Particle mystery: physicists confirm the muon is more magnetic than predicted

Muons twirl as they circulate on this ring-formed accelerator at Fermilab, adore urge vehicles ad infinitum spinning out.


By Adrian Cho

A likely chink in physicists’ working out of fundamental particles and forces now looks extra exact. Unusual measurements ascertain a fleeting subatomic particle called the muon will be ever so a minute extra magnetic than belief predicts, a crew of additional than 200 physicists reported this week. That microscopic anomaly—appropriate 2.5 parts in 1 billion—is a welcome risk to particle physicists’ prevailing belief, the odd mannequin, which has long defined rather unheard of the whole lot they’ve viewed at atom smashers and left them pining for one thing fresh to puzzle over.

“On fable of the 1970s we’ve been purchasing for a crack in the odd mannequin,” says Alexey Petrov, a theorist at Wayne Utter University. “This is able to perhaps perhaps perhaps also be it.” However Sally Dawson, a theorist at Brookhaven National Laboratory, notes the final consequence is peaceful no longer definitive. “It does nothing for our working out of physics other than to advise we now delight in got to wait barely longer to sight whether it’s miles exact.”

For a long time, physicists delight in measured the magnetism of the muon, a heavier, unstable cousin of the electron, which behaves adore a minute bar magnet. They assign aside muons in a vertical mag­netic self-discipline that makes them twirl horizontally adore minute compass needles. The frequency at which the muons twirl unearths how magnetic they are, which in precept can expose fresh particles, even ones too wide to be blasted into existence at an atom smasher adore Europe’s Gargantuan Hadron Collider.

That’s because, due to quantum uncertainty, the muon sits amid a haze of alternative particles and antiparticles flitting in and out of existence. These “digital” particles can’t be seen at as soon as, nonetheless they’ll have an effect on the muon’s properties. Quantum mechanics and Albert Einstein’s belief of special relativity predict the muon must delight in a mosey odd magnetism. Familiar odd mannequin particles flitting in regards to the muon fabricate bigger that magnetism by about 0.1%. And unknown particles lurking in the vacuum could perhaps perhaps add another, unpredictable increment of alternate.

In 2001, researchers with the Muon g-2 experiment, then at Brookhaven, reported that the muon used to be a contact extra magnetic than the odd mannequin predicts. The discrepancy used to be fully about 2.5 situations the combined theoretical and experimental uncertainties. That’s nowhere approach physicists’ odd for claiming a discovery: 5 situations the whole uncertainty. However it indubitably used to be a spirited ticket of most fresh particles appropriate past their take.

Continual anomaly

Two measurements fetch the same extra magnetism in the muon, perhaps a ticket of unknown fresh particles.

–2 –1 2006 Moderate 2021 0 1 2 3 4 Distinction between measured and theoretical values (parts per billion) Uncertainty Favorite mannequin prediction

GRAPHIC: V. ALTOUNIAN/SCIENCE; Knowledge: B. Abi et al., Phys. Rev. Lett., 126, 141801 (2021)

So in 2013, researchers hauled the experiment to Fermi National Accelerator Laboratory (Fermilab) in Illinois, where they would perhaps perhaps perhaps obtain purer beams of muons. By the point the revamped experiment began to rob info in 2018, the odd mannequin predictions of the muon’s magnetism had improved and the distinction between the experimental results and belief had risen to three.7 situations the whole uncertainty.

Now, the g-2 crew has launched the predominant consequence from the revamped experiment, utilizing 1 one year’s worth of information. And the fresh consequence agrees almost exactly with the susceptible one, the crew launched this day at a symposium at Fermilab. The concordance presentations the susceptible consequence used to be neither a statistical fluke nor the fabricated from some undetected flaw in the experiment, says Chris Polly, a Fermilab physicist and co-spokesperson for the g-2 crew. “On fable of I used to be a graduate pupil on the Brookhaven experiment, it used to be absolutely an amazing sense of relief for me,” he says.

Collectively, the fresh and susceptible results widen the difference with the odd mannequin prediction to 4.2 situations the experimental and theoretical errors. That’s peaceful no longer rather adequate to advise a clear discovery. However in a self-discipline by which same hints of most fresh physics approach and mosey, the magnetism of the muon has remained a nearly singular puzzle, says Graham Kribs, a theorist on the University of Oregon. “There’s nothing else that’s if truth be told style of standing out for which the whole community is adore, ‘Be conscious, we furthermore must deal with this.’”

The total g-2 crew shared a 2nd of reality when, on 25 February, the experimenters first published the fresh consequence to themselves. The experiment entails measuring the price at which the muons twirl to comely precision. And to construct themselves from subconsciously steerage the dimension to a tag they’d gain, experimenters relied on a clock ticking at a secret frequency known to totally two other folks, each exterior the collaboration. Handiest on the very end of the diagnosis did they open the envelopes containing the secret frequency—on a Zoom meeting thanks to COVID-19 restrictions. “Positively there used to be this ambiance of vulgar stress,” says Hannah Binney, a graduate pupil and crew member from the University of Washington, Seattle. Interior seconds, she says, researchers susceptible the secret frequency to identify that the fresh consequence matched the susceptible one.

The instantaneous responses to the fresh consequence is recurrently twofold, Petrov says. First, with the experimental value confirmed, physicists will likely assign aside a question to the theoretical estimate anew. Starting up in 2017, extra than 130 theorists met in a series of workshops to hammer out a consensus value for the odd mannequin prediction, which they printed in November 2020. However Petrov says the calculation is a posh “hodgepodge” that employs rather a pair of ideas—together with extrapolating from collider results—to fable for rather a pair of sorts of odd mannequin particles flitting in and out of the vacuum. Theorists will now redouble their efforts to validate the consensus value and to fabricate computational ideas that will allow them to calculate it from first ideas, Petrov says.

And, obviously, others will talk in confidence to concoct fresh theories that will mosey past the odd mannequin and expose the muon’s additional magnetism. “Right here’s going to be a self-discipline day for theorists,” Petrov predicts. Their musings will be a tad untimely, as g-2 experimenters are peaceful taking info and hope to slice the experimental uncertainty by 75% inside a pair of years. So the discrepancy could perhaps perhaps peaceful move. However, on the probability that the muon is truly signaling the presence of one thing fresh, many theorists will be alive to to open up. 

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