tag:theconversation.com,2011:/us/topics/muon-53998/articlesMuon – The Conversation2023-10-17T19:03:53Ztag:theconversation.com,2011:article/2056282023-10-17T19:03:53Z2023-10-17T19:03:53ZNew technique uses near-miss particle physics to peer into quantum world − two physicists explain how they are measuring wobbling tau particles<figure><img src="https://images.theconversation.com/files/532985/original/file-20230620-21-sf8wvl.jpg?ixlib=rb-1.1.0&rect=464%2C501%2C4206%2C3241&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The Large Hadron Collider at CERN can be used to study many kinds of fundamental particles, including mysterious and rare tau particles.</span> <span class="attribution"><a class="source" href="https://www.gettyimages.com/detail/photo/abstract-neon-circles-digital-fractal-black-royalty-free-image/1191907046?phrase=particle+physics&adppopup=true">Oxygen/Moment via Getty Images</a></span></figcaption></figure><p>One way physicists seek clues to unravel the mysteries of the universe is by smashing matter together and inspecting the debris. But these types of destructive experiments, while incredibly informative, have limits. </p>
<p>We are two scientists who <a href="https://www.colorado.edu/physics/dennis-perepelitsa">study nuclear</a> and <a href="https://www.phy.cam.ac.uk/staff/dr-jesse-liu">particle physics</a> using CERN’s Large Hadron Collider near Geneva, Switzerland. Working with an international group of nuclear and particle physicists, our team realized that hidden in the data from previous studies was a remarkable and innovative experiment. </p>
<p>In a new paper published in Physical Review Letters, we developed a new method with our colleagues for measuring <a href="https://doi.org/10.1103/PhysRevLett.131.151802">how fast a particle called the tau wobbles</a>.</p>
<p>Our novel approach looks at the times incoming particles in the accelerator whiz by each other rather than the times they smash together in head-on collisions. Surprisingly, this approach enables far more accurate measurements of the tau particle’s wobble than previous techniques. This is the first time in nearly 20 years scientists have measured this wobble, known as the <a href="https://doi.org/10.1088/1742-6596/912/1/012001">tau magnetic moment</a>, and it may help illuminate tantalizing cracks <a href="https://theconversation.com/the-standard-model-of-particle-physics-may-be-broken-an-expert-explains-182081">emerging in the known laws of physics</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A diagram showing a particle wobbling off of a vertical axis." src="https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=319&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=319&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=319&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=401&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=401&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531776/original/file-20230613-26-1ofchy.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=401&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Electrons, muons and taus all wobble in a magnetic field like a spinning top. Measuring the wobbling speed can provide clues into quantum physics.</span>
<span class="attribution"><span class="source">Jesse Liu</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Why measure a wobble?</h2>
<p>Electrons, the building blocks of atoms, have two heavier cousins called the <a href="https://www.britannica.com/science/subatomic-particle/Charged-leptons-electron-muon-tau">muon and the tau</a>. Taus are the heaviest in this family of three and the most mysterious, as they exist only for minuscule amounts of time.</p>
<p>Interestingly, when you place an electron, muon or tau inside a magnetic field, these particles wobble in a manner similar to how a spinning top wobbles on a table. This wobble is called a particle’s magnetic moment. It is possible to predict how fast these particles should wobble using the <a href="https://home.cern/science/physics/standard-model">Standard Model of particle physics</a> – scientists’ best theory of how particles interact.</p>
<p>Since the 1940s, physicists have been interested in measuring magnetic moments to reveal intriguing <a href="https://doi.org/10.1103/PhysRev.74.250">effects in the quantum world</a>. According to quantum physics, clouds of particles and antiparticles are constantly <a href="https://www.symmetrymagazine.org/article/july-2009/virtual-particles">popping in and out of existence</a>. These fleeting fluctuations slightly alter how fast electrons, muons and taus wobble inside a magnetic field. By measuring this wobble very precisely, physicists can peer into this cloud to uncover possible hints of undiscovered particles. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A chart showing the basic particles." src="https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=574&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=574&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=574&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=722&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=722&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531789/original/file-20230613-15-4hjd2s.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=722&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Electrons, muons and taus are three closely related particles in the Standard Model of particle physics – scientists’ current best description of the fundamental laws of nature.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg#/media/File:Standard_Model_of_Elementary_Particles.svg">MissMJ, Cush/Wikimedia Commons</a></span>
