A fleeting subatomic particle may be exposing flaws in a major physics theory

One of the biggest questions in particle physics is whether the field itself tells an incomplete picture of the universe. At Fermilab, a US Department of Energy facility in suburban Chicago, particle physicists are trying to resolve this identity crisis. There, members of the Muon g–2 (pronounced as “g minus 2”) Collaboration have been carefully measuring a peculiar particle known as a muon. Last week, they released their updated results: the muon—a heavier, more ephemeral counterpart of the electron—may be under the influence of something unknown.

If accurate, it’s a sign that the theories forming the foundation of modern particle physics don’t tell the whole story. Or is it? While the Collaboration’s scientists have been studying muons, theoretical researchers have been re-evaluating their numbers, leaving doubt whether such an error exists.

“Either way, there’s something that’s not understood, and it needs to be resolved,” says Ian Bailey, a particle physicist at Lancaster University in the UK and a member of the Muon g–2 collaboration.

The tried and tested basic law of modern particle physics—what scientists call the Standard Model—enshrines the muon as one of our universe’s fundamental building blocks. Muons, like electrons, are subatomic particles that carry negative electrical charge; unlike electrons, muons decay after a few millionths of a second. Still, scientists readily encounter muons in the wild. Earth’s upper atmosphere is laced with muon rain, spawned by high-energy cosmic rays striking our planet. 

But if the muon doesn’t always look like physicists expect it to look, that is a sign that the Standard Model is incomplete, and some hitherto unknown physics is at play. “The muon, it turns out, is predicted to have more sensitivity to the existence of new physics than…the electron,” says Bailey.

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Also like electrons, muons spin like whirling tops, which creates a magnetic field. The titular g defines how quickly it spins. In isolation, a muon’s g has a value of 2. In reality, muons don’t exist in isolation. Even in a vacuum, muons are hounded by throngs of short-lived “virtual particles” that pop in and out of quantum existence, influencing a muon’s spin.

The Standard Model should account for these particles, too. But in the 2000s, scientists at Brookhaven National Laboratory measured g and found that it was subtly but significantly greater than the Standard Model’s prediction. Perhaps the Brookhaven scientists had gotten it wrong—or, perhaps, the muon was at the mercy of particles or forces the Standard Model doesn’t consider.

Breaking the Standard Model would be one of the biggest moments in particle physics history, and particle physicists don’t take such disruption lightly. The Brookhaven scientists  moved their experiment to Fermilab in Illinois, where they could take advantage of a more powerful particle accelerator to mass-produce muons. In 2018, the Muon g–2 experiment began. 

Three years later, the experimental collaboration released their first results, suggesting that Brookhaven hadn’t made a mistake or seen an illusion. The results released last week add data from two additional runs in 2018 and 2019, corroborating what was published in 2021 and improving its precision. Their observed value for g—around 2.0023—diverges from what theory would predict after the eighth decimal place.

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“We’ve got a true value of the magnetic anomaly pinned down nicely,” says Lawrence Gibbons, a particle physicist at Cornell University and a member of the Muon g–2 collaboration.

Had this result come out several years ago, physicists might have heralded it as definitive proof of physics beyond the Standard Model. But today, it’s not so straightforward. Few affairs of the quantum world are simple, but the spanner in these quantum works is the fact that the Standard Model’s prediction itself is blurry.

“There has been a change coming from the theory side,” says Bailey.

Physicists think that the “virtual particles” that pull at a muon’s g do so with different forces. Some particles yank with electromagnetism, whose influence is easy to calculate. Others do so via the strong nuclear force (whose effects we mainly notice because it holds particles together inside atomic nuclei). Computing the strong nuclear force’s influence is nightmarishly complex, and theoretical particle physicists often substituted data from past experiments in their calculations. 

Recently, however, some groups of theorists have adopted a technique known as “lattice quantum chromodynamics,” or lattice QCD, which allows them to crunch strong nuclear force numbers on computers. When scientists feed lattice QCD numbers into their g predictions, they produce a result that’s more in line with Muon g–2’s results.

Adding to the confusion is that a different particle experiment located in Siberia—known as CMD-3—produced a result that also makes the Muon g–2 discrepancy disappear. “That one is a real head scratcher,” says Gibbons.

The Muon g–2 Collaboration isn’t done. Crunching through three times as much data, collected between 2021 and 2023, remains on the collaboration’s to-do list. Once they analyze all that data, which may be ready in 2025, physicists believe they can make their g minus 2 estimate twice as precise. But it’s not clear whether this refinement would settle things, as theoretical physicists race to update their predictions. The question of whether or not muons really are misbehaving remains an open one.

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