Particle mystery deepens as physicists confirm that the muon is more magnetic than predicted Science

By Adrian Cho7 April 2021, 11:00

A potential sound in physicists’ understanding of fundamental particles and forces now seems more real. New measurements confirm that a volatile subatomic particle called the muon could ever be as magnetic as the theory predicts, a team of more than 200 physicists reported this week. That small deviation – only 2.5 parts in a billion – is a welcome threat to the prevailing theory of particle physicists, the standard model, which for a long time explains everything they have seen in atomic reports and makes them lean towards something new. to puzzles.

“Since the 1970s, we’re been looking for a crack in the standard model,” says Alexey Petrov, a theorist at Wayne State University. “It could be that.” But Sally Dawson, a theorist at the Brookhaven National Laboratory, notes that the result is not yet definitive. “It does nothing for our understanding of physics other than to say we have to wait a little longer to see if it’s real.”

Physicists have been measuring the magnetism of the muon for decades, a heavier, unstable cousin of the electron, which behaves like a small bar magnet. They place muons in a vertical magnetic field that causes them to rotate horizontally like compass needles. The frequency at which the muzzles rotate reveals how magnetic they are, which in principle may indicate new particles, even too massive to be inflated by an atomic breaker such as the Great Hadron Collider of Europe.

This is because the muon, thanks to quantum uncertainty, sits in the middle of a haze of other particles and antiparticles that flutter in and out. These ‘virtual’ particles can not be observed directly, but they can affect the muon’s properties. Quantum mechanics and Albert Einstein’s theory of special relativity predict that the muon must have a certain basic magnetism. Known standard model particles fluttering around the muon increase the magnetism by about 0.1%. And unknown particles lurking in the vacuum can give another unpredictable increase in change.

In 2001, researchers using the Muon g-2 experiment, then in Brookhaven, reported that the muon was a touch more magnetic than the standard model predicted. The difference was only about 2.5 times the theoretical and experimental uncertainty. It is nowhere near the standard of physicists to claim a discovery: five times the total uncertainty. But it was a tantalizing hint of new particles just out of their grasp.

Persistent anomaly

–2 –1 2006 Average 2021 0 1 2 3 4 Difference between measured and theoretical values ​​(parts per billion) Insecurity Standard model prediction

So in 2013, researchers dragged the experiment to Fermi National Accelerator Laboratory (Fermilab) in Illinois, where they could get pure beam muuns. By the time the refurbished experiment started taking data in 2018, the standard model predictions of the muzzle magnetism had improved and the difference between the experimental results and theory increased to 3.7 times the total uncertainty.

Now the g-2 team has announced the first result of the refurbished experiment using 1 year of data. And the new result almost exactly matches the old one, the team announced today at a symposium on Fermilab. The agreement shows that the old result was not a statistical luck nor was it the result of an unmarked error in the experiment, says Chris Polly, a physicist from Fermilab and co-spokesperson for the g-2 team. . “Because I was a graduate student at the Brookhaven experiment, it was definitely an overwhelming sense of relief for me,” he says.

Together, the new and old results increase the disagreement with the standard prediction to 4.2 times the experimental and theoretical errors. It is still not quite enough to claim a definite discovery. But in a field where similar tips of new physics come and go, the muzzle magnetism has remained an almost simple mystery, says Graham Kribs, a theorist at the University of Oregon. “There is nothing else that really stands out for what the whole community looks like: ‘Remember, we have to deal with it too.’ ‘

The entire g-2 team shared a moment of truth when the experiments first revealed the new result to themselves on February 25th. The experiment involves measuring the rate at which the muons rotate to excellent precision. And to refrain from sending the measurement subconsciously to a value they prefer, experiments rely on a clock that taps on a secret frequency that only two people know, both outside the collaboration. Only at the very end of the analysis did they open the envelopes with the secret frequency – at a Zoom meeting due to COVID-19 restrictions. “There was definitely this atmosphere of extreme tension,” says Hannah Binney, a graduate student and team member from the University of Washington, Seattle. According to her, researchers used the secret frequency within seconds to find out that the new result matched the old one.

The immediate reaction to the new result is likely to be twofold, says Petrov. First, with the experimental value confirmed, physicists are likely to question the theoretical estimate anew. As of 2017, more than 130 theorists have met in a series of workshops to hammer out a consensus value for the standard model forecast, which they published in November 2020. But Petrov says the calculation is a complicated ‘mix’ that uses different methods. —Including extrapolation of the collision results – to account for different types of standard model particles fluttering in and out of the vacuum. Theorists will now double their efforts to validate the consensus value and develop calculation methods that enable them to calculate it from the first principles, Petrov says.

And of course, other new theories will begin to emerge that go beyond the standard model and will explain the extra magnetism of the muon. “This is going to be a field day for theorists,” Petrov predicts. Their speculation may still be premature, as g-2 experiments are still taking data and hoping to reduce experimental uncertainty by 75% within a few years. The contradiction can therefore still fade. But, if the muon really indicates the presence of something new, many theorists will be eager to get started.