When hundreds of physicists gathered on a Zoom call in late February to discuss their experiment’s results, none of them knew what they had found. Like doctors in a clinical trial, the researchers at the Muon g-2 experiment blinded their data, concealing a single variable that prevented them from being biased about or knowing—for years—what the information they were working with actually meant.
But when the data were unveiled over Zoom, the physicists knew the wait had been worth it: their results are further evidence that new physics is hiding in muons, the bulkier cousins of electrons. “That was the point at which we knew the results. Until then we had no idea,” says Rebecca Chislett, a physicist at University College London, who is part of the Muon g-2 collaboration. “It was exciting and nerve-wracking and a bit of a relief.”
Despite its remarkable success in explaining the fundamental particles and forces that make up the universe, the Standard Model’s description remains woefully incomplete. It does not account for gravity, for one thing, and it is similarly silent about the nature of dark matter, dark energy and neutrino masses. To explain these phenomena and more, researchers have been hunting for new physics—physics beyond the Standard Model—by looking for anomalies in which experimental results diverge from theoretical predictions.
Muon g-2 is an experiment at Fermi National Laboratory in Batavia, Ill, that aims to precisely measure how magnetic muons are by watching them wobble in a magnetic field. If the experimental value of these particles’ magnetic moment differs from the theoretical prediction—an anomaly—that deviation could be a sign of new physics, such as some subtle and unknown muon-influencing particle or force. The newly updated experimental value for muons, reported on Wednesday in Physical Review Letters, deviates from theory by only a minuscule value (0.00000000251) and has a statistical significance of 4.2 sigma.* But even that tiny amount could profoundly shift the direction of particle physics.
“My first impression is ‘Wow,’” says Gordan Krnjaic, a theoretical physicist at Fermilab, who was not involved in the research. “It’s almost the best possible case scenario for speculators like us.... I’m thinking much more that it’s possibly new physics, and it has implications for future experiments and for possible connections to dark matter.”
Not everyone is as sanguine. Numerous anomalies have cropped up only to disappear, leaving the Standard Model victorious and physicists jaded about the prospects of breakthrough discoveries.
“My feeling is that there’s nothing new under the sun,” says Tommaso Dorigo, an experimental physicist at the University of Padua in Italy, who was also not involved with the new study. “I think that this is still more likely to be a theoretical miscalculation.... But it is certainly the most important thing that we have to look into presently.”
Muons are almost identical to electrons. The two particles have the same electric charge and other quantum properties, such as spin. But muons are some 200 times heavier than electrons, which causes them to have a short lifetime and to decay into lighter particles. As a result, muons cannot play electrons’ pivotal role in forming structures: molecules and mountains alike—indeed, essentially all chemical bonds among atoms—endure thanks to electrons’ stability.
When German physicist Paul Kunze first observed the muon in 1933, he wasn’t sure what to make of it. “He showed this track that was neither an electron nor a proton, which he called—my translation—‘a particle of uncertain nature,’” says Lee Roberts, a physicist at Boston University and an experimentalist at Muon g-2. The newfound particle was a curious complication to the otherwise limited cast of subatomic particles, which led physicist Isidor Isaac Rabi to famously wonder, “Consider the muon. Who ever ordered that?” The ensuing deluge of exotic particles discovered in the decades that followed showed that the muon was actually part of a larger ensemble, but history has nonetheless been kind to Rabi’s befuddlement: it turns out there might indeed be something strange about the muon.
In 2001 the E821 experiment at Brookhaven National Laboratory in Upton, N.Y., found hints that muons’ magnetic moment diverged from theory. At the time, the finding was not robust enough because it had a statistical significance of only 3.3 sigma: that is, if there were no new physics, then scientists would still expect to see a difference that large once out of 1,000 runs of an experiment because of pure chance. The result was short of five sigma—a one-in-3.5-million fluke—but enough to pique researchers’ interest for future experiments.
With a statistical significance of 4.2 sigma, researchers cannot yet say they have made a discovery. But the evidence for new physics in muons—in conjunction with anomalies recently observed at the Large Hadron Collider Beauty (LHCb) experiment at CERN near Geneva—is tantalizing.
