New Physics Just Dropped (Maybe)

Photo: Reidar Hahn/FermiLab via US Department of Energy

On Wednesday, researchers with Fermilab, located just west of Chicago, announced that a result 20 years in the making could upend physicists‘ understanding of how the universe works.

They released the first results of the Muon g-2 experiment, which since 2018 has measured a heavier cousin of an electron called the muon. Muons spin in a magnetic field, and other subatomic particles affect how they move. The stronger a magnetic field, the faster a muon wobbles. By observing the spin, scientists can measure how fast the muons are wobbling. But when the experiment was run, researchers found that the muons might be ever so slightly more magnetic than theory predicts. The anomaly is small — just 2.5 parts in 1 billion — but it may be enough to require an explanation for what’s causing the faster wobble in the form of entirely new elementary particles. If that happened, it would challenge the Standard Model of particle physics, a rulebook for how the universe works.

“My first impression is ‘Wow,’” Gordan Krnjaic, a theoretical physicist at Fermilab who was not involved in the research, told Scientific American. “It’s almost the best possible case scenario for speculators like us. I’m thinking much more that it’s possibly new physics, and it has implications for future experiments and for possible connections to dark matter.”

Muons were a curious complication to the otherwise limited cast of subatomic particles when they were discovered in the 1930s. (After they were found in cosmic rays, it so confounded scientists that a physicist famously asked, “Who ordered that?”) Indeed, research suggests there might be something strange about muons. Back in 2001, an earlier run of the Muon g-2 experiment at Brookhaven National Laboratory found a slight discrepancy between experiment and theory. But the experiment didn’t take enough data and fell short of the critical threshold required to claim discovery. And so, after 10 years, Fermilab decided to pick up the search. And this time, physicists would rerun the experiment with a stronger muon beam.

Physicists constructed a ring of magnets in the shape of a Hula-Hoop, 50 feet across and 6 feet high, designed to measure how particles wobble like a faltering spinning top in the presence of a magnetic field. Scientists blasted particles around a magnetized track that keeps them in existence long enough, tiny fractions of a second, for researchers to get a closer look at them.

There is currently a 1 in 40,000 chance that Fermilab’s result could simply be a fluke, equating to a statistical level of confidence described as 4.1 sigma. By the strict standards of particle physics, a level of 5 sigma, or a 1 in 3.5 million chance of the observation being a coincidence, is needed to claim a “discovery.”

Researchers who worked on the Muon g-2 experiment need another year or two to finish analyzing the results of all of the laps around the track. “So far, we’ve only analyzed 6% of the data, and when we combine the results of all of the runs, we’ll get an even better measurement,” Sudeshna Ganguly, associate scientist at Fermilab, told Gizmodo. “It’s super exciting to be a part of this.”


New Physics Just Dropped (Maybe)