tag:theconversation.com,2011:/id/topics/standard-model-of-particle-physics-1312/articles
Standard model of 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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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>
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<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/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>
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<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>
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<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>
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<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>
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</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/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/203010
2023-05-15T15:43:17Z
2023-05-15T15:43:17Z
Theory of everything: how progress in physics depends on asking the right questions
<figure><img src="https://images.theconversation.com/files/524343/original/file-20230504-29-15yttd.jpg?ixlib=rb-1.1.0&rect=152%2C197%2C5838%2C5793&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Calabi-Yau manifold: a proposed structure of extra dimensions of space in string theory. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/calabiyau-manifold-structure-extra-dimensions-space-1228700050">vchal/Shutterstock</a></span></figcaption></figure><iframe src="https://embed.acast.com/638f4b009a65b10011b94c5e/64353c62de066f001110361d" frameborder="0" width="100%" height="190px"></iframe>
<p><iframe id="tc-infographic-807" class="tc-infographic" height="100px" src="https://cdn.theconversation.com/infographics/807/1668471fb1e76a459995c87bd439c36b04b754ac/site/index.html" width="100%" style="border: none" frameborder="0"></iframe></p>
<p>When I began my undergraduate physics degree (around 20 years ago), “What is the <a href="https://theconversation.com/great-mysteries-of-physics-do-we-really-need-a-theory-of-everything-203534">theory of everything</a>?” was a question that I heard often. It was used as a label for how theoretical physicists were trying to develop a deeper understanding of the elementary building blocks of our universe and the forces that govern their dynamics.</p>
<p>But is it a good question? Is it helpful in guiding scientists towards the discoveries that will advance our understanding to the next level? After all, good science relies on asking good questions. Or is it just <a href="https://bigthink.com/starts-with-a-bang/theories-of-everything/">“wishful thinking”</a>?</p>
<p>Arguably, the question “What is the theory of everything?” reminds us that good science doesn’t have to start with the best questions. Let me explain what I mean.</p>
<p>Suppose we play a game. I have a deck of cards, and each card is printed with the name and a photograph of a different animal. I choose a card, and your job is to ask questions to find out which animal I have chosen. Of course, to ask a discerning question, you first need to know something about animals.</p>
<p>The first time you play, you may not be familiar with which animals are in the deck, and your first question is “Does it live in the sea?”. My answer is “No,” and the game continues. Then it is your turn to pick a card. You look carefully through the deck to make your choice, and you realise that it only contains land animals. “Does it live in the sea?” seemed like a good question to start with, but it was not.</p>
<p>We take turns, and the more we play, the quicker we seem to figure out which card has been chosen. Why? We have become better at asking good questions.</p>
<p>The role that questions play in scientific research is similar. We start from some level of understanding, and we ask questions based on that level of understanding to try to improve it. As our understanding builds, we refine our questions and get more insightful answers.</p>
<hr>
<figure class="align-left ">
<img alt="" src="https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=600&fit=crop&dpr=1 600w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=600&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=600&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=754&fit=crop&dpr=1 754w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=754&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/513939/original/file-20230307-20-pgea9d.png?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"></span>
</figcaption>
</figure>
<p><em>This is article is accompanied by a podcast series called <a href="https://podfollow.com/great-mysteries-of-physics">Great Mysteries of Physics</a> which uncovers the greatest mysteries facing physicists today – and discusses the radical proposals for solving them.</em></p>
<hr>
<p>This is how progress is made. The same is true of asking “What is the theory of everything?”: the goodness of a scientific question is not immutable.</p>
<h2>Why a ‘theory of everything’?</h2>
<p>The <a href="https://home.cern/tags/standard-model#:%7E:text=The%20Standard%20Model%20of%20particle,of%20scientists%20around%20the%20world.">Standard Model of Particle Physics</a>, one <a href="https://theconversation.com/the-standard-model-of-particle-physics-may-be-broken-an-expert-explains-182081">of the pillars of modern science</a>, is a success of reductionism - the idea that things can be explained by breaking them down into smaller parts.</p>
<p>The model, which is written in a mathematical language called <a href="https://www.damtp.cam.ac.uk/user/tong/whatisqft.html">quantum field theory</a>, describes how elementary particles move around and interact with one another. It explains the nature of three out of four of the known fundamental forces: electromagnetism, and the weak and strong forces that govern processes on subatomic scales. It does not include gravity, the fourth force.</p>
<p>The model accounts for <a href="https://theconversation.com/great-mysteries-of-physics-4-does-objective-reality-exist-202550">quantum mechanics</a>, which describes the probabilistic nature of the dynamics of subatomic particles, and Einstein’s special theory of relativity, which describes what happens when relative speeds are close to the speed of light – no small achievement.</p>
<p>The assumption in asking “What is the theory of everything?” is that the Standard Model will one day be found to be embedded within a larger structure (with more elemental ingredients) that provides us with a unified explanation of the fundamental forces including gravity. Gravity, in fact, is this question’s ultimate focus. </p>
<p>But the question “What is the theory of everything?” gives very little guidance as to what such a theory of everything might look like. We need some better questions.</p>
<p>Now, there are good reasons to expect that such a unified explanation of the fundamental forces might exist: the Standard Model includes the celebrated Higgs mechanism, from which the <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</a> arises. It explains why fundamental particles known as the W and Z bosons, which transmit the weak force, acquire a mass. It also explains why the photon, which transmits the electromagnetic force, does not.</p>
<figure class="align-center ">
<img alt="CMS experiment at Cern." src="https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=494&fit=crop&dpr=1 754w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=494&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/524347/original/file-20230504-17-wd6j6c.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=494&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">CMS experiment at Cern.</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>As a result, electromagnetism and the weak force, which is involved in the nuclear fusion that powers stars, behave differently at low energies: the electromagnetic force acts over very large distances, whereas the weak force acts only over very short distances. The Higgs mechanism also explains why, at higher energies, these two forces start to behave as a single “electroweak” force. This is called electroweak unification.</p>
<p>Now, if electromagnetism and the weak force combine in this way, why not all the forces in the Standard Model? Unifying these two with the strong force, the force that holds the ingredients of atomic nuclei together, is the aim of grand unified theories. Theoretical ideas such as <a href="https://home.cern/science/physics/supersymmetry">supersymmetry</a>, which postulates a symmetry between force carriers and matter particles, suggest that <a href="https://bigthink.com/starts-with-a-bang/theories-of-everything/">the strength of these three forces could get tantalisingly close at high enough energies</a>.</p>
<p>And if the electromagnetic, weak and strong forces turn out to be unified, why not gravity, too?</p>
<p>Gravity is described by <a href="https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042">Einstein’s General Theory of Relativity</a>, which applies on large scales or at low energies. But if we want a consistent quantum theory of gravity that applies on the smallest scales, quantum field theory isn’t enough. We need mathematical frameworks that can consistently incorporate both general relativity and quantum mechanics.</p>
<p>The “everything” in a “theory of everything” refers to all the known forces of nature: electromagnetism, the weak force, the strong force, and gravity (and new, <a href="https://theconversation.com/new-physics-latest-results-from-cern-further-boost-tantalising-evidence-170133">hypothetical forces</a>, too) and the particles that they act between. The “theory” refers to the existence of some common mathematical framework that describes all of the “everything”.</p>
<p>One such common mathematical framework is <a href="https://theconversation.com/stephen-hawking-had-pinned-his-hopes-on-m-theory-to-fully-explain-the-universe-heres-what-it-is-93440">string theory</a>, which supposes that the most fundamental building blocks of the universe are tiny strings that vibrate in extra spatial dimensions beyond the three of our everyday experience. </p>
<h2>Better questions</h2>
<p>Questions are the guide to scientific inquiry. The question “What is the theory of everything?” only speculates at a destination, but it gives very little direction.</p>
<p>Frameworks such as supersymmetry and string theory were not developed to answer the question “What is the theory of everything?” directly. They were motivated by better questions about what a theory of all the fundamental forces needs to explain and what it might look like, questions like: Why is there a huge discrepancy between the energy scales of the Standard Model and quantum gravity? Why do quantum mechanics and general relativity seem to be incompatible?</p>
<p>But the “whys” that theoretical physicists ask develop as our understanding develops, and the questions that we are now posing are getting us even closer than ever to an understanding of all the known forces of nature. </p>
<p>These new “whys” hint at <a href="https://doi.org/10.48550/arXiv.2006.06872">remarkable connections between very different areas of physics and mathematics</a>: Why does the physics of holograms seem to help us to understand gravity? Why does this seem to be connected to the properties of large collections of random numbers? Why do the rules of quantum information seem to explain the physics of black holes?</p>
<p>But this is not a case of “out with the old and in with the new”. Instead, these new questions have been reached by building on what has been learnt from developing and studying possible “Theories of Everything”, like string theory.</p>
<p>And these new questions are good questions. The exciting thing is that they still may not be the best questions, and having them to guide us doesn’t necessarily mean that we know where we will end up. That is what scientific discovery is all about.</p>
<p><em>You can listen to Great Mysteries of Physics via any of the apps listed above, our <a href="https://feeds.acast.com/public/shows/638f4b009a65b10011b94c5e">RSS feed</a>, or find out how else to listen here. You can also read a <a href="https://cdn.theconversation.com/static_files/files/2634/MoP__Ep6_-_Theory_of_Everything_TRANSCRIPT.docx.pdf?1681292977">transcript of the episode here</a>.</em></p><img src="https://counter.theconversation.com/content/203010/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Peter Millington is a Senior Research Fellow in the Particle Theory Group at the University of Manchester, UK, where he holds a UK Research and Innovation Future Leaders Fellowship and a Royal Society International Exchanges Grant. Peter Millington is a Member of the Institute of Physics, UK and serves on the Institute of Physics High Energy Particle Physics Group Committee.</span></em></p>
Good questions guide good science, but that doesn’t mean we know where we’ll end up.
