Particle mystery deepens, as physicists confirm that the muon is more magnetic than predicted - Science Magazine

That small anomaly—just 2.5 parts in 1 billion—is a welcome threat to particle physicists’ prevailing theory, the standard model, which has long explained pretty much everything they’ve seen at atom smashers and left them pining for something new to puzzle over.

“Since the 1970s we’ve been looking for a crack in the standard model,” says Alexey Petrov, a theorist at Wayne State University.

For decades, physicists have measured the magnetism of the muon, a heavier, unstable cousin of the electron, which behaves like a tiny bar magnet.

That’s because, thanks to quantum uncertainty, the muon sits amid a haze of other particles and antiparticles flitting in and out of existence.

Quantum mechanics and Albert Einstein’s theory of special relativity predict the muon should have a certain basic magnetism.

Familiar standard model particles flitting about the muon increase that magnetism by about 0.1%.

In 2001, researchers with the Muon g-2 experiment, then at Brookhaven, reported that the muon was a touch more magnetic than the standard model predicts.

Two measurements find the same excess magnetism in the muon, perhaps a hint of unknown new particles.

By the time the revamped experiment started to take data in 2018, the standard model predictions of the muon’s magnetism had improved and the difference between the experimental results and theory had risen to 3.7 times the total uncertainty.

“Because I was a graduate student on the Brookhaven experiment, it was certainly an overwhelming sense of relief for me,” he says.

But in a field in which similar hints of new physics come and go, the magnetism of the muon has remained an almost singular puzzle, says Graham Kribs, a theorist at the University of Oregon.

The immediate responses to the new result will likely be twofold, Petrov says.

Starting in 2017, more than 130 theorists met in a series of workshops to hammer out a consensus value for the standard model prediction, which they published in November 2020.

But Petrov says the calculation is a complicated “hodgepodge” that employs a variety of methods—including extrapolating from collider results—to account for different types of standard model particles flitting in and out of the vacuum.

Theorists will now redouble their efforts to validate the consensus value and to develop computational methods that would enable them to calculate it from first principles, Petrov says.

And, of course, others will begin to concoct new theories that would go beyond the standard model and explain the muon’s extra magnetism.

“This is going to be a field day for theorists,” Petrov predicts.