Most physics experiments reuse parts. For example, the Large Hadron Collider is based in the tunnel designed for, and previously occupied by, its predecessor, the Large Electron-Positron Collider. But the experimentalists behind Muon g-2 took matters further than most when, instead of building a new magnet, they shipped the 50-foot ring from Brookhaven on a 3,200-mile trip to its new home at Fermilab.
The magnet occupies a central place in Muon g-2. A beam of positive pions—lightweight particles made from an up quark and a down antiquark—decay into muons and muon neutrinos. The muons are collected and channeled into an orderly circular path around the magnet, which they will circle, at most, a few thousand times before they decay into positrons. By detecting the direction of muon decays, physicists can extract information about how the particles interacted with the magnet.
How does this process work? Imagine each muon as a tiny analog clock. As the particle circles the magnet, its hour hand goes around and around at a rate predicted by theory. When the muon’s time is up, it decays into a positron that is emitted in the direction of the hour hand. But if that hand turns at a rate different from theory—say, a tick too fast—the positron decay will end up pointing in a slightly different direction. (In this analogy, the hour hand corresponds to the muon’s spin, a quantum property that determines the direction of the muon decay.) Detect enough deviating positrons, and you have an anomaly.
What an anomaly implies is ambiguous. There might be something not accounted for by the Standard Model, and it could be a difference between electrons and muons. Or there could be a similar effect in electrons that is too small to currently see. (The mass of a particle is related to how much it can interact with heavier unknown particles, so muons, which have about 200 times the mass of electrons, are much more sensitive.)
Muon g-2 began collecting data for its first run in 2017, but the results did not come out until now because processing that information was an arduous task. “Although people might have wanted to see the result come out early, this just reflects a long period of doing our due diligence to understand things,” says Brendan Kiburg, a Fermilab physicist, who is part of the collaboration.
Alone, Muon g-2’s experimental value does not indicate much. To have meaning, it has to be compared against the latest theoretical prediction, which itself was the work of about 130 physicists.
The necessity for all that brainpower comes down to this: When a muon travels through space, that space is not really empty. Instead it is a sizzling and swarming soup of an infinite number of virtual particles that can pop in and out of existence. The muon has some small chance of interacting with these particles, which tug on it, influencing how it behaves. Calculating the virtual particles’ effect on the muon’s spin—the rate at which its hour hand turns—requires a series of equally arduous and incredibly precise theoretical determinations.
All of this means the theoretical prediction for muons has its own uncertainty, which theorists have been trying to whittle down. One way is via lattice quantum chromodynamics (QCD), a technique that relies on massive computational power to numerically solve the effects of the virtual particles on muons. According to Aida X. El-Khadra, a physicist at the University of Illinois at Urbana-Champaign, who was not involved with the experimental result, about half a dozen groups are all in hot pursuit of the problem.
The fun is just beginning. In the coming days and weeks, a torrent of theoretical papers will attempt to make more sense of the new result. Models that introduce new particles such as the Z' boson and the leptoquark will be updated in light of the new information. While some physicists speculate about what, exactly, the muon anomaly could mean, the effort to reduce uncertainties and push the anomaly above five sigma is ongoing.
Data from Muon g-2’s second and third runs are expected in about 18 months, according to Kiburg and Chislett, and that information could push the anomaly past the five-sigma threshold—or decrease its significance. If it is not decisive, researchers at J-PARC (Japan Proton Accelerator Research Complex), a physics lab in Tokai, Japan, may have an answer. They plan to independently corroborate the Muon g-2 result using a slightly different method to observe muon behavior. Meanwhile theorists will continue to refine their predictions to reduce the uncertainty of their own measurements.
Even if all of these efforts confirm there is new physics at work in muons, however, they will not be able to reveal what, exactly, that new physics is. The needed tool to reveal its nature may be a new collider—something many physicists are clamoring for via proposals such as the International Linear Collider and the High-Luminosity LHC. In the past few months, interest has surged around a muon collider, which multiple papers predict would guarantee physicists the ability to determine the properties of the unknown particle or force affecting the muon.
Even those who are skeptical about the significance of the new result cannot help but find a silver lining. “It is good for particle physics,” Dorigo says, “because particle physics has been dead for a little while.”
*Editor’s Note: The author of this article is related to Robert Garisto, a handling editor at Physical Review Letters, but they had no communications about the paper prior to its publication.