Peter Millington, Senior Research Fellow and UKRI Future Leaders Fellow in the Particle Theory Group, Department of Physics and Astronomy, University of Manchester
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/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/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/162687
2021-06-18T12:21:25Z
2021-06-18T12:21:25Z
Cern: how we’re probing the universe’s origins using record precision measurements
<figure><img src="https://images.theconversation.com/files/407198/original/file-20210618-18-1h7sua8.jpg?ixlib=rb-1.1.0&rect=0%2C35%2C5973%2C3898&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Cern has measure a tiny mass difference by colliding huge amounts of particles.</span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/fragmentation-highenergy-collisions-between-atomic-subatomic-1443036710">Jurik Peter/Shutterstock</a></span></figcaption></figure><p>What happened at the beginning of the universe, in the very first moments? The truth is, we don’t really know because it takes huge amounts of energy and precision to recreate and understand the cosmos on such short timescales in the lab. But scientists at the Large Hadron Collider (LHC) at CERN, Switzerland aren’t giving up. </p>
<p>Now our <a href="https://home.cern/news/news/physics/lhcb-measures-tiny-mass-difference-between-particles">LHCb experiment</a> has measured one of the smallest difference in mass between two particles ever, which will allow us to discover much more about our enigmatic cosmic origins. </p>
<p>The Standard Model of particle physics describes <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">the fundamental particles</a> which make up the universe, and the forces that act between them. The elementary particles include quarks, of which there are six – up, down, strange, charm, top and bottom. Similarly there are six “leptons” which include the electron, a heavier cousin called the muon, and the still heavier tau, each of which has an associated neutrino. There are also “antimatter partners” of all quarks and leptons which are identical particles apart from an opposite charge. </p>
<p>The Standard Model is experimentally verified to an incredible degree of accuracy but has some significant shortcomings. 13.8 billion years ago, the universe was created in the Big Bang. The theory suggests this event should have produced equal amounts of matter and “antimatter”. Yet today, the universe <a href="https://theconversation.com/explainer-what-is-antimatter-53414">is almost entirely made up of matter</a>. And that’s lucky, because antimatter and matter annihilate in a flash of energy when they meet.</p>
<p>One of the biggest open questions in physics today is why is there more matter than antimatter. Were there processes at play in the early universe that favoured matter over antimatter? To get closer to the answer, we have studied a process where matter transforms into antimatter and vice versa. </p>
<p>Quarks are bound together to form particles called baryons – including the protons and neutrons that make up the atomic nucleus – or mesons, which consist of quark-antiquark pairs. Mesons with zero electric charge continually undergo a phenomenon called mixing by which they spontaneously change into their antimatter particle, and vice versa. In this process, the quark turns into an anti-quark and the anti-quark turns into a quark.</p>
<p>It can do this because of quantum mechanics, which <a href="https://theconversation.com/physicists-prove-quantum-spookiness-and-start-chasing-schrodingers-cat-48190">governs the universe</a> on the tiniest of scales. According to this counter-intuitive theory, particles can be in many different states at the same time, essentially being a mix of many different particles – a feature called superposition. It is only when you measure its state that it “picks” one of them. A type of meson called D0, for example, which contains charm quarks, is in a superposition of two normal matter particles called D1 and D2. The rate at which the D0 meson turns into its anti-particle and back again, an oscillation, depends on the difference in masses of D1 and D2. </p>
<h2>Tiny masses</h2>
<p>It is difficult to measure mixing in D0 mesons, but <a href="https://www.desy.de/f/seminar/Staric.pdf">it was done</a> for the first time in 2007. However, until now, nobody has reliably measured the mass difference between D1 and D2 that determines how quickly the D0 oscillates into its antiparticle. </p>
<figure class="align-center ">
<img alt="Figure of the D1 and D2 meson." src="https://images.theconversation.com/files/407197/original/file-20210618-24-1844qru.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/407197/original/file-20210618-24-1844qru.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=333&fit=crop&dpr=1 600w, https://images.theconversation.com/files/407197/original/file-20210618-24-1844qru.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=333&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/407197/original/file-20210618-24-1844qru.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=333&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/407197/original/file-20210618-24-1844qru.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=419&fit=crop&dpr=1 754w, https://images.theconversation.com/files/407197/original/file-20210618-24-1844qru.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=419&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/407197/original/file-20210618-24-1844qru.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=419&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">The D1 and D2 mesons, which are a manifestation of the quantum superposition of the D0 particle and its antiparticle.</span>
<span class="attribution"><span class="source">Cern</span></span>
</figcaption>
</figure>
<p>Our latest discovery, <a href="https://home.cern/news/news/physics/lhcb-measures-tiny-mass-difference-between-particles">announced at the Charm conference</a>, changes this. We measured a parameter that corresponds to a mass difference of 6.4x10<sup>-6</sup> electron Volts (a measure of energy) or 10<sup>-38</sup> grams – one of the smallest mass differences between two particles ever measured.</p>
<p>We then calculated that the oscillation between the D0 and its antimatter partner takes around 630 picoseconds (1 ps = 1 millionth millionth of a second). This may seem fast, but the D0 meson doesn’t live long – it isn’t stable in the lab and falls apart (decays) into other particles after only 0.4 picoseconds. So it will typically disappear long before this oscillation occurs, posing a serious experimental challenge. </p>
<p>The key is precision. We know from theory that these oscillations follow the path of a a familiar type of wave (sinusoidal). Measuring the start of the wave very precisely, we can infer its full period as we know its shape. The measurement therefore had to reach record precision on several fronts. This is made possible by the unprecedented amount of charm particles produced at the LHC. </p>
<p>But why is this important? To understand why the universe produced less antimatter than matter we need to learn more about the asymmetry in the production of the two, a process known as CP-violation. It has already been shown that some unstable particles decay in a different way to their corresponding antimatter particle. This may have contributed <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">to the abundance of matter in the universe</a> – with <a href="https://www.nobelprize.org/prizes/physics/2008/kobayashi/lecture/">previous discoveries</a> of it leading to Nobel Prizes.</p>
<p>We also want to find CP-violation in the process of mixing. If we start with millions of D0 particles and millions of D0 antiparticles, will we end up with more D0 normal matter particles after some time? Knowing the oscillation rate is a key step towards this goal. While we did not find an asymmetry this time, our result and further precision measurements can help us find it in the future. </p>
<p>Next year, the LHC will switch on after a long shut down and the new upgraded LHCb detector will take much more data, boosting the sensitivity of these measurements further. Meanwhile, theoretical physicists are working on new calculations to interpret this result. The LHCb physics programme will also be complemented by the <a href="https://www.belle2.org">Belle-II experiment</a> in Japan. These are exciting prospects for investigating matter-antimatter asymmetry and the oscillations of mesons.</p>
<p>While we cannot yet completely solve the mysteries of the universe, our latest discovery has put the next piece in the puzzle. The new upgraded LHCb detector will open the door to an era of precision measurements that have the potential to uncover yet unknown phenomena – and perhaps physics beyond the Standard Model.</p><img src="https://counter.theconversation.com/content/162687/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Martha Hilton receives funding from STFC (Science Technology Facilities Council). </span></em></p><p class="fine-print"><em><span>Nathan Jurik received funding from the STFC (Science Technology Facilities Council).</span></em></p><p class="fine-print"><em><span>Sascha Stahl 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>
Record precision measurements at Cern may help explain why the universe has more matter than antimatter.