</figcaption>
</figure>
<h2>Testing electrons, muons and taus</h2>
<p>In 1948, theoretical physicist Julian Schwinger first calculated how the quantum cloud <a href="https://doi.org/10.1103/PhysRev.73.416">alters the electron’s magnetic moment</a>. Since then, experimental physicists have measured the speed of the electron’s wobble to an extraordinary <a href="https://doi.org/10.1038/s41586-020-2964-7">13 decimal places</a>. </p>
<p>The heavier the particle, the more its wobble will change because of undiscovered new particles lurking in its quantum cloud. Since electrons are so light, this limits their sensitivity to new particles.</p>
<p>Muons and taus are much heavier but also far shorter-lived than electrons. While muons exist only for mere microseconds, scientists at Fermilab near Chicago measured the muon’s magnetic moment to <a href="https://news.fnal.gov/2021/04/first-results-from-fermilabs-muon-g-2-experiment-strengthen-evidence-of-new-physics/">10 decimal places</a> in 2021. They found that muons wobbled noticeably faster than Standard Model predictions, suggesting unknown particles may be appearing in the muon’s quantum cloud.</p>
<p>Taus are the heaviest particle of the family – 17 times more massive than a muon and 3,500 times heavier than an electron. This makes them much more <a href="https://doi.org/10.1103/PhysRevD.64.035003">sensitive to potentially undiscovered particles</a> in the quantum clouds. But taus are also the hardest to see, since they live for just a millionth of the time a muon exists.</p>
<p>To date, the best measurement of the tau’s magnetic moment was made in 2004 using <a href="https://home.cern/science/accelerators/large-electron-positron-collider">a now-retired electron collider</a> at CERN. Though an incredible scientific feat, after multiple years of collecting data that experiment could measure the speed of the tau’s wobble to only <a href="https://doi.org/10.1140/epjc/s2004-01852-y">two decimal places</a>. Unfortunately, to test the Standard Model, physicists would need a measurement <a href="https://doi.org/10.1142/S0217732307022694">10 times as precise</a>.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Diagram showing two particles nearly colliding." src="https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=319&fit=crop&dpr=1 600w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=319&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=319&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=401&fit=crop&dpr=1 754w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=401&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/531773/original/file-20230613-29-zwf5pp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=401&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Instead of colliding two nuclei head-on to create tau particles, two lead ions can whiz past each other in a near miss and still produce taus.</span>
<span class="attribution"><span class="source">Jesse Liu</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Lead ions for near-miss physics</h2>
<p>Since the 2004 measurement of the tau’s magenetic moment, physicists have been seeking new ways to measure the tau wobble.</p>
<p>The Large Hadron Collider usually smashes the nuclei of two atoms together – that is why it is called a collider. These head-on collisions create a <a href="https://cds.cern.ch/record/2841509">fireworks display of debris</a> that can include taus, but the noisy conditions preclude careful measurements of the tau’s magnetic moment.</p>
<p>From 2015 to 2018, there was an experiment at CERN that was designed primarily to allow nuclear physicists to study <a href="https://home.cern/science/physics/heavy-ions-and-quark-gluon-plasma">exotic hot matter</a> created in head-on collisions. The particles used in this experiment were lead nuclei that had been stripped of their electrons – called lead ions. Lead ions are electrically charged and produce <a href="https://doi.org/10.1038/nphys4208">strong electromagnetic fields</a>. </p>
<p>The electromagnetic fields of lead ions contain particles of light called photons. When two lead ions collide, their photons can also collide and convert all their energy into a single pair of particles. It was these photon collisions that scientists used to <a href="https://doi.org/10.1103/PhysRevLett.121.212301">measure muons</a>.</p>
<p>These lead ion experiments ended in 2018, but it wasn’t until 2019 that one of us, Jesse Liu, teamed up with particle physicist Lydia Beresford in Oxford, England, and realized the data from the same lead ion experiments could potentially be used to do something new: measure the tau’s magnetic moment. </p>
<p><a href="https://doi.org/10.1103/PhysRevD.102.113008">This discovery was a total surprise</a>. It goes like this: Lead ions are so small that they often miss each other in collision experiments. But occasionally, the ions pass very close to each other without touching. When this happens, their accompanying photons can still smash together while the ions continue flying on their merry way. </p>
