New data on tiny muon particle deepens mystery in physics

A U.S. research lab has announced one of the most precise measurements ever of how a subatomic particle behaves, teeing up a showdown that could either vindicate one of science’s most powerful theories, or reveal previously unseen particles and forces in the universe.

In a seminar, scientists at the Energy Department’s Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, unveiled their latest measurement of how a particle called the muon wobbles within a magnetic field. Their measurement – by far the world’s best – puts numbers on a key piece of this wobble down to the ninth digit.

Fermilab scientists are measuring the muon’s behavior to such extraordinary precision because the particle provides a powerful test of the Standard Model, the modern theory that describes how the universe’s known particles and forces work on a fundamental level.

For all its success, the Standard Model is jarringly incomplete. It offers no description of gravity, nor does it account for “dark matter,” the mysterious stuff that gravitationally outmuscles ordinary matter at cosmic scales. And according to the Standard Model, the universe ought to contain equal amounts of regular matter and its counterpart, antimatter. But those substances annihilate on contact, so if this were true, none of our atoms would exist.

“All we’re trying to do is try to identify solid evidence – you know, a pointer – for what these new types of interactions or particles are,” said Muon g-2 member Mark Lancaster, a physicist at the University of Manchester. “We know they have to exist, because if they didn’t exist, we wouldn’t be here.”

The latest data from Fermilab’s Muon g-2 experiment has been analyzed by an international team of more than 180 scientists. It backs up previous results seen as evidence that the muon could be disobeying the Standard Model. If such misbehavior were confirmed, it would provide a tantalizing, long-sought clue as to what lies beyond our current understanding.

However, as precise as Fermilab’s new measurement is, today isn’t a champagne moment for physicists seeking the unknown. To claim an ironclad deviation from the Standard Model, everyone has to agree on what that theory predicts, and in a dramatic twist, they no longer do.

A flurry of papers published since 2021 has unsettled the previous Standard Model prediction for the muon, leaving theorists with several gnarly discrepancies to resolve – and experimentalists ready to watch the show.

“I’m sort of, like, leaning back on my chair with my popcorn,” said Muon g-2 member Paolo Girotti, a postdoctoral researcher at Italy’s National Institute of Nuclear Physics.