Martha Hilton, PhD candidate in Particle Physics, University of Manchester
Nathan Jurik, Research Fellow of Particle Physics, Syracuse University
Sascha Stahl, Research staff at CERN, CERN
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>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, Head of Audio
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, Head of Audio
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>
<hr>
<p>
<em>
<strong>
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>
</strong>
</em>
</p>
<hr>
<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.
tag:theconversation.com,2011:article/157464
2021-03-23T08:15:51Z
2021-03-23T08:15:51Z
Evidence of brand new physics at Cern? Why we’re cautiously optimistic about our new findings
<figure><img src="https://images.theconversation.com/files/390883/original/file-20210322-23-1liv6sm.jpg?ixlib=rb-1.1.0&rect=218%2C114%2C7450%2C4150&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Particle collisions are starting to reveal unexpected results. </span> <span class="attribution"><a class="source" href="https://www.shutterstock.com/image-illustration/particles-collision-hadron-collider-astrophysics-concept-1406326886">vchal/Shutterstock</a></span></figcaption></figure><p>When Cern’s gargantuan accelerator, the Large Hadron Collider (LHC), fired up ten years ago, hopes abounded that new particles would soon be discovered that could help us unravel physics’ deepest mysteries. Dark matter, microscopic black holes and hidden dimensions <a href="https://theconversation.com/from-black-holes-to-dark-matter-an-astrophysicist-explains-26019">were just some</a> of the possibilities. But aside from the <a href="https://theconversation.com/explainer-the-higgs-boson-particle-280">spectacular discovery</a> of the Higgs boson, the project has <a href="https://theconversation.com/could-the-higgs-nobel-be-the-end-of-particle-physics-18978">failed to</a> yield any clues as to what might lie beyond the <a href="https://theconversation.com/explainer-standard-model-of-particle-physics-2539">standard model of particle physics</a>, our current best theory of the micro-cosmos.</p>
<p>So our <a href="http://arxiv.org/abs/2103.11769">new paper</a> from LHCb, <a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">one of the four giant LHC experiments</a>, will probably set physicists’ hearts beating just a little faster. After analysing trillions of collisions produced over the last decade, we may be seeing evidence of something altogether new – potentially the carrier of a brand new force of nature.</p>
<p>But the excitement is tempered by extreme caution. The standard model has withstood every experimental test thrown at it since it was assembled in the 1970s, so to claim that we’re finally seeing something it can’t explain requires extraordinary evidence. </p>
<h2>Strange anomaly</h2>
<p>The standard model describes nature on the smallest of scales, comprising <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">fundamental particles</a> known as leptons (such as electrons) and quarks (which can come together to form heavier particles such as protons and neutrons) and the forces they interact with. </p>
<p>There are many different kinds of quarks, some of which are unstable and can decay into other particles. The new result relates to an experimental anomaly that was <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.151601">first hinted at in 2014</a>, when LHCb physicists spotted “beauty” quarks decaying in unexpected ways. </p>
<p>Specifically, beauty quarks appeared to be decaying into leptons called “muons” less often than they decayed into electrons. This is strange because the muon is in essence a carbon-copy of the electron, identical in every way except that it’s around 200 times heavier. </p>
<p>You would expect beauty quarks to decay into muons just as often as they do to electrons. The only way these decays could happen at different rates is if some never-before-seen particles were getting involved in the decay and tipping the scales against muons.</p>
<p>While the 2014 result was intriguing, it wasn’t precise enough to draw a firm conclusion. Since then, a number of other anomalies have appeared in related processes. They have all individually been too subtle for researchers to be confident that they were genuine signs of new physics, but tantalisingly, they all seemed to be pointing in a similar direction. </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>The big question was whether these anomalies would get stronger as more data was analysed or melt away into nothing. In 2019, LHCb performed the <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.191801">same measurement</a> of beauty quark decay again but with extra data taken in 2015 and 2016. But things weren’t much clearer than they’d been five years earlier.</p>
<h2>New results</h2>
<p>Today’s result doubles the existing dataset by adding the sample recorded in 2017 and 2018. To avoid accidentally introducing biases, the data was analysed “blind” – the scientists couldn’t see the result until all the procedures used in the measurement had been tested and reviewed.</p>
<p><a href="https://www.imperial.ac.uk/people/mitesh.patel">Mitesh Patel</a>, a particle physicist at Imperial College London and one of the leaders of the experiment, described the excitement he felt when the moment came to look at the result. “I was actually shaking,” he said. “I realised this was probably the most exciting thing I’ve done in my 20 years in particle physics.” </p>
<p>When the result came up on the screen, the anomaly was still there – around 85 muon decays for every 100 electron decays, but with a smaller uncertainty than before.</p>
<p>What will excite many physicists is that the uncertainty of the result is now over “three sigma” – scientists’ way of saying that there is only around a one in a thousand chance that the result is a random fluke of the data. Conventionally, particle physicists call anything over three sigma “evidence”. However, we are still a long way from a confirmed “discovery” or “observation” – that would require five sigma.</p>
<p>Theorists have shown it is possible to explain this anomaly (and others) by recognising the existence of brand new particles that are influencing the ways in which the quarks decays. One possibility is a fundamental particle called a “Z prime” – in essence a carrier of a brand new force of nature. This force would be extremely weak, which is why we haven’t seen any signs of it until now, and would interact with electrons and muons differently. </p>
<p>Another option is the hypothetical “<a href="https://home.cern/news/news/physics/hunt-leptoquarks">leptoquark</a>” – a particle that has the unique ability to decay to quarks and leptons simultaneously and could be part of a larger puzzle that explains why we see the particles that we do in nature. </p>
<h2>Interpreting the findings</h2>
<p>So have we finally seen evidence of new physics? Well, maybe, maybe not. We do a lot of measurements at the LHC, so you might expect at least some of them to fall this far from the standard model. And we can never totally discount the possibility that there’s some bias in our experiment that we haven’t properly accounted for, even though this result has been checked extraordinarily thoroughly. Ultimately, the picture will only become clearer with more data. LHCb is undergoing a major upgrade to dramatically increase the rate it can record collisions. </p>
<p>Even if the anomaly persists, it will probably only be fully accepted once an independent experiment confirms the results. One exciting possibility is that we might be able to detect the new particles responsible for the effect being created directly in the collisions at the LHC. Meanwhile, the <a href="https://www.belle2.org">Belle II experiment</a> in Japan should be able to make similar measurements.</p>
<p>What then, could this mean for the future of fundamental physics? If what we are seeing is really the harbinger of some new fundamental particles then it will finally be the breakthrough that physicists have been yearning for for decades. </p>
<p>We will have finally seen a part of the larger picture that lies beyond the standard model, which ultimately could allow us to unravel any number of established mysteries. These include the nature of the invisible dark matter that fills the universe, or the nature of the Higgs boson. It could even help theorists unify the fundamental particles and forces. Or, perhaps best of all, it could be pointing at something we have never even considered. </p>
<p>So, should we be excited? Yes. Results like this don’t come around very often, the hunt is definitely on. But we should be cautious and humble too; extraordinary claims require extraordinary evidence. Only time and hard work will tell if we have finally seen the first glimmer of what lies beyond our current understanding of particle physics.</p><img src="https://counter.theconversation.com/content/157464/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Harry Cliff works for the University of Cambridge. He receives funding from STFC. He is a member of the LHCb collaboration and a 'user' at CERN. </span></em></p><p class="fine-print"><em><span>Konstantinos Alexandros Petridis receives funding from STFC. He works for the University of Bristol and is a member of the LHCb collaboration at CERN.</span></em></p><p class="fine-print"><em><span>Paula Alvarez Cartelle works for the University of Cambridge. She receives funding from STFC. She is a member of the LHCb collaboration at CERN. </span></em></p>
If the finding really is the result of new fundamental particles then it will finally be the breakthrough that physicists have been yearning for for decades.
