Another nail may seal the coffin of the muon as the site of new physics. A team of physicists performed high-precision calculations of the muon’s properties in simulations and found that the particle’s properties are more consistent with the Standard Model than previously thought.
The team is called BMW Collaboration and its The research is currently hosted on the preprint server arXiv, which means it has not yet been published in a peer-reviewed journal. The team’s previous findings were published in nature In 2021, “weakening[ed] The long-standing difference between experiment and theory. In other words, their work brings experimental physics closer to theoretical predictions when it comes to our understanding of muons.
In the new paper, the team performed large-scale lattice quantum chromodynamics (QCD) simulations with a finer lattice than previous work, producing more precise calculations. Essentially, the team took QCD as input, placed a grid in space and time, and ran simulations. Their results predict an unusual magnetic moment for muons that differs by only 0.9 standard deviations from the experimental average measured for this property.
Muons and the Standard Model
The muon is an elementary particle with a mass approximately 207 times that of the electron. For about 20 years, scientists have considered muons as a potential site for discovering new physics. The problem lies in the measurement of the particle’s anomalous magnetic moment (g-2), a property that describes the quantum mechanical contribution to the particle’s wobble in the presence of a magnetic field. The muon’s G-2 disagrees with the predictions of the Standard Model of particle physics, the set of fundamental theories that have underpinned physics for the past 50 years.
Unlike large experiments that measure g-2 through particle collisions, the team’s study “doesn’t require any experimental input. It just requires activating the underlying theory, which is QCD,” said study co-author , a theoretical particle physicist at the University of California, San Diego. Zoltan Fodor told Gizmodo in a phone interview. “You end up with what you see today in our data: The results are completely consistent with the experimental results.”
In other words, the team’s results show that the apparent gap between the anomalous magnetic moments predicted by muons and those predicted by the Standard Model is not as large as previous findings had suggested.
Key experimental results reveal new physics
The European Organization for Nuclear Research (CERN) first measured the muon’s anomalous magnetic moment in the 1960s, but the results were not precise. In 2006, Brookhaven National Laboratory’s E821 experiment announced the final measurement results of muon g-2, which differed from the predictions of the Standard Model by more than two standard deviations. After subsequent calculations, the difference was more than three standard deviations.
Andreas Crivellin, a theoretical physicist at the University of Zurich and the Paul Scherrer Institute, told Gizmodo: “It’s not that easy to explain the muon’s g-2 with new physics. “It doesn’t happen naturally; you rather try to find a pattern that makes a huge difference to you.
The statistical milestone at which physicists believe a true discovery has been made—showing that the probability of a 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 magnetic moment that was 4.2 standard deviations from the Standard Model. The gap between the numbers has widened further since the Brookhaven results were released. But last year, experimental results from the Russian accelerator CMD-3 seemed to narrow the difference between the numbers. Two steps forward, one step back, it depends on how you look at it.
“The lattice first-principles calculations and the CMD-3 measurements are both consistent, but neither points to new physics,” Crivellin said. “I’m not convinced that the g-2 of the muon really has a sizable new physics effect. Too much hope.”
Where does this leave us?
There are other ways to explore the properties of muons. In 2022, Gizmodo asked several physicists what the next big breakthrough in particle physics might be, given the relative quiet since the Higgs boson was observed in 2012. Collider – “If we have a question about muons, let’s use a muon collider to find out,” they said.
Just last week, another research group published an analysis of a muon beam experiment that could pave the way for future muon colliders. But building a new collider can be expensive and time-consuming.
In existing experiments, more data is always useful, and retesting previous results with more precision can indicate whether the Standard Model continues to hold. Fermilab’s Muon g-2 experiment is expected to release final results next year. If previous results are any indication, next year’s data will be another data point in the muon saga, not its final chapter.
