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

Last modified on April 08, 2021

Muons twirl as they circulation on this ring-shaped accelerator at Fermilab, like flee autos perpetually spinning out.


By Adrian Cho

A probable chink in physicists’ understanding of important particles and forces now appears to be like further proper. Current measurements confirm a fleeting subatomic particle referred to as the muon might be ever so a diminutive bit further magnetic than concept predicts, a staff of extra than 200 physicists reported this week. That tiny anomaly—factual 2.5 elements in 1 billion—is a welcome risk to particle physicists’ prevailing concept, the typical mannequin, which has lengthy defined pretty primary all the items they’ve seen at atom smashers and left them pining for one factor new to puzzle over.

“As a results of 1970s we’ve been purchasing for a crack inside the typical mannequin,” says Alexey Petrov, a theorist at Wayne Speak College. “This might maybe even be it.” However Sally Dawson, a theorist at Brookhaven National Laboratory, notes the consequence is mute not definitive. “It does nothing for our understanding of physics diversified than to verbalize now we should wait a diminutive bit longer to seek for if it is proper.”

For a few years, physicists preserve measured the magnetism of the muon, a heavier, unstable cousin of the electron, which behaves like a diminutive bar magnet. They construct muons in a vertical magazine­netic space that makes them twirl horizontally like diminutive compass needles. The frequency at which the muons twirl finds how magnetic they're, which in perception can level to new particles, even ones too broad to be blasted into existence at an atom smasher like Europe’s Colossal Hadron Collider.

That’s as a result of, because of quantum uncertainty, the muon sits amid a haze of diversified particles and antiparticles flitting interior and outside of existence. These “digital” particles can’t be seen immediately, nonetheless they'll impact the muon’s properties. Quantum mechanics and Albert Einstein’s concept of particular relativity predict the muon will should preserve a certain long-established magnetism. Acquainted long-established mannequin particles flitting with reference to the muon fabricate greater that magnetism by about 0.1%. And unknown particles lurking inside the vacuum might maybe add one different, unpredictable increment of commerce.

In 2001, researchers with the Muon g-2 experiment, then at Brookhaven, reported that the muon was a splash further magnetic than the typical mannequin predicts. The discrepancy was prime about 2.5 circumstances the blended theoretical and experimental uncertainties. That’s nowhere advance physicists’ long-established for claiming a discovery: 5 circumstances the full uncertainty. However it was a interesting hint of recent particles factual earlier their clutch.

Persistent anomaly

Two measurements get hold of the a related extra magnetism inside the muon, seemingly a splash of unknown new particles.

–2 –1 2006 Average 2021 0 1 2 3 4 Inequity between measured and theoretical values (elements per billion) Uncertainty Same outdated mannequin prediction

GRAPHIC: V. ALTOUNIAN/SCIENCE; Information: 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, the set they will safe purer beams of muons. By the level the revamped experiment started to clutch information in 2018, the typical mannequin predictions of the muon’s magnetism had improved and the adaptation between the experimental outcomes and concept had risen to a few.7 circumstances the full uncertainty.

Now, the g-2 staff has launched the first consequence from the revamped experiment, the utilization of 1 yr’s value of recordsdata. And the new consequence agrees virtually precisely with the worn one, the staff introduced at the present time at a symposium at Fermilab. The concordance reveals the worn consequence was neither a statistical fluke nor the manufactured from some undetected flaw inside the experiment, says Chris Polly, a Fermilab physicist and co-spokesperson for the g-2 staff. “Because of I used to be a graduate pupil on the Brookhaven experiment, it was indubitably an superior sense of discount for me,” he says.

Together, the new and worn outcomes widen the incompatibility with the typical mannequin prediction to 4.2 circumstances the experimental and theoretical errors. That’s mute not pretty adequate to say a certain discovery. However in a space through which a related hints of recent physics come and sure, the magnetism of the muon has remained an virtually singular puzzle, says Graham Kribs, a theorist at the College of Oregon. “There’s nothing else that’s genuinely bag of standing out for which the whole neighborhood is like, ‘Keep in thoughts, we moreover should deal with this.’”

The basic g-2 staff shared a second of reality when, on 25 February, the experimenters first revealed the new consequence to themselves. The experiment entails measuring the sure at which the muons twirl to dazzling precision. And to take care of themselves from subconsciously steering the measurement to a value they’d clutch, experimenters relied on a clock ticking at a secret frequency identified to prime two people, each outdoors the collaboration. Handiest at the very discontinue of the evaluation did they start the envelopes containing the secret frequency—on a Zoom assembly because of COVID-19 restrictions. “No doubt there was this ambiance of vulgar stress,” says Hannah Binney, a graduate pupil and staff member from the College of Washington, Seattle. Within seconds, she says, researchers used the secret frequency to find out that the new consequence matched the worn one.

The speedy responses to the new consequence is in total twofold, Petrov says. First, with the experimental value confirmed, physicists will seemingly ask the theoretical estimate anew. Starting in 2017, further than 130 theorists met in a collection of workshops to hammer out a consensus value for the typical mannequin prediction, which they revealed in November 2020. However Petrov says the calculation is a flowery “hodgepodge” that employs a differ of methods—together with extrapolating from collider outcomes—to memoir for diversified types of long-established mannequin particles flitting interior and outside of the vacuum. Theorists will now redouble their efforts to validate the consensus value and to fabricate computational methods that might maybe allow them to calculate it from first ideas, Petrov says.

And, of route, others will originate as much as concoct new theories that might maybe sure earlier the typical mannequin and show the muon’s further magnetism. “

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