Muons might not behave as expected, but scientists can’t agree on what to expect.

Physicists have precisely measured how muons, subatomic particles, wobble in a magnetic field, enhancing our understanding of the muon’s internal magnet. Researchers from the Muon g−2 experiment reported these findings on August 10 during a seminar at Fermilab in Batavia, Illinois.

Previous measurements of muon magnetism have conflicted with theoretical predictions based on the standard model of particle physics. This model is a cornerstone of modern physics, describing subatomic particles and the forces that govern them.

Many physicists hoped that the muon discrepancy might indicate a flaw in the standard model, potentially leading to new insights about the universe. However, recent scientific developments have complicated the theoretical predictions of the muon's magnetic strength, making it challenging to determine if the new measurements signal new physics or unresolved issues with the predictions.

Muons belong to the same particle family as electrons but are about 200 times more massive. These short-lived particles act like tiny magnets, each generating its own magnetic field. The strength of this magnetism is influenced by quantum physics, as the vacuum of space teems with transient particles known as "virtual" particles, which fleetingly exist and affect the muon’s magnetism. This adjustment, known as the anomalous magnetic moment or "g−2," is where the discrepancies arise.

Unknown particles could potentially alter the value of g−2 measured by scientists, generating excitement among physicists whenever deviations from the standard model's predictions are detected.

“The muons’ behavior that we’re measuring is affected by all of the forces and particles in the universe,” explains Brynn MacCoy, a Muon g−2 researcher and physicist at the University of Washington in Seattle. “It’s basically giving us this direct window into how the universe works.”