out, tell us what you think about the show or what questions we should be asking academics, find us on Twitter <a href="https://twitter.com/TC_Audio">@TC_Audio</a> or on Instagram at <a href="https://www.instagram.com/theconversationdotcom/?hl=en">theconversationdotcom</a>. Or you can email us on podcast@theconversation.com </p>
<p>Gemma: Thanks to all the academics who’ve spoken to us for this episode. And thanks too to Miriam Frankel, Abby Beall, Catesby Holmes, Nehal El-Hadi and Stephen Khan. And thanks to Alice Mason, Imriel Morgan and Sharai White for helping with our social media and promotion. </p>
<p>Gemma: This episode of The Conversation Weekly is co-produced by Mend Mariwany and me, with sound design by Eloise Stevens. Our theme music is by Neeta Sarl. </p>
<p>Dan: Thanks for listening everyone and we’ll talk to you next week.</p><img src="https://counter.theconversation.com/content/158244/count.gif" alt="The Conversation" width="1" height="1" />
A transcript of episode 9 of The Conversation Weekly podcast, including an update on the situation for Rohingya refugees in Myanmar living in camps in Bangladesh.
Gemma Ware, Host, The Conversation Weekly Podcast
Daniel Merino, Associate Breaking News Editor and Co-Host of The Conversation Weekly Podcast
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/158198
2021-04-01T10:34:16Z
2021-04-01T10:34:16Z
A new force of nature? The inside story of fresh evidence from Cern that’s exciting physicists – podcast
<figure><img src="https://images.theconversation.com/files/393101/original/file-20210401-23-1g34zox.jpg?ixlib=rb-1.1.0&rect=93%2C108%2C4966%2C3267&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Scientists think they may have found a new clue about the subatomic world around us. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/close-colorful-atomic-particle-background-science-733895305">Ezume Images via Shutterstock</a></span></figcaption></figure><p>In this episode of <a href="https://theconversation.com/uk/topics/the-conversation-weekly-98901">The Conversation Weekly</a>, the inside story of how scientists working at Cern’s Large Hadron Collider found tantalising new evidence that could mean we have to rethink what we know about the universe. And an update on the situation for Rohingya refugees from Myanmar living in Bangladesh after a deadly fire swept through a refugee camp there.</p>
<iframe src="https://embed.acast.com/60087127b9687759d637bade/606490a356dcab18893447f3?cover=true&ga=false" frameborder="0" allow="autoplay" width="100%" height="110"></iframe>
<p><iframe id="tc-infographic-561" class="tc-infographic" height="100" src="https://cdn.theconversation.com/infographics/561/4fbbd099d631750693d02bac632430b71b37cd5f/site/index.html" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>In late March, particle physicists working at the Large Hadron Collider (LHC), a massive particle accelerator at Cern in Geneva, announced, <a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">tentatively, that they’d had a bit of a breakthrough</a>. If what they think they’ve seen is proven correct, it could mean evidence for brand new physics – perhaps even a new force of nature. </p>
<p>We get the inside story on what these experimental physicists have been looking for from Harry Cliff, a particle physicist at the University of Cambridge who works on the LHCb, one of Cern’s four giant experiments. Cliff explains that what they’re doing is like looking for an animal in a jungle: “We might see a footprint and go, OK, there’s clearly something out here. We don’t necessarily know exactly what kind of animal it is just from its footprint, but it gives us a clue about where to then look next.”</p>
<p>And Celine Boehm, professor and head of physics at the University of Sydney, explains the bigger picture of where this all fits into the world of theoretical physics, including the ongoing hunt for dark matter. She says there are always real-world consequences for breakthroughs in fundamental physics and our understanding of the universe. If it turns out to be a new force, she predicts: “I’m sure we’re going to be able to harness it. It may take centuries, maybe more, but we will be using it eventually.”</p>
<p>In our second story, we find out more about the situation for hundreds of thousands of Rohingya refugees from Myanmar, living in camps in Cox’s Bazar in Bangladesh. On March 22, a devastating <a href="https://www.theguardian.com/global-development/2021/mar/24/ive-lost-everything-once-again-rohingya-recount-horror-of-coxs-bazar-blaze">fire ripped through one of the camps</a>, leaving thousands with no shelter. Rubayat Jesmin, a PhD candidate at Binghamton University in New York, explains the context behind the persecution of the Rohingya in Myanmar, what life is like in the camps, and why the issue of repatriation is so complex, particularly after the recent military coup in Myanmar. </p>
<p>And Nehal El-Hadi, science and technology editor at The Conversation in Toronto, gives us some recommended reading. </p>
<p>The Conversation Weekly is produced by Mend Mariwany and Gemma Ware, with sound design by Eloise Stevens. Our theme music is by Neeta Sarl. You can find us on Twitter <a href="https://twitter.com/TC_Audio">@TC_Audio</a> or on Instagram at <a href="https://www.instagram.com/theconversationdotcom/?hl=en">theconversationdotcom</a>. We’d love to hear what you think of the show too. You can email us on podcast@theconversation.com</p>
<p>A transcript of this episode <a href="https://theconversation.com/what-a-possible-new-breakthrough-at-cern-could-reveal-about-the-structure-of-the-universe-158244">is available here</a>. </p>
<p>News clips in this episode are from <a href="https://www.youtube.com/watch?v=Hyz1R8rfDRQ">Cern</a>, <a href="https://www.youtube.com/watch?v=KT7QUzJg0aM">BBC News</a>, <a href="https://www.youtube.com/watch?v=asgfmtMidzk">NBC News</a>, <a href="https://www.youtube.com/watch?v=6X2Ym-OHlMI">Al Jazeera</a>, <a href="https://www.youtube.com/watch?v=VxLM2bYXt6g">DW News</a>, <a href="https://www.youtube.com/watch?v=OD_XW9uxVfw">CNN</a> and <a href="https://www.youtube.com/watch?v=uOluteEbCTM">WION News</a>.</p>
<p><em>You can listen to The Conversation Weekly via any of the apps listed above, our <a href="https://feeds.acast.com/public/shows/60087127b9687759d637bade">RSS feed</a>, or find out how else to <a href="https://theconversation.com/how-to-listen-to-the-conversations-podcasts-154131">listen here</a>.</em></p><img src="https://counter.theconversation.com/content/158198/count.gif" alt="The Conversation" width="1" height="1" />
Plus why the situation for Rohingya Muslims living in Bangladesh has gone from bad to worse. Listen to episode 9 of The Conversation Weekly podcast.