<p>These photon collisions can create a variety of particles – like the muons in the previous experiment, and also taus. But without the chaotic fireworks produced by head-on collisions, these near-miss events are far quieter and ideal for measuring traits of the elusive tau.</p>
<p>Much to our excitement, when the team looked back at data from 2018, indeed these lead ion near misses were creating tau particles. There was a new experiment hidden in plain sight!</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="A long tube in an underground tunnel." src="https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=399&fit=crop&dpr=1 600w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=399&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=399&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=502&fit=crop&dpr=1 754w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=502&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/532999/original/file-20230620-8426-na9es5.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=502&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Large Hadron Collider accelerates particles to incredibly high speeds before trying to smash particles together, but not all attempts result in successful collisions.</span>
<span class="attribution"><a class="source" href="http://cds.cern.ch/record/1211045">Maximilien Brice/CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<h2>First measurement of tau wobble in two decades</h2>
<p>In April 2022, the CERN team announced that we had found <a href="https://atlas.cern/updates/briefing/observation-taupair-heavy-ions">direct evidence of tau particles created</a> during lead ion near misses. Using that data, the team was also able to measure the tau magnetic moment – the first time such a measurement had been done since 2004. The final results were published on Oct. 12, 2023.</p>
<p>This landmark result measured the tau wobble to two decimal places. Much to our astonishment, this method tied the previous best measurement using only one month of data recorded in 2018.</p>
<p>After no experimental progress for nearly 20 years, this result opens an entirely new and important path toward the tenfold improvement in precision needed to test Standard Model predictions. Excitingly, more data is on the horizon. </p>
<p>The Large Hadron Collider just restarted <a href="https://home.cern/news/news/experiments/lhc-lead-ion-collision-run-starts">lead ion data collection on Sept. 28, 2023</a>, after routine maintenance and upgrades. Our team plans to quadruple the sample size of lead ion near-miss data by 2025. This increase in data will double the accuracy of the measurement of the tau magnetic moment, and improvements to analysis methods may go even further.</p>
<p>Tau particles are one of physicists’ best windows to the enigmatic quantum world, and we are excited for surprises that upcoming results may reveal about the fundamental nature of the universe.</p><img src="https://counter.theconversation.com/content/205628/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Jesse Liu is supported by a Junior Research Fellowship at Trinity College, University of Cambridge. </span></em></p><p class="fine-print"><em><span>Dennis V. Perepelitsa receives research funding from the U.S. Department of Energy, Office of Science.</span></em></p>Physicists uncovered a new experiment hidden in old data from the Large Hadron Collider. Using this innovative approach, the team has unlocked an entirely new way to study quantum physics.Jesse Liu, Research Fellow in Physics, University of CambridgeDennis V. Perepelitsa, Associate Professor of Physics, University of Colorado BoulderLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/2112802023-08-10T15:41:30Z2023-08-10T15:41:30ZIs 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 LiverpoolCe Zhang, Postdoctoral research associate, University of LiverpoolElia Bottalico, Postdoctoral Research Associate, University of LiverpoolSaskia Charity, Postdoctoral Research Associate, Particle Physics, University of LiverpoolLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1578292021-04-09T15:31:39Z2021-04-09T15:31:39ZProof 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 StateLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1585642021-04-08T16:35:08Z2021-04-08T16:35:08ZHow 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>
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<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 LiverpoolMark Lancaster, Professor of Physics, University of ManchesterLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1314152020-02-11T17:58:12Z2020-02-11T17:58:12ZHow a ‘muon accelerator’ could unravel some of the universe’s greatest mysteries<figure><img src="https://images.theconversation.com/files/314757/original/file-20200211-146674-fyj6w5.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">The supernova remnant Cassiopeia A.</span> <span class="attribution"><span class="source">NASA</span></span></figcaption></figure><p>The fact that we are here at all is one of the greatest puzzles of physics. We are made from normal particles such as electrons, but every such particle also has an antimatter companion that is virtually identical to itself, but with the opposite charge. When matter and antimatter come into contact, they annihilate each other in a flash of light.</p>