Harry Cliff, Particle physicist, University of Cambridge
Konstantinos Alexandros Petridis, Senior lecturer in Particle Physics, University of Bristol
Paula Alvarez Cartelle, Lecturer of Particle Physics, University of Cambridge
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/155800
2021-03-03T13:42:01Z
2021-03-03T13:42:01Z
Cern: scientists discover four new particles – here’s why they matter
<figure><img src="https://images.theconversation.com/files/387513/original/file-20210303-21-1n3noa5.jpg?ixlib=rb-1.1.0&rect=36%2C0%2C6061%2C2216&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">LHCb experiment.</span> <span class="attribution"><a class="source" href="https://flickr.com/photos/r00s/48815389756/in/photolist-2hnDCEY-2hnBNZG-2hmMrsd-2hmMmXw-fZeRQ5-2hmJUUj-2hmLGp8-2hmLHUT-2hmLHsF-2hmLHGi-2hmMqga-2hmLJwK-2hmLJoU-2hmMsU6-2hmJVQc-2hmJURJ-2hmMqNN-2hmMr78-iuoMA-iuoRs-iuoUn-iuoPy-iuoSQ-XiwTJn-iuoWp-47Psud-qwDFna-fZrjWE-fZoMQT-fZrjcy-ybxjis-grz4Da-grz4Kn-fZrqK3-fZrZHe-fZrFoG-fZrqBM-fZrBbo-fZruMv-fZrJGc-grz4F4-fZrtHr-fZrjpW-fZrgm2-fZroYN-fZrymW-fZrmBo-grypt4-daC7Lj-pS6vpG">Roͬͬ͠͠͡͠͠͠͠͠͠͠͠sͬͬ͠͠͠͠͠͠͠͠͠aͬͬ͠͠͠͠͠͠͠ Menkman/Flickr</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>This month is a time to celebrate. Cern has just announced the discovery
of <a href="https://lhcb-public.web.cern.ch/Welcome.html#ccus">four brand new particles</a> at the Large Hadron Collider (LHC) in Geneva. This means that the LHC has now found <a href="https://home.cern/news/news/physics/59-new-hadrons-and-counting">a total of 59 new particles</a>, in addition to the <a href="https://theconversation.com/could-the-higgs-nobel-be-the-end-of-particle-physics-18978">Nobel prize-winning Higgs boson</a>, since it started colliding protons – particles that make up the atomic nucleus along with neutrons – in 2009. Excitingly, while some of these new particles were expected based on our established theories, some were altogether more surprising.</p>
<p>The LHC’s goal is to explore the structure of matter at the shortest distances and highest energies ever probed in the lab – testing our current best theory of nature: <a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">the Standard Model of Particle Physics</a>. And the LHC has delivered the goods – it enabled scientists <a href="https://theconversation.com/could-the-higgs-nobel-be-the-end-of-particle-physics-18978">to discover the Higgs boson</a>, the last missing piece of the model. That said, the theory is still far from being fully understood. </p>
<p>One of its most troublesome features is its description of the strong force which holds the atomic nucleus together. The nucleus is made up of protons and neutrons, which are in turn each composed of three <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html">tiny particles called quarks</a> (there are six different kinds of quarks: up, down, charm, strange, top and bottom). If we switched the strong force off for a second, all matter would immediately disintegrate into a soup of loose quarks – a state that existed for a fleeting instant at the beginning of the universe.</p>
<p>Don’t get us wrong: the theory of the strong interaction, pretentiously called
“<a href="https://cerncourier.com/a/the-history-of-qcd/">quantum chromodynamics</a>”, is on very solid footing. It describes how quarks interact through the strong force by exchanging particles called gluons. You can think of gluons as analogues of the more familiar photon, the particle of light and carrier of the electromagnetic force. </p>
<p>However, the way gluons interact with quarks makes the strong force behave very differently from electromagnetism. While the electromagnetic force gets weaker as you pull two charged particles apart, the strong force actually gets stronger as you pull two quarks apart. As a result, quarks are forever locked up inside particles called hadrons – particles made of two or more quarks – which includes protons and neutrons. Unless, of course, you smash them open at incredible speeds, as we are doing at Cern.</p>
<p>To complicate matters further, all the particles in the standard model <a href="https://theconversation.com/cern-discovery-sheds-light-on-the-great-mystery-of-why-the-universe-has-less-antimatter-than-matter-147226">have antiparticles</a> which are nearly identical to themselves but with the opposite charge (or other quantum property). If you pull a quark out of a proton, the force will eventually be strong enough to create a quark-antiquark pair, with the newly created quark going into the proton. You end up with a proton and a brand new “meson”, a particle made of a quark and an antiquark. This may sound weird but according to quantum mechanics, which rules the universe on the smallest of scales, particles can pop out of empty space.</p>
<p>This has been shown <a href="https://theconversation.com/explainer-quarks-12003">repeatedly by experiments</a> – we have never seen a lone quark. An unpleasant feature of the theory of the strong force is that calculations of what would be a simple process in electromagnetism can end up being impossibly complicated. We therefore cannot (yet) prove theoretically that quarks can’t exist on their own. Worse still, we can’t even calculate which combinations of quarks would be viable in nature and which would not.</p>
<figure class="align-center ">
<img alt="Illustration of a tetraquark." src="https://images.theconversation.com/files/387473/original/file-20210303-15-po0fxj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/387473/original/file-20210303-15-po0fxj.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/387473/original/file-20210303-15-po0fxj.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/387473/original/file-20210303-15-po0fxj.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/387473/original/file-20210303-15-po0fxj.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/387473/original/file-20210303-15-po0fxj.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/387473/original/file-20210303-15-po0fxj.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">Illustration of a tetraquark.</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>When quarks were first discovered, scientists realised that several combinations should be possible in theory. This included pairs of quarks and antiquarks (mesons); three quarks (baryons); three antiquarks (antibaryons); two quarks and two antiquarks (tetraquarks); and four quarks and one antiquark (pentaquarks) – as long as the number of quarks minus antiquarks in each combination was a multiple of three.</p>
<p>For a long time, only baryons and mesons were seen in experiments. But in 2003, the Belle experiment in Japan <a href="https://arxiv.org/abs/hep-ex/0308029">discovered a particle</a> that didn’t fit in anywhere. It turned out to be the first of a long series of tetraquarks. In 2015, the LHCb experiment at the LHC <a href="https://theconversation.com/heres-what-you-need-to-know-about-the-large-hadron-colliders-latest-discovery-pentaquarks-44721">discovered two pentaquarks</a>. The four new particles we’ve discovered recently <a href="https://arxiv.org/abs/2103.01803">are all tetraquarks</a> with a charm quark pair and two other quarks. All these objects are particles in the same way as the proton and the neutron are particles. But they are not fundamental particles: quarks and electrons are the true building blocks of matter.</p>
<h2>Charming new particles</h2>
<p>The LHC has now discovered 59 new hadrons. These include the tetraquarks most recently discovered, but also new mesons and baryons. All these new particles contain heavy quarks such as “charm” and “bottom”. </p>
<p>These hadrons are interesting to study. They tell us what nature considers acceptable as a bound combination of quarks, even if only for very short times. They also tell us what nature does not like. For example, why do all tetra- and pentaquarks contain a charm-quark pair (with just one exception)? And why are there no corresponding particles with strange-quark pairs? There is currently no explanation.</p>
<figure class="align-center ">
<img alt="Illustration of a pentaquark bound like a molecule." src="https://images.theconversation.com/files/387474/original/file-20210303-12-14v8dnv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/387474/original/file-20210303-12-14v8dnv.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/387474/original/file-20210303-12-14v8dnv.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/387474/original/file-20210303-12-14v8dnv.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/387474/original/file-20210303-12-14v8dnv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/387474/original/file-20210303-12-14v8dnv.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/387474/original/file-20210303-12-14v8dnv.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">Are pentaquarks molecules?</span>
<span class="attribution"><span class="source">CERN</span></span>
</figcaption>
</figure>
<p>Another mystery is how these particles are bound together by the strong force. One school of theorists considers them to be compact objects, like the proton or the neutron. Others claim they are akin to “molecules” formed by two loosely bound hadrons. Each newly found hadron allows experiments to measure its mass and other properties, which tell us something about how the strong force behaves. This helps bridge the gap between experiment and theory. The more hadrons we can find, the better we can tune the models to the experimental facts.</p>
<p>These models are crucial to achieve the ultimate goal of the LHC: find physics beyond the standard model. Despite its successes, the standard model is certainly not the last word in the understanding of particles. It is for instance <a href="https://theconversation.com/stephen-hawking-had-pinned-his-hopes-on-m-theory-to-fully-explain-the-universe-heres-what-it-is-93440">inconsistent with cosmological models</a> describing the formation of the universe. </p>
<p>The LHC is searching for new fundamental particles that could explain
these discrepancies. These particles could be visible at the LHC, but hidden
in the background of particle interactions. Or they could show up as small quantum mechanical effects in known processes. In either case, a better understanding of the strong force is needed to find them. With each new hadron, we improve our knowledge of nature’s laws, leading us to a better description of the most fundamental properties of matter.</p><img src="https://counter.theconversation.com/content/155800/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Harry Cliff is a member of the LHCb collaboration.</span></em></p><p class="fine-print"><em><span>Patrick Koppenburg 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>
The theory of tiny particles isn’t complete. But new discoveries are helping scientists expand it.
