Another nail may seal the coffin of the muon as a venue for new physics. A team of physicists taking highly precise calculations of the muon’s properties in simulations found the particle’s properties more in line with the Standard Model than previously believed.
The team is called the BMW Collaboration and its research is currently hosted on the pre-print server arXiv, meaning it has not yet been published in a peer-reviewed journal. The team’s previous findings, published in Nature in 2021, “weaken[ed] the long-standing discrepancy between experiment and theory.” In other words, their work has brought experimental physics closer in line with theoretical predictions when it comes to our understanding of the muon.
In its new paper, the team performed large-scale lattice quantum chromodynamic (QCD) simulations on finer lattices than in its previous work, yielding a more precise calculation. In essence, the team took QCD as an input, put a grid on spacetime, and simulated it. Their results predicted an anomalous magnetic moment of the muon that was just 0.9 standard deviations off the experimental average for measurements of the property.
The muon and the Standard Model
The muon is an elementary particle about 207 times as massive as the electron. For about 20 years, scientists have considered the muon a potential venue for the discovery of new physics. The issue lies with measurements of the particle’s anomalous magnetic moment, or g-2, a property which describes quantum mechanics’ contribution to the particles’ wobble in the presence of a magnetic field. G-2 of the muon disagreed with predictions of the Standard Model of particle physics, the foundational set of theories undergirding physics for the last 50 years.
Unlike large experiments measuring g-2 through particle collisions, the team’s research “doesn’t need any experimental input. It just needs the activation of the underlying theory, which is QCD,” said study co-author Zoltan Fodor, a theoretical particle physicist at the University of California in San Diego, in a call with Gizmodo. “You end up with what you see on our figures today: that the result agrees completely with the experimental result.”
In other words, the team’s findings suggest the apparent gulf between the muon’s predicted anomalous magnetic moment and that predicted by the Standard Model is not as large as previous findings suggested.
Major experimental results suggested new physics
The anomalous magnetic moment of the muon was first measured at CERN in the 1960s, but the measurement was imprecise. In 2006, the E821 experiment at Brookhaven National Laboratory released its final measurements of g-2 of the muon, which differed from Standard Model predictions by more than two standard deviations, swelling to a difference of more than three standard deviations after subsequent calculations.
“Explaining g-2 of the muon with new physics is not that easy,” said Andreas Crivellin, a theoretical physicist at the University of Zurich and the Paul Scherrer Institute, in a call with Gizmodo. “It’s not something that comes out naturally; you rather have to work to find a model that gives you a sizable effect.”
The statistical milestone at which physicists believe a true discovery has been made—indicating that the probability of the result occurring by chance under 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 disagreed with the Standard Model by 4.2 standard deviations. The gulf between the figures widened since the Brookhaven result. But last year, experimental results from CMD-3, an accelerator in Russia, seemed to make the discrepancy between the figures shrink. Two steps forward, one step back, depending on how you look at it.
“This first principle calculation from the lattice and the CMD-3 measurement both agree and both don’t point towards new physics,” Crivellin said. “I am not very hopeful that there is really a sizable new physics effect in g-2 of the muon.”
Where does this leave us?
There are other ways of exploring the muon’s properties. 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’s use muons to find out,” they said.
Just last week, a different team of researchers published its 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-intensive.
With existing experiments, more data is always useful, and retesting previous results in more precise ways could indicate whether the Standard Model continues to hold up. Fermilab’s Muon g-2 experiment is expected to release its final result next year. If previous results are any indication, next year’s figure will be another data point in the muon’s saga, not its final chapter.
