One of many largest questions in particle physics is whether or not the sphere itself tells an incomplete image of the universe. At Fermilab, a US Division of Power facility in suburban Chicago, particle physicists try to resolve this identification disaster. There, members of the Muon g–2 (pronounced as “g minus 2”) Collaboration have been rigorously measuring a peculiar particle referred to as a muon. Final week, they released their up to date outcomes: the muon—a heavier, extra ephemeral counterpart of the electron—could also be below the affect of one thing unknown.
If correct, it’s an indication that the theories forming the inspiration of recent particle physics don’t inform the entire story. Or is it? Whereas the Collaboration’s scientists have been learning muons, theoretical researchers have been re-evaluating their numbers, leaving doubt whether or not such an error exists.
“Both manner, there’s one thing that’s not understood, and it must be resolved,” says Ian Bailey, a particle physicist at Lancaster College within the UK and a member of the Muon g–2 collaboration.
The tried and examined primary legislation of recent particle physics—what scientists name the Standard Model—enshrines the muon as certainly one of our universe’s basic constructing blocks. Muons, like electrons, are subatomic particles that carry adverse electrical cost; not like electrons, muons decay after a number of millionths of a second. Nonetheless, scientists readily encounter muons within the wild. Earth’s higher environment is laced with muon rain, spawned by high-energy cosmic rays placing our planet.
But when the muon doesn’t all the time appear to be physicists count on it to look, that could be a signal that the Normal Mannequin is incomplete, and a few hitherto unknown physics is at play. “The muon, it seems, is predicted to have extra sensitivity to the existence of latest physics than…the electron,” says Bailey.
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Additionally like electrons, muons spin like whirling tops, which creates a magnetic discipline. The titular g defines how shortly it spins. In isolation, a muon’s g has a price of two. In actuality, muons don’t exist in isolation. Even in a vacuum, muons are hounded by throngs of short-lived “digital particles” that pop out and in of quantum existence, influencing a muon’s spin.
The Normal Mannequin ought to account for these particles, too. However within the 2000s, scientists at Brookhaven Nationwide Laboratory measured g and located that it was subtly however considerably larger than the Normal Mannequin’s prediction. Maybe the Brookhaven scientists had gotten it unsuitable—or, maybe, the muon was on the mercy of particles or forces the Normal Mannequin doesn’t contemplate.
Breaking the Normal Mannequin could be one of many largest moments in particle physics historical past, and particle physicists don’t take such disruption calmly. The Brookhaven scientists moved their experiment to Fermilab in Illinois, the place they might benefit from a extra highly effective particle accelerator to mass-produce muons. In 2018, the Muon g–2 experiment started.
Three years later, the experimental collaboration released their first outcomes, suggesting that Brookhaven hadn’t made a mistake or seen an phantasm. The outcomes launched final week add information from two extra runs in 2018 and 2019, corroborating what was revealed in 2021 and enhancing its precision. Their noticed worth for g—round 2.0023—diverges from what concept would predict after the eighth decimal place.
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“We’ve received a real worth of the magnetic anomaly pinned down properly,” says Lawrence Gibbons, a particle physicist at Cornell College and a member of the Muon g–2 collaboration.
Had this outcome come out a number of years in the past, physicists may need heralded it as definitive proof of physics past the Normal Mannequin. However right this moment, it’s not so easy. Few affairs of the quantum world are easy, however the spanner in these quantum works is the truth that the Normal Mannequin’s prediction itself is blurry.
“There was a change coming from the idea aspect,” says Bailey.
Physicists suppose that the “digital particles” that pull at a muon’s g achieve this with completely different forces. Some particles yank with electromagnetism, whose affect is simple to calculate. Others achieve this through the sturdy nuclear pressure (whose results we primarily discover as a result of it holds particles collectively inside atomic nuclei). Computing the sturdy nuclear pressure’s affect is nightmarishly complicated, and theoretical particle physicists typically substituted information from previous experiments of their calculations.
Just lately, nevertheless, some teams of theorists have adopted a method referred to as “lattice quantum chromodynamics,” or lattice QCD, which permits them to crunch sturdy nuclear pressure numbers on computer systems. When scientists feed lattice QCD numbers into their g predictions, they produce a outcome that’s extra according to Muon g–2’s outcomes.
Including to the confusion is {that a} completely different particle experiment situated in Siberia—referred to as CMD-3—produced a outcome that additionally makes the Muon g–2 discrepancy disappear. “That one is an actual head scratcher,” says Gibbons.
The Muon g–2 Collaboration isn’t executed. Crunching by way of thrice as a lot information, collected between 2021 and 2023, stays on the collaboration’s to-do record. As soon as they analyze all that information, which can be prepared in 2025, physicists consider they’ll make their g minus 2 estimate twice as exact. Nevertheless it’s not clear whether or not this refinement would settle issues, as theoretical physicists race to replace their predictions. The query of whether or not or not muons actually are misbehaving stays an open one.