Patrick Koppenburg, Research Fellow in Particle Physics, Dutch National Institute for Subatomic Physics
Harry Cliff, Particle physicist, University of Cambridge
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/155433
2021-02-18T17:31:37Z
2021-02-18T17:31:37Z
It’s no Large Hadron Collider, but our new particle accelerator is the size of a large room
<figure><img src="https://images.theconversation.com/files/385027/original/file-20210218-20-1451rd2.png?ixlib=rb-1.1.0&rect=7%2C0%2C1552%2C1031&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">A prototype of our novel plasma-based particle accelerator</span> <span class="attribution"><span class="source">EuPRAXIA Conceptual Design Report</span></span></figcaption></figure><p>In 2010, when scientists were preparing to smash the first particles together within the Large Hadron Collider (LHC), sections of the media fantasised that the EU-wide experiment might <a href="https://www.forbes.com/sites/startswithabang/2016/03/11/could-the-lhc-make-an-earth-killing-black-hole/">create a black hole</a> that could swallow and destroy our planet. How on Earth, columnists fumed, could scientists justify such a dangerous indulgence in the pursuit of abstract, theoretical knowledge?</p>
<p>But particle accelerators are much more than enormous toys for scientists to play with. They have practical uses too, though their sheer size has, so far, prevented their widespread use. Now, as part of a large-scale European collaboration, my team has <a href="https://doi.org/10.1140/epjst/e2020-000127-8">published a report</a> that explains in detail how a far smaller particle accelerator could be built – closer to the size of a large room, rather than a large city. </p>
<p>Inspired by the technological and scientific know-how of machines like the LHC, our particle accelerator is designed to be as small as possible so it can be put to immediate practical use in industry, in healthcare and in universities.</p>
<h2>Collider scope</h2>
<p>The biggest collider in the world, the LHC, uses particle acceleration to achieve the astonishing speeds at which it collides particles. This system was used to measure the sought-after <a href="https://www.sciencedirect.com/science/article/pii/S037026931200857X">Higgs boson particle</a> – one of the most elusive particles predicted by the Standard Model, which is our current model to describe the structure and operation of the universe.</p>
<p>Less giant and glamorous particle accelerators have been around since the early 1930s, performing useful jobs as well as causing collisions to help our understanding of fundamental science. Accelerated particles are used to generate radioactive materials and strong bursts of radiation, which are crucial for healthcare processes such as radiotherapy, nuclear medicine and CT scans.</p>
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Read more:
<a href="https://theconversation.com/five-ways-particle-accelerators-have-changed-the-world-without-a-higgs-boson-in-sight-54187">Five ways particle accelerators have changed the world (without a Higgs boson in sight)</a>
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<p>The typical downside to accelerators is that they tend to be bulky, complex to run and often prohibitively expensive. The LHC represents a pinnacle of experimental physics, but it is 27 kilometres (17 miles) in circumference and cost <a href="https://home.cern/sites/home.web.cern.ch/files/2018-07/factsandfigures-en_0.pdf">6.5 billion Swiss francs (£5.2 billion)</a> to build and test. The accelerators currently installed in select hospitals are smaller and cheaper, but they still cost tens of millions of pounds, and require 400x400m of space for installation. As such, only large regional hospitals can afford the money and the space to host a radiotherapy department.</p>
<p>Why exactly do accelerators need to be so big? The simple answer is that if they were any smaller, they’d break. Since they’re based on solid materials, ramping up the power too much would tear the system apart, creating a very expensive mess.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A large yellow circle drawn over an aerial view of fields" src="https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=450&fit=crop&dpr=1 600w, https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=450&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=450&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=566&fit=crop&dpr=1 754w, https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=566&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/385016/original/file-20210218-12-19epl5i.jpg?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>
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<span class="caption">The Large Hadron Collider is a vast looped system on the France-Switzerland border.</span>
<span class="attribution"><span class="source">Cern</span></span>
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<h2>Need for speed</h2>
<p>We set out to find a way to make smaller, cheaper particle accelerators for use in a wider range of hospitals – from the large and regional to the small and provincial.</p>
<p>Our team worked on the premise that to accelerate particles you actually have two options: either give them a strong boost over a short distance, or lots of small nudges over a long one – which is how the LHC works.</p>
<p>It’s a bit like reaching 100mph in a vehicle: you can either slowly accelerate in a truck over a long period of time, or you can put your foot down in a sports car and get there in a matter of seconds. Conventional accelerators are a bit like trucks: reliable and docile, but slow. We’ve been searching for the sports car alternative.</p>
<p>We found that alternative in plasma. The beauty of plasma is that it’s just composed of an ionised gas: a gas that’s been broken down to its tiniest components. As such, it doesn’t have the same limit on the power that can be applied to it as a solid system. In effect, you can’t break something that is already broken.</p>
<figure class="align-center ">
<img alt="A man holds a clear component in front of his eye. Behind him is a large yellow pipe" src="https://images.theconversation.com/files/385020/original/file-20210218-28-aii8mb.png?ixlib=rb-1.1.0&rect=8%2C5%2C1830%2C1196&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/385020/original/file-20210218-28-aii8mb.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=393&fit=crop&dpr=1 600w, https://images.theconversation.com/files/385020/original/file-20210218-28-aii8mb.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=393&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/385020/original/file-20210218-28-aii8mb.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=393&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/385020/original/file-20210218-28-aii8mb.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=494&fit=crop&dpr=1 754w, https://images.theconversation.com/files/385020/original/file-20210218-28-aii8mb.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=494&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/385020/original/file-20210218-28-aii8mb.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=494&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">A researcher holding a section of our novel particle accelerator. Behind is the corresponding section in a traditional accelerator.</span>
<span class="attribution"><span class="source">EuPRAXIA Conceptual Design Report</span>, <span class="license">Author provided</span></span>
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<p>It’s in this sense that plasmas can sustain much higher accelerating powers – up to a thousand times larger than a solid-state accelerator. The higher the power, the shorter time and distance it takes to accelerate particles, and this leads to smaller, cheaper accelerators. </p>
<p>Our accelerator uses powerful lasers to “shake” the plasmas it contains, moving their particles about in a way that creates waves. It’s a little like the wake left behind by a boat (the laser) on a lake (the plasma). Like a surfer, a beam placed on one of these waves can then be pushed forward by it, constantly accelerating. These waves within plasmas are very small (sub-millimetre) and very powerful, which is what allows the overall accelerator to be extremely small.</p>
<h2>Plasma perks</h2>
<p>Plasma-based particle accelerators like ours will need 100 times less space than existing designs, reducing the space required for installation from 400x400m to just 40x40m. The hardware needed to build our accelerator is cheaper to install, run and maintain. Overall, we expect our plasma accelerator to reduce the cost of installing an accelerator in a hospital by a factor of ten.</p>
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<a href="https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Four different scanned images of a mouse" src="https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=909&fit=crop&dpr=1 600w, https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=909&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=909&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1142&fit=crop&dpr=1 754w, https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1142&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/385089/original/file-20210218-28-1ia04lk.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1142&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">A mouse embryo scanned with our machine (left column) and traditional scans (right column).</span>
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<p>Besides these two advantages, our accelerator can perform certain new functions that existing accelerators cannot. For instance, plasma-based accelerators can provide <a href="https://www.youtube.com/watch?v=a8ueGqLPy1I">detailed X-rays of biological samples</a> with <a href="https://www.pnas.org/content/115/25/6335/tab-figures-data">far greater clarity</a> than those that exist today. By providing a better image of the inside of a human body, this could help doctors find cancer at a much earlier stage, dramatically increasing the chance of successfully treating the illness. </p>
<p>The same ultra-high resolution imaging can also help spot the early signs of cracks and defects on machinery, at nanometer scale. Faults related to such defects are regarded as one of the “six big losses” well known to manufacturers. Their early detection by our accelerator could help extend the lifetime of high-precision, high-quality components in heavy industry and manufacturing.</p>
<h2>Accelerator rollout</h2>
<p>The European Strategy Forum on Research Infrastructures is assessing the design report, with a decision expected in summer 2021. If successful, construction of the first two prototypes is expected to be completed by 2030, with access to external users to be granted immediately after.</p>
<p>Several years of interdisciplinary research were needed for us to form the first detailed and realistic design of a machine of this kind. Our plasma accelerator is the most recent example of how obscure, abstract, fundamental physics can enter into our everyday lives – cutting research costs, improving manufacturing and helping to save lives.</p>
<p><em>This article was updated on February 23 2021 to clarify that the EuPRAXIA particle accelerator is designed to perform a different set of tasks than those performed by the Large Hadron Collider.</em></p><img src="https://counter.theconversation.com/content/155433/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Gianluca Sarri receives funding from the Engineering and Physical Sciences Research Council (EPSRC) and the Science and Technology Facility Council (STFC). </span></em></p>
The compact accelerators are 100 times smaller than traditional ones, and could easily fit inside hospitals and laboratories.
