By Hannah Pell

On April 7, 2021, the physicists at Fermi National Laboratory announced the first results of their muon g-2 experiment (“g minus 2”), suggesting that muons may behave in ways not predicted by the Standard Model still incomplete theory of fundamental particles and their interactions.

You can think of a muon as a tiny top. They act like they have an internal magnet that rotates in response to an applied magnetic field. The strength of the inner magnet of a muon is known as the “g-factor”, a dimensionless quantity that characterizes the magnetic moment and the angular momentum of a particle. The experiment is referred to as “g minus 2” because both the theoretical value and the new experimental average of the muon magnetic moment are slightly greater than 2. However, they are not equivalent; Although the difference between them is incredibly small, it has been observed to be abnormal.

Photo credit: Reidar Hahn / Fermilab.

The muon g-2 experiment involves sending a pion beam that circulates around a storage ring at almost the speed of light and decays into muons and muon neutrinos. Detectors measure how fast the muons move or “wobble”. Since muons are heavier than their electron cousins ​​(about 200 times as massive), muons can be a more useful probe for the new physics. However, they are extremely unstable and short-lived (muons exist for about two millionths of a second), and their interactions with a “quantum foam” of virtual particles appearing in and out of existence in such a short period of time affect the g out factor.

An anomalous value of the muon magnetic moment was first measured two decades ago at Brookhaven National Laboratory and offers an encouraging path to new physics beyond the Standard Model. (The first Muon g-2 experiment was carried out at CERN in 1959.) In 2013, the storage ring magnet was transported from Brookhaven to Fermilab (via a 3,200 mile roundabout) for a more accurate measurement (the results from Fermilab are 15% more accurate than the first results in Brookhaven). The statistical significance of the data from Fermilab combined with the data from Brookhaven is 4.2σ or standard deviations. Although the probability of these results is about 1 in 40,000, 4.2σ is below the 5σ standard required to be considered an official discovery.

Is the discrepancy due to a bug in the standard model? It is certainly possible; Physicists have long been aware of their shortcomings, particularly the lack of explanation of the theory for neutrino masses, gravity, or dark matter. But is it likely? It can be too early to tell.

“We’re just starting this experiment,” said Fermilab physicist and Muon g-2 co-spokesman Chris Polly during the press conference. “There will be a lot more data to come.” The Muon g-2 experiment has completed three runs of data collection periods (Run-1, Run-2, and Run-3), another is currently running (Run-4) and a fifth is planned (Run-5).

On the day of the announcement, several open access articles were published in Physical Review Letters, Physical Review A, and Physical Review D. Another manuscript appears in Physical Review Accelerators and Beams, but is available on arXiv.


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