First results from Muon g-2 experiment strengthen evidence of new physics

The results confirm a discrepancy that has been gnawing at researchers for decades.

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Like an electron, the muon is 200 times more massive than occurs naturally when cosmic rays strike Earth’s atmosphere. Also, Fermilab can produce them in large numbers via particle accelerators.

While circulating in the Muon g-2 magnet, muons interact with a quantum foam of subatomic particles popping in and out of existence. Interactions with these short-lived particles influence the estimation of the g-factor, making the muons’ precession accelerate or hinder somewhat.

The Standard Model predicts this so-called anomalous magnetic moment exceptionally precisely. But if the quantum foam contains additional forces or particles not accounted for by the Standard Model, that would tweak the muon g-factor further.

In the first kind of Muon g-2 experiment, physicists found that muons are behaving completely differently. The investigation conducted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory suggests that muons behaving in a way that is not predicted by scientists’ best theory, the Standard Model of particle physics.

The results of this study affirm a disparity that has been troubling specialists for quite a long time.

Renee Fatemi, a physicist at the University of Kentucky and the simulations manager for the Muon g-2 experiment, said, “This quantity we measure reflects the muon’s interactions with everything else in the universe. But when the theorists calculate the same quantity, we don’t get the same answer using all of the known forces and particles in the Standard Model. This is strong evidence that the muon is sensitive to something that is not in our best theory.”

In 2001, scientists experimented at DOE’s Brookhaven National Laboratory, which predicted that the muon’s behavior disagreed with the Standard Model. This new experiment strongly agrees with Brookhaven’s value and diverges from theory with the most precise measurement to date.

The accepted theoretical values for the muon are: g-factor: 2.00233183620(86) anomalous magnetic moment: 0.00116591810(43) [uncertainty in parentheses]

The new experimental world-average results announced by the Muon g-2 collaboration today are: g-factor: 2.00233184122(82) anomalous magnetic moment: 0.00116592061(41)

The joined outcomes from Fermilab and Brookhaven show a difference with theory at a significance of 4.2 sigmas, somewhat short of the five sigma (or standard deviations) that scientists need to guarantee a disclosure yet time convincing proof of new physics. The possibility that the outcomes are a substantial variance is around 1 in 40,000.

In this experiment, scientists used a 50-foot-diameter superconducting magnetic storage ring. The Muon g-2 experiment sends a beam of muons into the storage ring, where they circulate thousands of times at nearly the speed of light. Detectors lining the ring allow scientists to determine how fast the muons are precessing.

In 2013, the magnetic storage rin was transported to the Chicago suburbs. Scientists could take advantage of Fermilab’s particle accelerator and produce the most intense beam of muons in the United States. Over the next four years, scientists assembled the experiment, tuned and calibrated an incredibly uniform magnetic field, developed new techniques, instrumentation, and simulations, and thoroughly tested the entire system.

In 2018, the Fermilab experiment collected more data than all previous muon g-factor experiments. With more than 200 scientists from 35 institutions in seven countries, the Muon g-2 collaboration has now finished analyzing the motion of more than 8 billion muons from that first run.

Fermilab scientist Chris Polly, a co-spokesperson for the current experiment and was a lead graduate student on the Brookhaven experiment, said, “After the 20 years that have passed since the Brookhaven experiment ended, it is so gratifying to be resolving this mystery finally.”

Scientists are now performing data analysis on the second and third runs of the experiment. The fourth run is ongoing, and a fifth run is planned. The data obtained by combining all results may give an even more precise measurement of the muon’s wobble, revealing with greater certainty whether new physics is hiding within the quantum foam.

Fermilab Deputy Director of Research Joe Lykken said“Pinning down the subtle behavior of muons is a remarkable achievement that will guide the search for physics beyond the Standard Model for years to come. This is an exciting time for particle physics research, and Fermilab is at the forefront.”

Journal Reference:
  1. B. Abi et al. Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm. DOI: 10.1103/PhysRevLett.126.141801