The Standard Model, scientists’ best description of the makeup and behavior of the Universe yet, very precisely predicts the g-factor of a fundamental particle called muon — a value that tells physicists how this particle behaves in a magnetic field. In the 1990s, the E821 experiment at Brookhaven National Laboratory indicated that g-2 differed from the theoretical prediction by a few parts per million. This miniscule difference hinted at the existence of unknown interactions between the muon and the magnetic field. The new results from the Muon g-2 experiment at Fermilab National Accelerator Laboratory strongly agree with Brookhaven’s, strengthening the evidence that there is new physics to discover. The combined results from the two experiments show a difference from the Standard Model at a significance of 4.2 sigma, slightly less than the 5 sigma that scientists require to claim a discovery.
The Muon g-2 ring sits in its detector hall amidst electronics racks, the muon beamline, and other equipment. Image credit: Reidar Hahn.
Discovered by Caltech physicists in 1936, a muon is about 200 times as massive as its cousin, the electron.
Muons occur naturally when cosmic rays strike Earth’s atmosphere, and particle accelerators can produce them in large numbers.
Like electrons, they act as if they have a tiny internal magnet. In a strong magnetic field, the direction of the muon’s magnet precesses are much like the axis of a spinning top or gyroscope.
The strength of the internal magnet determines the rate that the muon precesses in an external magnetic field and is described by a number that physicists call the g-factor. This number can be calculated with ultra-high precision.
“Today is an extraordinary day, long awaited not only by us but by the whole international physics community,” said Muon g-2 experiment co-spokesperson Dr. Graziano Venanzoni, a physicist at the Italian National Institute for Nuclear Physics.
“A large amount of credit goes to our young researchers who, with their talent, ideas and enthusiasm, have allowed us to achieve this incredible result.”
“This is an incredibly exciting result,” said Dr. Ran Hong, a postdoctoral researcher at Argonne National Laboratory.
“These findings could have major implications for future particle physics experiments and could lead to a stronger grasp on how the Universe works.”
This infographic lays out some of muon’s basic stats alongside fun facts. Image credit: Diana Brandonisio.
As the muons circulate in the Muon g-2 magnet, they also interact with a quantum foam of subatomic particles popping in and out of existence.
Interactions with these short-lived particles affect the value of the g-factor, causing the muons’ precession to speed up or slow down very slightly.
The Standard Model predicts this so-called anomalous magnetic moment extremely 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.
“This quantity we measure reflects the interactions of the muon with everything else in the Universe,” said Muon g-2 experiment’s simulations manager Dr. Renee Fatemi, a physicist at the University of Kentucky.
“But when the theorists calculate the same quantity, using all of the known forces and particles in the Standard Model, we don’t get the same answer.”
“This is strong evidence that the muon is sensitive to something that is not in our best theory.”
“So far we have analyzed less than 6% of the data that the experiment will eventually collect,” said Dr. Chris Polly, a physicist at Fermilab.
“Although these first results are telling us that there is an intriguing difference with the Standard Model, we will learn much more in the next couple of years.”
“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,” said Dr. Joe Lykken, deputy director of research at Fermilab.
“This is an exciting time for particle physics research, and Fermilab is at the forefront.”
The results appear in three papers in the journal Physical Review Letters, the journal Physical Review A, and the journal Physical Review D.
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B. Abi et al. (Muon g-2 Collaboration). 2021. Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm. Phys. Rev. Lett 126 (14): 141801; doi: 10.1103/PhysRevLett.126.141801
T. Albahri et al. (Muon g-2 Collaboration). 2021. Magnetic-field measurement and analysis for the Muon g-2 Experiment at Fermilab. Phys. Rev. A 103 (4): 042208; doi: 10.1103/PhysRevA.103.042208
T. Albahri et al. (Muon g-2 Collaboration). 2021. Measurement of the anomalous precession frequency of the muon in the Fermilab Muon g-2 Experiment. Phys. Rev. D 103 (7): 072002; doi: 10.1103/PhysRevD.103.072002
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