Gemma Ware, Host, The Conversation Weekly Podcast
Daniel Merino, Associate Breaking News Editor and Co-Host of The Conversation Weekly Podcast
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/157871
2021-03-29T19:05:44Z
2021-03-29T19:05:44Z
New physics at the Large Hadron Collider? Scientists are excited, but it’s too soon to be sure
<figure><img src="https://images.theconversation.com/files/392159/original/file-20210329-15-ypnegu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">blank</span> </figcaption></figure><p>Last week, physicists at the Large Hadron Collider in Switzerland <a href="https://home.cern/news/news/physics/intriguing-new-result-lhcb-experiment-cern">announced</a> they <em>might</em> have discovered a brand new force of nature. Or, to be <a href="https://twitter.com/CERN/status/1374283651337355265">precise</a>, they unveiled “new results which, if confirmed, would suggest hints of a violation of the Standard Model of particle physics”.</p>
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<p>What does that mean? And why are they making such a big deal of it, while at the same time stopping short of claiming a new discovery?</p>
<p>The answers lie in the way particle physicists think about evidence and results, and what it would mean to find “a violation of the Standard Model”.</p>
<h2>So what?</h2>
<p>The Standard Model, devised between the 1950s and 1970s, has been enormously successful at explaining the behaviour of subatomic particles and three of the four fundamental forces we know about. The physicists at CERN think they’ve found a situation that the Standard Model can’t explain: where the model predicts a particle called a beauty quark should decay into other particles called muons and electrons at about the same rate, it looks like it <a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">actually decays</a> into electrons more often than muons.</p>
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Read more:
<a href="https://theconversation.com/evidence-of-brand-new-physics-at-cern-why-were-cautiously-optimistic-about-our-new-findings-157464">Evidence of brand new physics at Cern? Why we're cautiously optimistic about our new findings</a>
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<p>This is exciting, because we already know the Standard Model doesn’t tell the whole story about what’s happening in the universe. It’s very good at telling us about matter and energy. But it doesn’t provide an account of the so-called dark matter and dark energy scientists believe must exist to explain the large-scale behaviour of stars and galaxies.</p>
<p>The Standard Model is also tremendously difficult to reconcile with our best explanation of gravity, Einstein’s theory of general relativity. The Standard Model is at best a step along the road to a complete theory of everything. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/392155/original/file-20210329-15-dt7hp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/392155/original/file-20210329-15-dt7hp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=380&fit=crop&dpr=1 600w, https://images.theconversation.com/files/392155/original/file-20210329-15-dt7hp8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=380&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/392155/original/file-20210329-15-dt7hp8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=380&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/392155/original/file-20210329-15-dt7hp8.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=477&fit=crop&dpr=1 754w, https://images.theconversation.com/files/392155/original/file-20210329-15-dt7hp8.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=477&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/392155/original/file-20210329-15-dt7hp8.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=477&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
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<span class="caption">The decay of a beauty meson involving an electron and positron, observed at the LHCb experiment.</span>
<span class="attribution"><a class="source" href="https://cds.cern.ch/images/OPEN-PHO-EXP-2018-004-1">CERN</a></span>
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<p>To go beyond the Standard Model, we need new empirical data. What we really need is evidence showing some prediction of the Standard Model is wrong, but not a prediction so central to the theory that we need to rebuild from the ground up. </p>
<p>That’s why the decay of beauty quarks is so interesting. The unexpected behaviour points to an area where the theory could be modified without having to start from scratch. </p>
<h2>Sigmas and p-values</h2>
<p>The reason scientists are cautious about the result is because it’s what’s called a 3-sigma finding.</p>
<p>To explain, let’s imagine you’re looking for fairies in the bottom of your garden. You start by assuming there are no fairies – that’s called your <em>null hypothesis</em>. </p>
<p>You then gather some observations seeking to reject that hypothesis. After analysing your data, you find there is a 90% probability that <em>if</em> there were no fairies in the garden, you would make observations like the ones you in fact made. </p>
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Read more:
<a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">Explainer: what are fundamental particles?</a>