Gianluca Sarri, Reader (Associate Professor) at the School of Mathematics and Physics, Queen's University Belfast
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/147226
2020-12-21T12:05:25Z
2020-12-21T12:05:25Z
CERN: discovery sheds light on the great mystery of why the universe has less ‘antimatter’ than matter
<figure><img src="https://images.theconversation.com/files/374225/original/file-20201210-21-lsz3bz.jpg?ixlib=rb-1.1.0&rect=14%2C5%2C1982%2C739&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">There's a lot of matter in the universe, here the cat paw nebula of dust and gas.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>It’s one of the greatest puzzles in physics. All the particles that make up the matter around us, such electrons and protons, have <a href="https://theconversation.com/explainer-what-is-antimatter-53414">antimatter versions</a> which are nearly identical, but with mirrored properties such as the opposite electric charge. When an antimatter and a matter particle meet, they annihilate in a flash of energy. </p>
<p>If antimatter and matter are truly identical but mirrored copies of each other, they should have been produced in equal amounts in the Big Bang. The problem is that would have made it all annihilate. But today, there’s nearly no antimatter left in the universe – it appears only in some radioactive decays and in a small fraction of cosmic rays. So what happened to it? Using the <a href="https://home.cern/news/news/physics/lhcb-sees-new-form-matter-antimatter-asymmetry-strange-beauty-particles">LHCb experiment</a> at CERN to study the difference between matter and antimatter, we have <a href="https://arxiv.org/abs/2012.05319">discovered a new way</a> that this difference can appear.</p>
<p>The existence of antimatter was predicted by physicist <a href="https://www.nobelprize.org/prizes/physics/1933/dirac/biographical/">Paul Dirac</a>’s equation describing the motion of electrons in 1928. At first, it was not clear if this was just a mathematical quirk or a description of a real particle. But in 1932 Carl Anderson <a href="https://timeline.web.cern.ch/carl-anderson-discovers-positron">discovered</a> an antimatter partner to the electron – the positron – while studying cosmic rays that rain down on Earth from space. Over the next few decades physicists found that all matter particles have antimatter partners.</p>
<p>Scientists believe that in the very hot and dense state shortly after the Big Bang, there must have been processes that gave preference to matter over antimatter. This created a small surplus of matter, and as the universe cooled, all the antimatter was destroyed, or annihilated, by an equal amount of matter, leaving a tiny surplus of matter. And it is this surplus that makes up everything we see in the universe today.</p>
<p>Exactly what processes caused the surplus is unclear, and physicists have been on the lookout for decades. </p>
<h2>Known asymmetry</h2>
<p>The behaviour of quarks, which are the fundamental building blocks of matter along with leptons, can shed light on the difference between matter and antimatter. Quarks <a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/quark.html">come in many different kinds</a>, or “flavours”, known as up, down, charm, strange, bottom and top plus six corresponding anti-quarks. </p>
<p>The up and down quarks are what make up the protons and neutrons in the nuclei of ordinary matter, and the other quarks can be produced by high-energy processes – for instance by colliding particles in accelerators such as the Large Hadron Collider at CERN. </p>
<p>Particles consisting of a quark and an anti-quark are called mesons, and there are four neutral mesons (B<sup>0</sup><sub>S</sub>, B<sup>0</sup>, D<sup>0</sup> and K<sup>0</sup>) that exhibit a fascinating behaviour. They can spontaneously turn into their antiparticle partner and then back again, a phenomenon that was observed for the first time in the 1960. Since they are unstable, they will “decay” – fall apart – into other more stable particles at some point during their oscillation. This decay <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">happens slightly differently for mesons compared with anti-mesons</a>, which combined with the oscillation means that the rate of the decay varies over time.</p>
<p>The rules for the oscillations and decays are given by a theoretical framework called the <a href="https://www.nobelprize.org/prizes/physics/2008/summary/">Cabibbo-Kobayashi-Maskawa (CKM) mechanism</a>. It predicts that there is a difference in the behaviour of matter and antimatter, but one that is too small to generate the surplus of matter in the early universe required to explain the abundance we see today. </p>
<p>This indicates that there is something we don’t understand and that studying this topic may challenge some of our most fundamental theories in physics.</p>
<h2>New physics?</h2>
<p>Our recent result from the LHCb experiment is a study of neutral B<sup>0</sup><sub>S</sub> mesons, looking at their decays into pairs of charged K mesons. The B<sup>0</sup><sub>S</sub> mesons were created by colliding protons with other protons in the Large Hadron Collider where they oscillated into their anti-meson and back three trillion times per second. The collisions also created anti-B<sup>0</sup><sub>S</sub> mesons that oscillate in the same way, giving us samples of mesons and anti-mesons that could be compared. </p>
<p>We counted the number of decays from the two samples and compared the two numbers, to see how this difference varied as the oscillation progressed. There was a slight difference – with more decays happening for one of the B<sup>0</sup><sub>S</sub> mesons. And for the first time for
B<sup>0</sup><sub>S</sub> mesons, we observed that the difference in decay, or asymmetry, varied according to the oscillation between the B<sup>0</sup><sub>S</sub> meson and the anti-meson.</p>
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<img alt="" src="https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.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">
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<span class="caption">LHCb.</span>
<span class="attribution"><span class="source">Maximilien Brice et al./CERN</span></span>
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<p>In addition to being a milestone in the study of matter-antimatter differences, we were also able to measure the size of the asymmetries. This can be translated into measurements of several parameters of the underlying theory. Comparing the results with other measurements provides a consistency check, to see if the currently accepted theory is a correct description of nature. Since the small preference of matter over antimatter that we observe on the microscopic scale cannot explain the overwhelming abundance of matter that we observe in the universe, it is likely that our current understanding is an approximation of a more fundamental theory. </p>
<p>Investigating this mechanism that we know can generate matter-antimatter asymmetries, probing it from different angles, may tell us where the problem lies. Studying the world on the smallest scale is our best chance to be able to understand what we see on the largest scale.</p><img src="https://counter.theconversation.com/content/147226/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Lars Eklund works for Uppsala University and is affiliated with the University of Glasgow, which would both benefit from any publicity generated by this article. He has received funding from STFC in the UK . </span></em></p>
New physics may be needed to explain why there’s more matter than antimatter in the universe.
Lars Eklund, Professor of Particle Physics, University of Glasgow
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/127987
2019-12-10T00:04:01Z
2019-12-10T00:04:01Z
The X17 factor: a particle new to physics might solve the dark matter mystery
<figure><img src="https://images.theconversation.com/files/304869/original/file-20191203-67011-f5wdzi.jpg?ixlib=rb-1.1.0&rect=0%2C13%2C4551%2C4579&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Anomalies in nuclear physics experiments may show signs of a new force.</span> <span class="attribution"><span class="source">Shutterstock</span></span></figcaption></figure><p>A team of scientists in Hungary recently <a href="https://arxiv.org/abs/1910.10459">published a paper</a> that hints at the existence of a previously unknown subatomic particle. The team first <a href="https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.042501">reported</a> finding traces of the particle in 2016, and they now report more traces in a different experiment.</p>
<p>If the results are confirmed, the so-called X17 particle could help to explain dark matter, the mysterious substance scientists believe accounts for more than 80% of the mass in the universe. It may be the carrier of a “fifth force” beyond the four accounted for in the standard model of physics (gravity, electromagnetism, the weak nuclear force and the strong nuclear force). </p>
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Read more:
<a href="https://theconversation.com/why-do-astronomers-believe-in-dark-matter-122864">Why do astronomers believe in dark matter?</a>
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<h2>Smashing atoms</h2>
<p>Most researchers who hunt for new particles use enormous accelerators that smash subatomic particles together at high speed and look at what comes out of the explosion. The biggest of these accelerators is the Large Hadron Collider in Europe, where the Higgs boson – a particle scientists had been hunting for decades – was discovered in 2012.</p>
<p>Attila J. Krasznahorkay and his colleagues at ATOMKI (the Institute of Nuclear Research in Debrecen, Hungary) have taken a different approach, conducting smaller experiments that fire the subatomic particles called protons at the nuclei of different atoms.</p>
<p>In 2016, they looked at pairs of electrons and positrons (the antimatter version of electrons) produced when beryllium-8 nuclei went from a high energy state to a low energy state.</p>
<p>They found a deviation from what they expected to see when there was a large angle between the electrons and positrons. This anomaly could be best be explained if the nucleus emitted an unknown particle which later “split” into an electron and a positron. </p>
<p>This particle would have to be a boson, which is the kind of particle that carries force, and its mass would be around 17 million electron volts. That’s about as heavy as 34 electrons, which is fairly lightweight for a particle like this. (The Higgs boson, for example, is more than 10,000 times heavier.) </p>
<p>Because of its mass, Krasznahorkay and his team called the hypothetical particle X17. Now they have observed some strange behaviour in helium-4 nuclei which can also be explained by the presence of X17. </p>
<p>This latest anomaly is statistically significant – a seven sigma confidence level, which means there is only a very tiny possibility the result occurred by chance. This is well beyond the usual five-sigma standard for a new discovery, so the result would seem to suggest there is some new physics here. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/304866/original/file-20191203-66986-1ygqhma.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/304866/original/file-20191203-66986-1ygqhma.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=338&fit=crop&dpr=1 600w, https://images.theconversation.com/files/304866/original/file-20191203-66986-1ygqhma.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=338&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/304866/original/file-20191203-66986-1ygqhma.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=338&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/304866/original/file-20191203-66986-1ygqhma.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=424&fit=crop&dpr=1 754w, https://images.theconversation.com/files/304866/original/file-20191203-66986-1ygqhma.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=424&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/304866/original/file-20191203-66986-1ygqhma.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">The new research is led by Attila Krasznahorkay (right).</span>
<span class="attribution"><span class="source">Attila Krasznahorkay</span></span>
</figcaption>
</figure>