<p>Physics suggests that matter and antimatter <a href="https://theconversation.com/cern-study-sheds-light-on-one-of-physics-biggest-mysteries-why-theres-more-matter-than-antimatter-113947">were created in almost equal quantities</a> in the Big Bang. So how come there’s almost only matter left today – why didn’t the matter and antimatter annihilate each other to render the universe lifeless? Our new research has enabled scientists <a href="http://www.natureasia.com/en/research/highlight/13209">to build a new type of accelerator</a>, based on particles called muons, that could help us find out.</p>
<p>A muon <a href="https://theconversation.com/explainer-what-are-fundamental-particles-38339">is almost identical</a> to an electron, except that it is 207 times heavier. What’s more, it decays into other particles in two millionths of a second. This lifetime, although short for us, is quite a long time for an unstable fundamental particle, and this explains why muons make up almost all the charged cosmic rays which reach the Earth’s surface.</p>
<p>Much of our ability to investigate the structure of matter at the shortest distances depends on creating beams of particles and accelerate them to high energies. However there are only four stable particles which can be used in this way, the electron and its anti-particle (positron), and the proton and its antiparticle (antiproton).</p>
<p>Particle beams consisting of those have been used for many years, yet both of these pairs have drawbacks. The electron and its partner are very light – when we try to accelerate them, they radiate electromagnetic energy. This is can be useful for applications such as TV but makes it hard to reach the sort of energies which we need to improve our understanding of the universe. </p>
<p>Unlike electrons, the proton and the anti-proton are made up of more fundamental particles – quarks and gluons. In a collision between a proton and anti-proton it is these fundamental particles which actually collide, resulting in a lower energy crash than you would have got if protons had been truly fundamental particles.</p>
<figure class="align-center ">
<img alt="" src="https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=443&fit=crop&dpr=1 600w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=443&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=443&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=557&fit=crop&dpr=1 754w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=557&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/314754/original/file-20200211-146686-rytr8t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=557&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px">
<figcaption>
<span class="caption">Each particle also has an antiparticle, not included here.</span>
<span class="attribution"><a class="source" href="https://www.publicdomainpictures.net/en/view-image.php?image=35144&picture=fundamental-particles">Publicdomainpictures.net</a></span>
</figcaption>
</figure>
<p>The muons are heavy enough that they radiate much less energy,
but fundamental (not made up of smaller particles) so that all their energy is available for the investigation. When scientists created the <a href="https://theconversation.com/could-the-higgs-nobel-be-the-end-of-particle-physics-18978">Nobel-prize winning Higgs particle</a> with protons, they needed a machine 10km in diameter: the <a href="https://theconversation.com/explainer-how-does-an-experiment-at-the-large-hadron-collider-work-42846">Large Hadron Collider</a>. A muon machine, however, could achieve this with a circumference of just 200 metres.</p>
<p>The disadvantage of muons is that, unlike electrons and protons, they are unstable and need to be produced and then rapidly used before they all decay. We can produce muons by taking a narrow, high-intensity beam of protons and running it into a target made of a metal, such as titanium. This produces a beam of another fundamental particle called the pion. </p>
<p><a href="http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/hadron.html">Pions</a> form a beam which fans out. If the original proton beam looks like a laser pointer, the pion beam looks more like a torch beam – with intensity dropping rapidly with distance. The pions then decay to produce the muons, meaning the beam spreads even more – resembling a light bulb. </p>
<p>We cannot accelerate such a beam in a machine like the LHC, so we need to create a beam which spreads out much less. This is challenging given that we have only have two millionths of a second to produce, accelerate and collide it.</p>
<p>But our team of physicists and engineers, from around the world – known as the <a href="http://mice.iit.edu/">Muon Ionisation Cooling Experiment</a> (MICE) – has now shown that it is possible. We used a process known as cooling to help compress the beam. This involves passing the muons through a container with liquid hydrogen at -250°C, slowing the particles down. We then passed them through an electromagnetic cavity, which caused the beam to accelerate in the required direction.</p>
