By Hannah Pell
On 7 April 2021, physicists at Fermi National Laboratory announced the first results of their Muon g-2 (“g minus 2”) experiment, which have hinted that muons may behave in a way not predicted by the Standard Model, a self-consistent yet incomplete theory of fundamental particles and their interactions.
You can picture a muon like a tiny, spinning top; they act as if they have an internal magnet, twirling around in response to an applied magnetic field. The strength of a muon’s internal magnet is known as the “g-factor,” a dimensionless quantity characterizing the magnetic moment and angular momentum of a particle. The experiment is named “g minus 2” because both the theoretical value and 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 anomalous.
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| Image Credit: Reidar Hahn/Fermilab. |
The Muon g-2 experiment involves sending a beam of pions to circulate at nearly the speed of light around a storage ring, decaying into muons and muon neutrinos. Detectors measure how fast the muons are precessing, or “wobbling.” Because muons are heavier than their electron cousins (roughly 200 times more massive), muons can be a more useful probe for 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 that appear in and out of existence over such a brief timeframe affect the g-factor.
An anomalous value of the muon magnetic moment was first measured at Brookhaven National Laboratory two decades ago, offering an encouraging pathway to new physics beyond the Standard Model. (The first Muon g-2 experiment ever was constructed in 1959 at CERN). In 2013, the storage ring magnet was transported from Brookhaven to Fermilab (via a 3200 mile roundabout scenic route) in order to conduct a more precise measurement (Fermilab’s results are 15% more precise than the initial results obtained at Brookhaven). The statistical significance of Fermilab’s data combined with Brookhaven’s amounts to 4.2σ, or standard deviations. Although the odds of these results are about 1 in 40,000, 4.2σ falls short of the 5σ standard required to consider them an official discovery.
Is the discrepancy due to a flaw in the Standard Model? It’s certainly possible; physicists have been long aware of its shortcomings, particularly the theory’s lack of explanation for neutrino masses, gravity, or dark matter. But is it probable? It may be too early to tell.
“We’re really just getting started on this experiment,” said Fermilab physicist and Muon g-2 co-spokesperson Chris Polly during the press conference. “There’s much more data to come.” The Muon g-2 experiment has completed three runs of data-taking periods (Run-1, Run-2, and Run-3), an additional is currently ongoing (Run-4), and a fifth is planned (Run-5).
Several open-access articles were published on the day of the announcement in Physical Review Letters, Physical Review A, and Physical Review D, and one additional manuscript is forthcoming in Physical Review Accelerators and Beams, but is available on the arXiv.