Another nail could seal the muon’s coffin as a place for new physics. A team of physicists performing extremely precise calculations of the muon’s properties in simulations found that the particle’s properties are more consistent with the Standard Model than previously thought.
The team is called BMW Collaboration and his the study is currently hosted on the arXiv preprint server, meaning it has not yet been published in a peer-reviewed journal. The team’s previous findings, published in Nature in 2021 they are “weakening[ed] the longstanding discrepancy between experiment and theory. In other words, their work has brought experimental physics closer to theoretical predictions when it comes to our understanding of the muon.
In their new paper, the team performed large-scale lattice quantum chromodynamics (QCD) simulations on finer grids than in their previous work, yielding a more precise calculation. Essentially, the team took QCD as input, put a lattice on spacetime, and simulated it. Their results predicted an anomalous muon magnetic moment that was only 0.9 standard deviations from the experimental mean for measurements of the property.
The muon and the standard model
A muon is an elementary particle about 207 times more massive than an electron. For about 20 years, scientists have considered the muon as a potential site for discovering new physics. The problem lies in measurements of the particle’s anomalous magnetic moment, or g-2, a property that describes the contribution of quantum mechanics to the particle’s oscillation in the presence of a magnetic field. The muon’s G-2 disagrees with the predictions of the Standard Model of particle physics, the foundational set of theories supporting physics for the past 50 years.
Unlike large experiments measuring g-2 through particle collisions, the team’s study “doesn’t need any experimental data. It just needs an activation of the underlying theory, which is QCD,” study co-author Zoltan Fodor, a theoretical particle physicist at UC San Diego, told Gizmodo. “You end up with what you see in our numbers today: that the result completely matches the experimental result.”
In other words, the team’s findings suggest that the apparent gap between the muon’s predicted anomalous magnetic moment and that predicted by the Standard Model is not as large as previous findings suggested.
Large experimental results suggest new physics
The muon’s anomalous magnetic moment was first measured at CERN in the 1960s, but the measurement was inaccurate. In 2006, the E821 experiment at Brookhaven National Laboratory published its final g-2 measurements of the muon, which differed from the Standard Model predictions by more than two standard deviations, swelling to a difference of more than three standard deviations after further calculations.
“Explaining the muon’s g-2 with new physics is not so easy,” Andreas Krivelin, a theoretical physicist at the University of Zurich and the Paul Scherer Institute, told Gizmodo. “It’s not something that comes out naturally; rather, you have to work to find a pattern that gives you significant effect.
The statistical milestone at which physicists believe a true discovery has been made—which indicates that the probability of the result occurring by chance according to the Standard Model is extremely small—is five standard deviations, or “five sigma.”
In 2021, the Muon g-2 Collaboration announced a measurement of the muon’s magnetic moment that was inconsistent with the Standard Model by 4.2 standard deviations. The gap between the figures widened after the Brookhaven result. But last year, experimental results from CMD-3, an accelerator in Russia, appeared to make the discrepancy between the numbers shrink. Two steps forward, one step back, depends how you look at it.
“This first-principles lattice calculation and the CMD-3 measurement agree and do not point to new physics,” Krivelin said. “I’m not hopeful that there really is a significant new physical effect in the muon’s g-2.”
Where does that leave us?
There are other ways to study the properties of the muon. In 2022, Gizmodo asked several physicists what the next big breakthrough in particle physics might be, given the relative quiet since the observation of the Higgs boson in 2012. One physicist suggested a muon collider – “if we have a problem with muons, let we use muons to find out,” they said.
Just last week, a different team of researchers published their analysis of a muon beam experiment that could pave the way for muon colliders in the future. But building a new collider can be expensive and time-consuming.
With existing experiments, more data is always useful, and retesting previous results in more precise ways can show whether the Standard Model continues to hold. Fermilab’s Muon g-2 experiment is expected to publish its final result next year. If past results are any indication, next year’s figure will be another data point in the muon saga, not its final chapter.
