Muons spin as they circulate in this ring-shelf accelerator at Fermilab, just like race cars constantly turning out.
REIDAR HAHN / FERMILAB
A potential chink in physicists’ understanding of fundamental particles and forces now looks more real. New measurements confirm that a volatile subatomic particle called muon may be ever so slightly more magnetic than theory predicts, a team of more than 200 physicists reported this week. The small anomaly – only 2.5 parts of 1 billion – is a welcome threat to the prevailing theory of particle physicists, the standard model, which has long explained pretty much everything they have seen in atomic smashing, leaving them pining for something new to go over .
“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 Brookhaven National Laboratory, notes that the result is still not final. “It does nothing for our understanding of physics other than to say we’ll have to wait a little longer to see if that’s right.”
For decades, physicists have measured the magnetism of the muon, a heavier, unstable cousin of the electron that behaves like a small bar magnet. They place muons in a vertical magnetic field that causes them to twist horizontally like small compass needles. The frequency with which the muons spin reveals how magnetic they are, which in principle can point to new particles, even those that are too massive to be detonated in an atom smashing like Europe’s Large Hadron Collider.
This is because, thanks to quantum uncertainty, the muon sits in the middle of a fog of other particles and antiparticles flirting in and out of existence. These “virtual” particles can not be observed directly, but they can affect the properties of muons. Quantum mechanics and Albert Einstein’s theory of special relativity predict that the muon must have some basic magnetism. Known standard model particles fluttering around the muon increase magnetism by approx. 0.1%. And unknown particles lurking in the vacuum could add yet another unpredictable increase in change.
In 2001, researchers with the Muon g-2 experiment, then at Brookhaven, reported that muon was a touch more magnetic than the standard model predicts. The difference was only approx. 2.5 times the combined 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 reach.
Persistent anomaly
Two measurements find the same excess magnetism in the muon, perhs a hint of unknown new particles.
GRHIC: V. ALTOUNIAN /SCIENCE; Data: B. Abi et al., Phys. Priest Lett., 126, 141801 (2021)
So in 2013, the researchers pulled the experiment to the Fermi National Accelerator Laboratory (Fermilab) in Illinois, where they could get cleaner beams of muons. When the renewed experiment started taking data in 2018, the standard model predictions of muons magnetism had improved and the difference between the experimental results and the theory had increased to 3.7 times the total uncertainty.
Now the g-2 team has released the first result from the updated experiment using 1 year value of data. And the new result is almost exactly in line with the old one, the team announced today at a symposium at Fermilab. Consistency shows that the old result was neither a statistical fluke nor a product of some undetected error in the experiment, says Chris Polly, a Fermilab physicist and spokesman for the g-2 team. “Because I was a student at the Brookhaven experiment, it was definitely an overwhelming sense of relief for me,” he says.
Together, the new and old results extend the disagreement with the standard model prediction to 4.2 times the experimental and theoretical errors. It is still not quite enough to require a particular discovery. But in a field where similar hints of new physics come and go, magnetism in muon has remained an almost unambiguous puzzle, says Graham Kribs, a theorist at the University of Oregon. “There’s nothing else that really stands out that the whole community is like: ‘Remember, we have to deal with this too.'”
The entire g-2 team shared a moment of truth when the February 25 experiments first revealed the new result to themselves. The experiment involves measuring the speed at which the muons spin to exquisite precision. And to prevent them from subconsciously controlling the measurement to a value they prefer, experiments showed that a clock was ticking at a secret frequency that only two people knew, both outside the collaboration. Only at the end of the analysis did they open the envelopes that contained the secret frequency – at a Zoom meeting due to COVID-19 restrictions. “There was definitely this atmosphere of extreme excitement,” said Hannah Binney, a graduate student and team member from the University of Washington, Seattle. Within seconds, she says, researchers used the secret frequency to find out that the new result matched the old one.
The immediate response to the new result is likely to be twofold, Petrov said. First, with the experimental value confirmed, physicists are likely to question the theoretical estimate again. As of 2017, more than 130 theorists met in a series of workshops to hamper a consensus value for the standard model prediction, which they published in November 2020. But Petrov says the calculation is a complicated “hodgepodge” using a variety of methods – including extrusion from collision results – to account for different types of standard model particles that merge in and out of the vacuum. Theorists will now redouble their efforts to validate the consensus value and develop calculation methods that allow them to calculate it based on the first principles, Petrov says.
And of course, others are starting to put together new theories that would go beyond the standard model and explain muons extra magnetism. “This is going to be a field day for theorists,” Petrov predicts. Their considerations may be a bit premature, as g-2 experiments still take data and hope to reduce the experimental uncertainty by 75% within a few years. So the discrepancy could still fade. But with the chance that muon really signals the presence of something new, many theorists will be eager to get started.
