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Muon's magnetism could hint at a breakdown of the standard model of physics

A mysterious magnetic property of subatomic particles called muons suggests that new fundamental particles can be discovered.

In a meticulously accurate experiment, the rotations of muons within a magnetic field appear to challenge the predictions of the standard model of particle physics, which describes known particles and fundamental forces. The result strengthens the previous evidence that muons, the heavy relatives of electrons, behave unexpectedly.

“It’s a very big business,” says theoretical physicist Bhupal Dev of the University of Washington in St. Louis. "This could be the long-awaited sign of new physics we all look forward to."

The misbehavior of muons could point to the existence of new types of particles that alter the magnetic properties of muons. Muons behave like small magnets, each with a north and south pole. The force of that magnet is adjusted by transient quantum particles that constantly fly in and out of existence, adjusting the magnetism of the muon by an amount known as the magnetic anomaly of the muon. Physicists can predict the value of the magnetic anomaly by considering the contributions of all known particles. If fundamental particles are hidden, their additional effects on the magnetic anomaly could come off them.

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Muons and electrons have a familiar resemblance, but muons are about 200 times more massive. This makes muons more sensitive to the effects of hypothetical heavy particles. “The muon type gets to the right point,” says Aida El-Khadra of the University of Illinois at Urbana-Champaign.

To measure the magnetic subtleties of the muon, physicists released billions of particles around the huge screw – shaped magnet of the Muon g – 2 experiment at Fermilab in Batavia, Island. (SN: 19/9/18). Within that magnet, the orientation of the magnetic poles of the muons oscillated or precessed. Notably, the rate of this precession diverges slightly from the expectation of the standard model, as reported by physicists on April 7 in a virtual seminar and in an article published in Physical Review Letters.

“This is a really complex experiment,” says Tsutomu Mibe of the KEK High Energy Accelerator Research Organization in Japan. "This is an excellent job."

To avoid bias, the team worked under self-imposed secrecy, keeping the final number hidden from themselves while analyzing the data. By the time the answer was finally revealed, physicist Meghna Bhattacharya of the University of Mississippi at Oxford says, "She had goosebumps." The researchers found a magnetic muon anomaly of 0.00116592040, accurate to 46 millionths of a percent. The theoretical prediction sets the number at 0.00116591810. That discrepancy “hints at a new physics,” Bhattacharya says.

A previous measurement of this type, from an experiment conducted in 2001 at the Brookhaven National Laboratory in Upton, New York, also seemed to disagree with the theoretical predictions (SN: 15/02/01). When the new result is combined with the previous discrepancy, the measurement diverges from the prediction by a statistical measure of 4.2 sigma, tentatively approximated to the typical five-sigma reference to claim a discovery. “We have to wait for more data from the Fermilab experiment to be really convinced that this is a real discovery, but it’s getting more and more interesting,” says theoretical physicist Carlos Wagner of the University of Chicago.

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

Many physicists believe that this theoretical prediction is solid and is unlikely to change with further research. But some debate persists. Using a computational technique called QCD lattice for a particularly thorny part of the calculation gives an estimate that comes close to the experimentally measured value, physicist Zoltan Fodor and colleagues report on April 7 in Nature. If the calculation of Fodor and his colleagues is correct, “it could change the way we view the experiment,” says Fodor of Pennsylvania State University, perhaps facilitating the explanation of the experimental results with the standard model. But he notes that his team’s prediction would have to be confirmed by other calculations before it could be taken as seriously as the “gold standard” prediction.

While theoretical physicists continue to refine their predictions, experimental estimates will also improve: Muon g – 2 (pronounced gee-minus-two) so far physicists have only analyzed a fraction of their data. And Mibe and his colleagues are planning an experiment using a different technique at J-PARC, the Japan Accelerator Research Complex in Tokai, to begin in 2025.

If the discrepancy between the experiment and the prediction persists, scientists will have to find an explanation that goes beyond the standard model. Physicists already believe that the standard model cannot explain everything there is: the universe appears to be invaded by invisible dark matter, for example, that standard model particles cannot explain.

Some physicists speculate that the explanation for the muon's magnetic anomaly may be related to known puzzles in particle physics. For example, a new particle may simultaneously explain dark matter and the Muon g – 2 result. Or there may be a connection with unexpected features of certain particle decays observed in the LHCb experiment in the CERN particle physics laboratory near Geneva (SN : 20/04/17), recently reinforced by the new results published on on March 22nd.

The measurement of Muon g-2 will intensify such research, says Muon g-2 physicist Jason Crnkovic of the University of Mississippi. "This is an exciting result because it's going to generate a lot of conversations."

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