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One of the most ubiquitous subatomic particles in the universe, the muon, seems to be misbehaving.
Or at least, it isn’t behaving the way physicists expect. In fact, muons are deviating so much from what the laws of physics suggest that scientists are beginning to think their playbook is either incomplete, or there’s some force in the universe we don’t yet know about.
Muons are like fat electrons: They have a negative charge but are 207 times heavier than electrons. Thanks to their charge and a property known as spin, they act like tiny magnets. So when muons are immersed in another magnetic field, they experience an infinitesimal wobble.
But in a study released this week, physicists at the Fermilab in Illinois reported a discrepancy between how much muons should be wobbling and how much they actually did wobble during a lab experiment.
The difference is substantial enough that many scientists are convinced particles or forces we haven’t yet discovered must be involved. The finding, in other words, offers new evidence that something mysterious has played a role in shaping our universe — something that’s missing from the existing rules of physics.
“In this respect, the new measurement could indeed mark the start of a revolution of our understanding of nature,” Thomas Teubner, a theoretical physicist from the University of Liverpool and co-author of the new study, told Insider.
It’s possible that this unknown phenomenon is also linked to dark matter, the shadowy cousin of matter that was created just after the Big Bang and makes up a quarter of the universe.
When cosmic rays penetrate Earth’s atmosphere, they create muons. Several hundred muons strike your head every second. They can penetrate objects like an X-ray does — a few years ago, scientists used muons to discover a hidden chamber in Egypt’s Great Pyramid — but the particles only last for two-millionths of a second. After that, they decay into clusters of lighter particles.
During its brief existence, each muon remains oriented around a single point, in the same way a compass always points north. But when it encounters a magnetic field, a muon’s orientation shifts slightly away from that point. That crucial wobble, known as the g-factor, is what the Fermilab experiment is examining.
Fermilab is a US Department of Energy project with ties to the University of Chicago that’s devoted to the study of particle physics.
Scientists there can produce muons for study by running a beam of protons super quickly into metal using a particle accelerator. So the researchers behind the new study took these muons and funneled them inside a circular electromagnet 50 feet in diameter. The muons then traveled at nearly the speed of light around the circle more than 1,000 times.
When muons in the machine decay, ultra-sensitive detectors can measure which direction the resulting smaller particles are moving. Physicists can then use that information to calculate where each muon’s fixed point is.
It should be possible to calculate the precise amount muons will wobble using the Standard Model of physics, which encompasses everything we know about particles’ behavior. But the Fermilab team found that their muons’ wobble did not match those expectations.
Instead, it was off by one-third of one-millionth of a percent.
That difference may seem mind-bogglingly small, but Teubner said it’s actually “a milestone for particle physics.”
And it’s unlikely to be the result of error: The team found that there’s only a 1 in 40,000 chance the discrepancy in their measurement was due to random chance.
“This is strong evidence that the muon is sensitive to something that is not in our best theory,” Renee Fatemi, one of the Fermilab muon experiment managers, said in a press release.
This isn’t the first time muons have not behaved in the way science’s best theories would predict.
In 2001, the Brookhaven National Laboratory in New York ran a similar experiment using the same giant electromagnet. Those results also showed that muons’ wobble in the lab deviated from what it should have been. But those findings had a smaller statistical significance than Fermilab’s: There was a 1 in 1,000 chance it could have been a fluke.
Now, the Fermilab results confirm what Brookhaven physicists discovered 20 years ago — and that “has made the discrepancy which was already seen with the old result more intriguing,” Teubner said.
Fermilab is expected to release data from two more similar experiments within the next two years. A fourth experiment is also already underway, and fifth is in the works.
According to Teubner, it’s possible that some force that’s not in the Standard Model of physics could explain the muons’ whack-a-doo wobbles.
That force, he said, may also explain the existence of dark matter, and possibly even dark energy — which plays a key role in accelerating the expansion of the universe.
“Theorists would find it appealing to solve more than one problem at once,” Teubner said.
One hypothesis that could apply to both muons and dark matter, he added, is that muons and all other particles have almost identical partner particles that weakly interact with them. This concept is known as supersymmetry.
But Fermilab’s existing technologies aren’t sensitive enough to test that idea. Plus, Teubner added, it’s could be the case that the mysterious influence on muons isn’t linked to dark matter at all — which would mean the rules of physics are inadequate in more ways than one.