<h2>Checking and double checking</h2>
<p>However, the new announcement and the one in 2016 have been met with scepticism by the physics community – the kind of scepticism that did not exist when two teams simultaneously announced the discovery of the Higgs boson in 2012. </p>
<p>So why is it so hard for physicists to believe a new lightweight boson like this could exist? </p>
<p>First, experiments of this sort are difficult, and so is the analysis of the data. Signals can appear and disappear. Back in 2004, for example, the group in Debrecen found <a href="https://arxiv.org/abs/hep-ph/0511049">evidence</a> they interpreted as the possible existence of an even lighter boson, but when they repeated the experiment the signal was gone.</p>
<p>Second, one needs to make sure the very existence of X17 is compatible with the results from other experiments. In this case, both the 2016 result with beryllium and the new result with helium can be explained by the existence of X17 but an independent check from an independent group is still necessary. </p>
<p>Krasznahorkay and his group first reported weak evidence (at a three-sigma level) for a new boson in 2012 at <a href="http://inspirehep.net/record/1235778">a workshop</a> in Italy. </p>
<p>Since then the team has repeated the experiment using upgraded equipment and successfully reproduced the beryllium-8 results, which is reassuring, as are the new results in helium-4. These new results were presented at the <a href="http://hias.anu.edu.au/2019/">HIAS 2019</a> symposium at the Australian National University in Canberra. </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>
<h2>What does this have to do with dark matter?</h2>
<p>Scientists believe that most of the matter in the universe is invisible to us. So-called dark matter would only interact with normal matter very weakly. We can infer that it exists from its gravitational effects on distant stars and galaxies, but it has never been detected in the lab. </p>
<p>So where does X17 come in?</p>
<p>In 2003, in one of us (Boehm) showed that a particle like X17 could exist, in <a href="https://arxiv.org/abs/hep-ph/0305261">work co-authored with Pierre Fayet</a> and <a href="https://arxiv.org/abs/astro-ph/0208458">alone</a>. It would carry force between dark matter particles in much the same way photons, or particles of light, do for ordinary matter.</p>
<p>In one of the scenarios I proposed, lightweight dark matter particles could sometimes produce pairs of electrons and positrons in a way that is similar to what Krasznahorkay’s team has seen. </p>
<p>This scenario has led to many searches in low-energy experiments, which have ruled out a lot of possibilities. However, X17 has not yet been ruled out – in which case the Debrecen group might have indeed discovered how dark matter particles communicate with our world. </p>
<h2>More evidence required</h2>
<p>While the results from Debrecen are very interesting, the physics community will not be convinced a new particle has indeed been found until there is independent confirmation. </p>
<p>So we can expect many experiments around the world that are looking for a new lightweight boson to start hunting for evidence of X17 and its interaction with pairs of electrons and positrons.</p>
<p>If confirmation arrives, the next discovery might be the dark matter particles themselves.</p><img src="https://counter.theconversation.com/content/127987/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.</span></em></p>
A recent experiment with atomic nuclei is hard to square with our current understanding of physics.
Celine Boehm, Head of School for Physics, University of Sydney
Tibor Kibedi, Senior Fellow in Nuclear Physics, Australian National University
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/122230
2019-08-23T10:37:26Z
2019-08-23T10:37:26Z
Ghost particles: how galaxies helped us weigh the lightest neutrino – and why it matters
<figure><img src="https://images.theconversation.com/files/289201/original/file-20190823-170931-99idv2.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Crab Nebula is a remnant of a supernova, a source of neutrinos.</span> <span class="attribution"><span class="source">NASA, ESA, J. Hester and A. Loll (Arizona State University)</span></span></figcaption></figure><p>Even when you close your eyes at night, 100 billion neutrinos produced in the sun will pass through them – <a href="https://theconversation.com/benefits-of-knowing-more-about-neutrinos-which-pass-through-our-bodies-unnoticed-49025">travelling close to the speed of light, but never hitting anything</a>. Neutrinos are extremely elusive and only weakly interact with matter around them: nature’s true ghosts. Until very recently, these tiny particles were believed to be massless.</p>
<p>In the late 1990s, <a href="https://theconversation.com/how-the-neutrino-could-solve-great-cosmic-mysteries-and-win-its-next-nobel-prize-48789">researchers demonstrated</a> that neutrinos constantly change between three different types (flavours or species), which affects how they interact with matter. This is something they can only do if they have mass – a discovery that was <a href="https://theconversation.com/physics-duo-wins-the-nobel-prize-for-solving-longstanding-neutrino-puzzle-48702">granted the Nobel Prize in 2015</a>. From these particle physics experiments, we know that at least two of the three neutrino species have mass. </p>
<p>Yet little has been known about the mass of the lightest species – until now. Our new study, <a href="https://www.ucl.ac.uk/news/2019/aug/maximum-mass-lightest-neutrino-revealed-using-astronomical-big-data">published in Physical Review Letters</a>, shows that the lightest neutrino is at least 6m times lighter than the mass of an electron, at 0.086 electron volts (a unit of energy). Our technique may give exact masses for each neutrino in the future.</p>
<p>Neutrinos are peculiar. Thanks to the <a href="https://theconversation.com/physicists-prove-quantum-spookiness-and-start-chasing-schrodingers-cat-48190">strange rules of quantum mechanics</a>, the relationships between the flavours and their masses are complicated. In any beam of neutrinos, the three masses are always present but in different ratios. Each flavour of neutrino has a combination of the three masses and each neutrino mass has a combination of the three flavours.</p>
<h2>Massive significance</h2>
<p>Neutrinos matter to understanding space. Back to the 1940s, <a href="https://journals.aps.org/pr/pdf/10.1103/PhysRev.58.1117">a letter to Physical Review</a> by physicists <a href="https://cosmosmagazine.com/space/this-week-in-science-history-big-bang-theorist-george-gamow-dies">George Gamow</a> and <a href="https://en.wikipedia.org/wiki/M%C3%A1rio_Schenberg">Mario Schoenberg</a> (a personal hero of mine) suggested that neutrinos played a major role in stellar evolution and supernovas (exploding stars). This <a href="https://www.nasa.gov/feature/goddard/2017/the-dawn-of-a-new-era-for-supernova-1987a/">was confirmed</a> when scientists detected the first neutrinos from a supernova in 1987, providing a better understanding of supernovas and neutrinos alike.</p>
<p>On a cosmological scale, however, because these ghostly particles have mass, they tend to drag a little bit of matter with them thanks to gravity. And so the more massive the neutrinos are, the “fuzzier” the distribution of galaxies around us will be. This means that by observing galaxies around us, we can infer the mass of neutrinos. It is amazing to think that the largest structures of galaxies in the universe are sensitive to the tiniest particles known by physics. </p>
<p>The reason scientists are so keen to find out their mass, is that it matters to our ultimate understanding of reality. <a href="https://theconversation.com/the-standard-model-of-particle-physics-the-absolutely-amazing-theory-of-almost-everything-94700">The Standard Model of Particle Physics</a> is one of the most precise theories of fundamental particles that we have so far. However, this theory predicts that neutrinos should be massless.</p>
<p>Understanding neutrino masses is a key point to move ahead towards a new and improved theory of particle physics. It is quite possible that by doing so, other mysteries in physics, which also cannot be explained by the standard model, such as the nature of <a href="https://theconversation.com/the-experiments-trying-to-crack-physics-biggest-question-what-is-dark-energy-52917">dark energy</a> and <a href="https://theconversation.com/from-machos-to-wimps-meet-the-top-five-candidates-for-dark-matter-51516">dark matter</a>, would also be solved.</p>
<h2>Two camps</h2>
<p>The way we made our discovery matters, too. Our international team of researchers from the UK and Brazil combined cosmological data and particle physics experiments.</p>
<p>Each approach has its limitations. When cosmologists determine neutrino masses from observations of galaxy distributions, they can only determine a maximum mass for the sum of the three neutrinos. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=415&fit=crop&dpr=1 600w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=415&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=415&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=522&fit=crop&dpr=1 754w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=522&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/289196/original/file-20190823-170906-1wxxyui.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=522&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Hercules galaxy cluster.</span>
<span class="attribution"><span class="source">ESO/INAF-VST/OmegaCAM</span>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Particle physics experiments can directly study neutrinos, for example by <a href="https://www.symmetrymagazine.org/article/november-2012/how-to-make-a-neutrino-beam">creating a beam</a> of them in the lab or detecting neutrinos from space underground. But most of them are based on detecting how neutrino flavours mix. This doesn’t tell you the absolute mass for each particle, but only <a href="https://www.sciencedirect.com/science/article/pii/S0550321316000602?via%3Dihub">the difference in mass</a> for two out of three neutrino species. It also doesn’t tell us which of the two is the heaviest neutrino.</p>
<p>Sadly, many cosmologists often ignore results from particle physics. And some particle physicists are sceptical of cosmologists’ statistical techniques, claiming their way of using prior knowledge, such as estimating that the mass cannot be negative, influences their results.</p>
<p>We combined the two by creating a mathematical model to calculate the sum of the neutrino masses by studying the large scale structure of more than a million galaxies from the <a href="http://www.sdss3.org/surveys/boss.php">Baryon Oscillation Spectroscopic Survey (BOSS)</a>. We also had to include many other parameters that affect galaxy distribution such as dark matter and dark energy. We then fed in what we know from particle physics experiments, which told us in very exact terms what the relationships between the masses of the neutrinos should be. This allowed us to calculate an upper limit of the mass of the lightest particle.</p>
<p>In order to achieve these results, we had to push the boundaries of big data analysis in cosmology to their limits, using more than half a million computing hours to process the data. Luckily, we had a <a href="https://www.ucl.ac.uk/research-it-services/grace-launch">super computer</a> to help us. </p>
<p>Although current available data is not powerful enough to detect a lower-bound for the mass of the lightest neutrino – it could still be massless– this work demonstrates that applying a combined methodology is the way forward. </p>
<p><a href="https://www.desi.lbl.gov">The Dark Energy Spectroscopic Instrument (DESI)</a>, which will analyse ten times more galaxies than we had access to, will give a much more reliable estimate of the total mass for all the neutrinos – and that may make it possible to work out a minimum mass. Excitingly, this might just launch a new era in neutrino physics.</p><img src="https://counter.theconversation.com/content/122230/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Arthur Loureiro received funding from the National Council for Science and Technology (CNPq), Brazil.</span></em></p>
If we want an improved theory of particle physics, understanding neutrino masses is key.