<p>By repeating this several times it is possible to create a beam which spreads out much less and has a dense core. This beam can be injected into a particle accelerator to produce a high-energy muon beam. Such a beam can either be collided or left to circulate until the muons decay into an intense beam of neutrinos – far in excess of any neutrino beam which can currently be produced. </p>
<h2>Probing the universe</h2>
<p>In fact, a neutrino beam created from muons is part of a planned <a href="https://stfc.ukri.org/research/accelerator-science/neutrino-factory/">Neutrino Factory</a>, which would allow us to answer many questions related to the origin and evolution of the universe – such as the mysterious imbalance between matter and antimatter. </p>
<p>Neutrinos could also help us understand the details of how life-essential elements such as oxygen, carbon and silicon, which are formed in stars, spread across universe. These heavier elements were not produced in the Big Bang and yet are responsible for the planet we live on and all the life around us. We know that bursts of neutrinos, which are released in star explosions (supernovas), are responsible. </p>
<p>We could also collide two beams of muons in the same way as we collide protons at the LHC. The muons, being simpler than the protons, would allow for more precise determination of the properties of the Higgs particle, for example.</p>
<p>The properties of the muon also makes it an invaluable tool in the field of material physics. The ability to create more closely focused beams may improve the current measurements and open up new diagnostic methods.</p>
<p>Our method may also be used to help increase the intensity of any other charged particle beams. It has been a long project lasting over a dozen years, but it has been worth the effort when we consider what a powerful tool we have created.</p><img src="https://counter.theconversation.com/content/131415/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Paul Kyberd receives funding from STFC.</span></em></p>When scientists created the Higgs particle with protons, they needed the 10km-wide Large Hadron Collider. A muon machine could achieve it with a diameter of just 200 metres.Paul Kyberd, Senior Lecturer in Particle Physics Informatics, Brunel University LondonLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/947002018-05-23T10:39:19Z2018-05-23T10:39:19ZThe Standard Model of particle physics: The absolutely amazing theory of almost everything<figure><img src="https://images.theconversation.com/files/219824/original/file-20180521-14978-36nv6i.jpg?ixlib=rb-1.1.0&rect=174%2C0%2C977%2C649&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">How does our world work on a subatomic level?</span> <span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Varsha_ys.jpg">Varsha Y S</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span></figcaption></figure><p>The Standard Model. What a dull name for the most accurate scientific theory known to human beings.</p>
<p>More than a quarter of the Nobel Prizes in physics of the last century are direct inputs to or direct results of the Standard Model. Yet its name suggests that if you can afford a few extra dollars a month you should buy the upgrade. <a href="https://scholar.google.com/citations?user=eQiX0m4AAAAJ&hl=en&oi=ao">As a theoretical physicist</a>, I’d prefer The Absolutely Amazing Theory of Almost Everything. That’s what the Standard Model really is.</p>
<p>Many recall the excitement among scientists and media over the 2012 <a href="https://home.cern/topics/higgs-boson">discovery of the Higgs boson</a>. But that much-ballyhooed event didn’t come out of the blue – it capped a five-decade undefeated streak for the Standard Model. Every fundamental force but gravity is included in it. Every attempt to overturn it to demonstrate in the laboratory that it must be substantially reworked – and there have been many over the past 50 years – has failed. </p>
<p>In short, the <a href="https://home.cern/about/physics/standard-model">Standard Model</a> answers this question: What is everything made of, and how does it hold together?</p>
<h2>The smallest building blocks</h2>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=424&fit=crop&dpr=1 600w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=424&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=424&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=533&fit=crop&dpr=1 754w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=533&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/219828/original/file-20180521-14991-vlfgkx.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=533&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">But these elements can be broken down further.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Periodic_table_vectorial.png">Rubén Vera Koster</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>You know, of course, that the world around us is made of molecules, and molecules are made of atoms. Chemist <a href="https://www.famousscientists.org/dmitri-mendeleev/">Dmitri Mendeleev</a> figured out in the 1860s how to organize all atoms – that is, the elements – into the periodic table that you probably studied in middle school. But there are 118 different chemical elements. There’s antimony, arsenic, aluminum, selenium … and 114 more.</p>