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<p>This gives you what’s called a <em>p-value</em>. A 90% probability of observing the data you in fact observed if your null hypothesis were true is the same as a p-value of 0.9. </p>
<p>Basically, you’ve discovered you don’t have a strong reason to <em>reject</em> the assumption that your garden is fairy-free. That’s <em>not</em> the same thing as discovering a reason to believe that your null hypothesis is true. </p>
<p>The p-value is the probability of the evidence, given your null hypothesis, which is distinct from the probability that the null hypothesis is true, given your evidence. (In case this seems odd, consider that the probability that someone is funny given that they’re dad is <em>not</em> the same as the probability that someone is your dad given that they’re funny). </p>
<p>Sigma values like the “3-sigma” result correspond to p-values. At the LHC, the null hypothesis is the claim that the Standard Model is correct, and the observations are of particle interactions. </p>
<p>A 3-sigma result means there is a roughly 1 in 1,000 probability that observations at least as extreme as those gathered would occur, given the Standard Model. That’s substantially better than your quest to find fairies and does seem to call the Standard Model into question.</p>
<h2>Why so cautious?</h2>
<p>Physicists don’t usually crack open the champagne until they have a <em>5-sigma</em> result. </p>
<p>A 5-sigma result tells you there would be a chance of less than one in a million of your observation if the Standard Model were correct. That’s like wandering into your garden and chatting with a small being with wings: your “no fairies” hypothesis is starting to look quite shaky. </p>
<p>Why do physicists look for a 5-sigma event? There are several reasons. The first is historical: they’ve been stung before. In 2011 physicists claimed to have measured neutrinos travelling faster than the speed of light. This measurement exceeded 3-sigma, but it <a href="https://en.wikipedia.org/wiki/Faster-than-light_neutrino_anomaly">turned out</a> to be due to a faulty cable. </p>
<p>Physicist Tommaso Dorigo has been keeping a <a href="https://www.science20.com/a_quantum_diaries_survivor/true_and_false_discoveries_how_to_tell_them_apart-141024">diary</a> of measured events that reached or surpassed 3-sigma significance. He notes 6 previous claims that were later withdrawn. </p>
<p>Another reason for caution is the problem of multiple comparisons. If you carry out enough tests, you are bound to see something odd. </p>
<p>Suppose you flip a coin 100 times and get 50 heads and 50 tails. Now suppose you repeat the experiment 100 times (flipping the coin 10,000 times altogether). </p>
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Read more:
<a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">Explainer: how does an experiment at the Large Hadron Collider work?</a>
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<p>In some versions of the experiment you might see 20 heads and 80 tails. In some you see 10 heads and 90 tails. Both distributions are unlikely, assuming the coin is fair. </p>
<p>Do you therefore have evidence the coin is unfair? It seems doubtful. Even a fair coin will yield lopsided results sometimes. </p>
<p>The LHC is like a coin-flipping machine. It is constantly conducting experiments. To correct for this, physicists demand the very high 5-sigma standard. A 3-sigma result is noteworthy, but not yet a “discovery”.</p>
<p>Finally, there’s the adage that extraordinary claims require extraordinary evidence. The Standard Model is extremely well confirmed. It will take an extremely striking observation (such as on observation of an event that would be very unlikely if the standard model were true) to reduce confidence in the model.</p>
<h2>What’s next?</h2>
<p>The LHC is an extraordinarily complex experiment, and there are a lot of things that can go wrong with it. That makes it difficult to control for systematic errors.</p>
<p>So even reaching the 5-sigma level in itself might not be enough to confirm a new discovery. Indeed, three of the six withdrawn results documented by Dorigo reached the even higher <em>6-sigma</em> level. </p>
<p>To confirm a discovery, ideally the results need to be replicated using a different experimental set up (one that doesn’t risk also replicating the same errors), preferably more than once. That’s why the physicists at CERN are hoping their results will be replicated by the Belle experiment in Japan.</p>
<p>The announcement from CERN thus may seem a bit premature. But Dorigo’s diary provides reason to be optimistic. He points out that all of the withdrawn results from particle accelerator experiments reached levels of significance that are even numbers (4 or 6-sigma), whereas genuine discoveries reached levels that are odd numbers (3 or 5-sigma).</p>
<p>Dorigo suggests we should take observations with odd-numbered sigma values very seriously. He’s making a joke. But behind the joke is a sociological observation: physicists don’t tend to publish 3-sigma results unless they’re confident that they will lead to a discovery. The physicists at CERN clearly believe they’re onto something, and so should we.</p><img src="https://counter.theconversation.com/content/157871/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Sam Baron receives funding from Australian Research Council.</span></em></p>
A long-sought crack in the Standard Model of particle physics may have been spotted.
Sam Baron, Associate professor, Australian Catholic University
Licensed as Creative Commons – attribution, no derivatives.