Muon magnetism could hint at failure of Standard Model of physics

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A mysterious magnetic property of subatomic particles called muons suggests that new fundamental particles may remain hidden.

In a painstakingly precise experiment, the gyrations of muons in a magnetic field appear to defy the predictions of the Standard Model of particle physics, which describes particles and known fundamental forces. The result reinforces previous evidence that muons, the heavy parent of electrons, behave in unexpected ways.

“It’s a very big deal,” says theoretical physicist Bhupal Dev of Washington University in St. Louis. “It could be the long-awaited sign of a new physique we’ve all been hoping for.”

The poor behavior of muons could indicate the existence of new types of particles which modify the magnetic properties of muons. Muons behave like tiny magnets, each with a north pole and a south pole. The strength of this magnet is changed by transient quantum particles constantly moving in and out of existence, adjusting the muon’s magnetism by an amount known as the muon magnetic anomaly. Physicists can predict the magnitude of the magnetic anomaly by considering the contributions of all known particles. If fundamental particles are hiding, their additional effects on the magnetic anomaly could betray them.

Muons and electrons share a family resemblance, but muons are about 200 times more massive. This makes the muons more sensitive to the effects of hypothetical heavy particles. “The muon kind of hits the sweet spot,” says Aida El-Khadra of the University of Illinois at Urbana-Champaign.

To measure the magnetic subtleties of the muon, physicists projected billions of particles around the huge donut-shaped magnet of the Muon g-2 experiment at Fermilab in Batavia, Ill. (NS: 09/19/18). Inside this magnet, the orientation of the magnetic poles of the muons flickered or preceded. In particular, the rate of this precession deviated slightly from the expectations of the standard model, physicists report on April 7 in a virtual seminar, and in an article published in Physical examination letters.

“It’s a really complex experiment,” says Tsutomu Mibe of the KEK High Energy Accelerator Research Organization in Japan. “It’s a great job.”

To avoid bias, the team worked in self-imposed secrecy, keeping the final count hidden from itself while they analyzed the data. By the time the answer was finally revealed, says physicist Meghna Bhattacharya of the University of Mississippi at Oxford, “I had goose bumps.” The researchers found a muon magnetic anomaly of 0.00116592040, accurate to 46 millionths of a percent. The theoretical prediction sets the number at 0.00116591810. This discrepancy “hints at new physics,” Bhattacharya says.

A previous measurement of this type, from a 2001 experiment at the Brookhaven National Laboratory in Upton, NY, also seemed to disagree with theoretical predictions (NS: 02/15/01). When the new result is combined with the previous deviation, the measurement deviates from the prediction by a statistical measurement of 4.2 sigma – incredibly close to the typical five sigma benchmark for claiming a discovery. “We have to wait for more data from the Fermilab experiment to really be convinced that this is a real discovery, but it is becoming more and more interesting”, explains theoretical physicist Carlos Wagner of the University of Chicago .

According to quantum physics, muons are constantly emitting and absorbing particles in a frenzy that makes the theoretical calculations of the magnetic anomaly extremely complex. An international team of more than 170 physicists, co-led by El-Khadra, finalized the theoretical prediction in December 2020 at Physics reports.

Many physicists believe this theoretical prediction is sound and unlikely to change with further research. But some debates persist. Using a computational technique called a QCD lattice for a particularly thorny part of the computation gives an estimate that falls closer to the value measured experimentally, physicist Zoltan Fodor and his colleagues report on April 7 in Nature. If Fodor and colleagues’ calculation is correct, “it might change the way we view the experience,” says Fodor of Pennsylvania State University, which might make it easier to explain the results. experimental with the standard model. But he notes that his team’s prediction would need to be confirmed by further calculations before it is taken as seriously as the “gold standard” prediction.

As theoretical physicists continue to refine their predictions, experimental estimates will improve as well: muon g-2 (pronounced gee-minus-two) physicists have so far analyzed only a fraction of their data. . And Mibe and his colleagues are planning an experiment using a different technique at J-PARC, Japan’s proton accelerator research complex in Tokai, which will begin in 2025.

If the gap between experiment and prediction persists, scientists will need to find an explanation that goes beyond the Standard Model. Physicists already believe that the Standard Model cannot explain everything that exists: the universe seems to be permeated with invisible dark matter, for example, that the Standard Model particles cannot explain.

Some physicists believe that the explanation for the muon’s magnetic anomaly may be linked to known puzzles in particle physics. For example, a new particle could simultaneously explain dark matter and the Muon g-2 result. Or there may be a link with unexpected characteristics of some particle decays observed in the LHCb experiment at the CERN particle physics laboratory near Geneva (NS: 04/20/2017), recently reinforced by new results published on arXiv.org on March 22.

Measuring the g-2 muon will intensify this research, says Jason Crnkovic, a g-2 muon physicist at the University of Mississippi. “It’s an exciting result because it will generate a lot of conversations. “


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