<p>Physicists like things simple. We want to boil things down to their essence, a few basic building blocks. Over a hundred chemical elements is not simple. The ancients believed that everything is made of just five elements – <a href="https://en.wikipedia.org/wiki/Classical_element">earth, water, fire, air and aether</a>. Five is much simpler than 118. It’s also wrong. </p>
<p>By 1932, scientists knew that all those atoms are made of just three particles – neutrons, protons and electrons. The neutrons and protons are bound together tightly into the nucleus. The electrons, thousands of times lighter, whirl around the nucleus at speeds approaching that of light. Physicists <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1918/planck-bio.html">Planck</a>, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-bio.html">Bohr</a>, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1933/schrodinger-bio.html">Schroedinger</a>, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1932/heisenberg-bio.html">Heisenberg</a> and friends had invented a new science – <a href="https://en.wikipedia.org/wiki/Quantum_mechanics">quantum mechanics</a> – to explain this motion.</p>
<p>That would have been a satisfying place to stop. Just three particles. Three is even simpler than five. But held together how? The negatively charged electrons and positively charged protons are bound together by <a href="https://en.wikipedia.org/wiki/Electromagnetism">electromagnetism</a>. But the protons are all huddled together in the nucleus and their positive charges should be pushing them powerfully apart. The neutral neutrons can’t help. </p>
<p>What binds these protons and neutrons together? “Divine intervention” a man on a Toronto street corner told me; he had a pamphlet, I could read all about it. But this scenario seemed like a lot of trouble even for a divine being – keeping tabs on every single one of the universe’s 10⁸⁰ protons and neutrons and bending them to its will. </p>
<h2>Expanding the zoo of particles</h2>
<p>Meanwhile, nature cruelly declined to keep its zoo of particles to just three. Really four, because we should count the <a href="https://en.wikipedia.org/wiki/Photon">photon</a>, the particle of light that <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-bio.html">Einstein</a> described. Four grew to five when <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1936/anderson-bio.html">Anderson</a> measured electrons with positive charge – positrons – striking the Earth from outer space. At least <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1933/dirac-bio.html">Dirac</a> had predicted these first anti-matter particles. Five became six when the pion, which <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1949/yukawa-bio.html">Yukawa</a> predicted would hold the nucleus together, was found. </p>
<p>Then came the muon – 200 times heavier than the electron, but otherwise a twin. “Who ordered that?” <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1944/rabi-bio.html">I.I. Rabi</a> quipped. That sums it up. Number seven. Not only not simple, redundant.</p>
<p>By the 1960s there were hundreds of “fundamental” particles. In place of the well-organized periodic table, there were just long lists of baryons (heavy particles like protons and neutrons), mesons (like <a href="https://en.wikipedia.org/wiki/Hideki_Yukawa">Yukawa</a>’s pions) and leptons (light particles like the electron, and the elusive neutrinos) – with no organization and no guiding principles.</p>
<p>Into this breach sidled the Standard Model. It was not an overnight flash of brilliance. No Archimedes leapt out of a bathtub shouting “eureka.” Instead, there was a series of crucial insights by a few key individuals in the mid-1960s that transformed this quagmire into a simple theory, and then five decades of experimental verification and theoretical elaboration. </p>
<p><a href="https://home.cern/about/updates/2014/01/fifty-years-quarks">Quarks</a>. They come in six varieties we call flavors. Like ice cream, except not as tasty. Instead of vanilla, chocolate and so on, we have up, down, strange, charm, bottom and top. In 1964, <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1969/gell-mann-bio.html">Gell-Mann</a> and <a href="https://www.macfound.org/fellows/113/">Zweig</a> taught us the recipes: Mix and match any three quarks to get a baryon. Protons are two ups and a down quark bound together; neutrons are two downs and an up. Choose one quark and one antiquark to get a meson. A pion is an up or a down quark bound to an anti-up or an anti-down. All the material of our daily lives is made of just up and down quarks and anti-quarks and electrons.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=536&fit=crop&dpr=1 600w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=536&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=536&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=673&fit=crop&dpr=1 754w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=673&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/219958/original/file-20180522-51127-4tx5tr.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=673&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 elementary particles provides an ingredients list for everything around us.</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:Standard_Model_From_Fermi_Lab.jpg">Fermi National Accelerator Laboratory</a>, <a class="license" href="http://creativecommons.org/licenses/by/4.0/">CC BY</a></span>