Arthur Loureiro, Research Assistant in Cosmology and Astroparticle Physics, UCL
Licensed as Creative Commons – attribution, no derivatives.
tag:theconversation.com,2011:article/114211
2019-03-27T12:48:45Z
2019-03-27T12:48:45Z
Exotic particles containing five quarks discovered at the Large Hadron Collider
<figure><img src="https://images.theconversation.com/files/265980/original/file-20190326-36270-m9m2v3.png?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Illustration of the possible layout of the quarks in a pentaquark particle. </span> <span class="attribution"><span class="source">Daniel Dominguez/CERN</span></span></figcaption></figure><p>Everything you see around you is made up of <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">elementary particles</a> called quarks and leptons, which can combine to form bigger particles such as protons or atoms. But that doesn’t make them boring – these subatomic particles can also combine in exotic ways we’ve never spotted. Now CERN’s LHCb collaboration <a href="https://home.cern/news/news/physics/lhcb-experiment-discovers-new-pentaquark">has announced the discovery</a> of a clutch of new particles dubbed “pentaquarks”. The results can help unveil many mysteries of the theory of quarks, a key part of the standard model of particle physics.</p>
<p>Quarks were <a href="https://home.cern/news/news/physics/fifty-years-quarks">first proposed</a> to explain the untidy slew of new particles discovered in cosmic ray and collider experiments in the mid 20th century. This growing “zoo” of apparently fundamental particles caused consternation among physicists, who have a natural bias towards simplicity and order – and hate having to remember more than a few basic principles. The famous Italian physicist <a href="https://www.nobelprize.org/prizes/physics/1938/fermi/biographical/">Enrico Fermi</a> captured the mood of his colleagues <a href="https://en.wikiquote.org/wiki/Enrico_Fermi">when he said</a> “Young man, if I could remember the names of all these particles, I would have been a botanist”.</p>
<p>Fortunately, in the 1960s, the American physicist <a href="https://www.nobelprize.org/prizes/physics/1969/gell-mann/biographical/">Murray Gell-Mann</a> noticed patterns in the particle zoo, similar to those spotted by <a href="https://www.chemistryworld.com/features/the-father-of-the-periodic-table/3009828.article">Dimitri Mendeleev</a> when he drew up the periodic table of the chemical elements. Just as the periodic table implied the existence of things smaller than atoms, Gell-Mann’s theory suggested the existence of a new class of fundamental particles. Particle physicists were eventually able to explain the hundreds of particles in the zoo as being made up of a much smaller number of truly fundamental particles called quarks.</p>
<h2>Mystery hadrons</h2>
<p>There are six types of quarks in the standard model – down, up, strange, charm, bottom and top. These also have “antimatter” companions – it is believed that every particle has an antimatter version that is virtually identical to itself, but with the opposite charge. Quarks and antiquarks <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">get bound together</a> to make particles known as hadrons. </p>
<p>According to Gell-Mann’s model, there are two broad classes of hadrons. One is particles made of three quarks called <a href="http://www.particleadventure.org/hadrons.html">baryons</a> (which include the protons and neutrons that make up the atomic nucleus) and the other particles made of a quark and an antiquark known as <a href="https://www.britannica.com/science/meson">mesons</a>.</p>
<p>Until recently, baryons and mesons were the only types of hadrons that had been seen in experiments. However, back in the 1960s, Gell-Mann also raised the possibility of more exotic combinations of quarks, such as tetraquarks (two quarks and two antiquarks) and pentaquarks (four quarks and one antiquark). </p>
<p>In 2014, LHCb, which runs one of the four giant experiments at CERN’s Large Hadron Collider, <a href="https://theconversation.com/quirky-quark-combination-creates-exotic-new-particle-25465">published a result</a> showing that the snappily named Z(4430)<sup>+</sup> particle was a tetraquark. This started a flurry of interest in new exotic hadrons. Then, in 2015, LHCb <a href="https://theconversation.com/heres-what-you-need-to-know-about-the-large-hadron-colliders-latest-discovery-pentaquarks-44721">announced the discovery</a> of the first ever pentaquark, adding a brand new class of particle to the hadron family.</p>
<p>The results presented by LHCb today expand upon that first pentaquark discovery by finding additional such particles. This was possible thanks to a big chunk of new data recorded during the second run of the Large Hadron Collider. <a href="https://www.linkedin.com/in/liming-zhang-ab12b73b/">Liming Zhang</a>, an associate professor at Tsinghua University in Beijing and one of the physicists who made the measurement, said that “we now have ten times more data than in 2015, which allows us to see more exciting and finer structures than we could before.” When Liming and his colleagues examined the original pentaquark discovered in 2015, they were surprised to find that it had split in two. The original pentaquark was actually two separate pentaquark particles that had such similar masses that they originally looked like a single particle. </p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=400&fit=crop&dpr=1 600w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=400&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=400&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=503&fit=crop&dpr=1 754w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=503&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/264993/original/file-20190320-93063-r7pdv4.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">LHCb.</span>
<span class="attribution"><span class="source">Maximilien Brice et al./CERN</span></span>
</figcaption>
</figure>
<p>As if two pentaquarks for the price of one wasn’t exciting enough, LHCb also found a third pentaquark with a slightly smaller mass than the other two. All three pentaquarks are made of one down quark, two up quarks, a charm quark and a charm antiquark.</p>
<p>The big question now is: what is the precise internal structure of these pentaquarks? One option is that they are truly made of five quarks, with all of them mixed together evenly within a single hadron. Another possibility is that the pentaquarks are really a baryon and a meson stuck together to form a loosely bound molecule, similar to the way that protons and neutrons bind together inside the atomic nucleus.</p>
<p><a href="http://thecollege.syr.edu/people/faculty/pages/phy/skwarnicki-tomasz.html">Tomasz Skwarnicki</a>, a professor of physics at Syracuse University in New York who also worked on the measurement, told me that the new companion state “is at a mass which offers hints about internal structure of pentaquarks”. The most likely option is that these pentaquarks are baryon-meson molecules, he added. To be absolutely sure, physicists will need more experimental data, as well as further studies from theorists, meaning that the story of these pentaquarks is far from over. </p>
<p>These results complete a week of exciting new announcements from LHCb, which included the discovery of a <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">new kind of matter-antimatter asymmetry</a>. The LHC has yet to discover any particles beyond the standard model that could help to explain mysteries like dark matter, an invisible but unknown substance that makes up the majority of matter in the universe. </p>
<p>But these exciting measurements show that there is still lots to learn about the particles and forces of the standard model. It may be that our best chance of finding answers to the big questions facing fundamental physics in the 21st century lies in more detailed studies of the particles we already know about rather than discovering new ones. Either way, we still have a great deal to discover.</p><img src="https://counter.theconversation.com/content/114211/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Harry Cliff is a member of the LHCb Collaboration, though he was not directly involved in the work described in this article.</span></em></p>
The LHCb experiment at CERN has discovered three new ‘pentaquark’ particles being created in high energy particle collisions at the Large Hadron Collider.
Harry Cliff, Particle physicist, University of Cambridge
Licensed as Creative Commons – attribution, no derivatives.