</figcaption>
</figure>
<p>Simple. Well, simple-ish, because keeping those quarks bound is a feat. They are tied to one another so tightly that you never ever find a quark or anti-quark on its own. The theory of that binding, and the particles called gluons (chuckle) that are responsible, is called <a href="https://en.wikipedia.org/wiki/Quantum_chromodynamics">quantum chromodynamics</a>. It’s a vital piece of the Standard Model, but mathematically difficult, even posing an unsolved problem of basic mathematics. We physicists do our best to calculate with it, but we’re still learning how.</p>
<p>The other aspect of the Standard Model is “<a href="https://doi.org/10.1103/PhysRevLett.19.1264">A Model of Leptons</a>.” That’s the name of the landmark 1967 paper by <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/1979/weinberg-bio.html">Steven Weinberg</a> that pulled together quantum mechanics with the vital pieces of knowledge of how particles interact and organized the two into a single theory. It incorporated the familiar electromagnetism, joined it with what physicists called “the weak force” that causes certain radioactive decays, and explained that they were different aspects of the same force. It incorporated <a href="https://www.nobelprize.org/nobel_prizes/physics/laureates/2013/higgs-facts.html">the Higgs mechanism</a> for giving mass to fundamental particles. </p>
<p>Since then, the Standard Model has predicted the results of experiment after experiment, including the discovery of several varieties of quarks and of the <a href="https://en.wikipedia.org/wiki/W_and_Z_bosons">W and Z bosons</a> – heavy particles that are for weak interactions what the photon is for electromagnetism. The possibility that <a href="https://en.wikipedia.org/wiki/Neutrino#Mass">neutrinos aren’t massless</a> was overlooked in the 1960s, but slipped easily into the Standard Model in the 1990s, a few decades late to the party.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=385&fit=crop&dpr=1 600w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=385&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=385&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=484&fit=crop&dpr=1 754w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=484&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/219960/original/file-20180522-51095-vverdp.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=484&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">3D view of an event recorded at the CERN particle accelerator showing characteristics expected from the decay of the SM Higgs boson to a pair of photons (dashed yellow lines and green towers).</span>
<span class="attribution"><a class="source" href="https://commons.wikimedia.org/wiki/File:3D_view_of_an_event_recorded_with_the_CMS_detector_in_2012_at_a_proton-proton_centre_of_mass_energy_of_8_TeV.png">McCauley, Thomas; Taylor, Lucas; for the CMS Collaboration CERN</a>, <a class="license" href="http://creativecommons.org/licenses/by-sa/4.0/">CC BY-SA</a></span>
</figcaption>
</figure>
<p>Discovering the Higgs boson in 2012, long predicted by the Standard Model and long sought after, was a thrill but not a surprise. It was yet another crucial victory for the Standard Model over the dark forces that particle physicists have repeatedly warned loomed over the horizon. Concerned that the Standard Model didn’t adequately embody their expectations of simplicity, worried about its mathematical self-consistency, or looking ahead to the eventual necessity to bring the force of gravity into the fold, physicists have made numerous proposals for theories beyond the Standard Model. These bear exciting names like <a href="https://en.wikipedia.org/wiki/Grand_Unified_Theory">Grand Unified Theories</a>, <a href="https://en.wikipedia.org/wiki/Supersymmetry">Supersymmetry</a>, <a href="https://en.wikipedia.org/wiki/Technicolor_(physics)">Technicolor</a>, and <a href="https://en.wikipedia.org/wiki/String_theory">String Theory</a>. </p>
<p>Sadly, at least for their proponents, beyond-the-Standard-Model theories have not yet successfully predicted any new experimental phenomenon or any experimental discrepancy with the Standard Model.</p>
<p>After five decades, far from requiring an upgrade, the Standard Model is <a href="http://artsci.case.edu/smat50/">worthy of celebration</a> as the Absolutely Amazing Theory of Almost Everything.</p><img src="https://counter.theconversation.com/content/94700/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Glenn Starkman receives funding from the Office of Science of the US Department of Energy. He is affiliated with Case Western Reserve University. </span></em></p>A particle physicist explains just what this keystone theory includes. After 50 years, it’s the best we’ve got to answer what everything in the universe is made of and how it all holds together.Glenn Starkman, Distinguished University Professor of Physics, Case Western Reserve UniversityLicensed as Creative Commons – attribution, no derivatives.