A Crack in the Foundations of Particle Physics?

New measurements from Fermilab of a subatomic particle’s ‘wobble’ look like they challenge the Standard Model of Particle Physics. But the jury’s still out.

When Peter Higgs and François Englert were awarded the 2013 Nobel Prize in Physics for their theoretical predictions of the Higgs particle, it fittingly marked the end of a 40-year search for the final piece of the particle physics puzzle. CERN scientists had discovered the elusive Higgs particle just the previous year. Its existence in turn signalled the existence of the Higgs field, an invisible, undetectable energy field that pervades the universe and gives fundamental particles mass. The Standard Model of Particle Physics was complete.

But almost 10 years after this keystone discovery, Fermilab researchers might have put a spanner in the works. In 2021, they discovered a tiny but significant discrepancy between the predicted and measured magnetic moment or ‘wobble’ of muons; elementary particles similar to electrons but 207 times heavier. This result backs up a previous measurement in 2001 at the Brookhaven National Laboratory, which hinted that muons seem to wobble slightly faster than the Standard Model predicts.

Physics Beyond the Standard Model

The difference we are talking about is miniscule. An international consortium of 132 theoretical physicists calculated the muon’s gyromagnetic ratio or ‘g-factor’ – the number relating the particle’s magnetic moment to its other properties – to be 2.00233183620(86). The Fermilab result is 2.00233184122(82). Though the two values match to the seventh decimal place, if correct, the result suggests there is more physics to discover beyond the Standard Model.

Could the tiny discrepancy from theory just be a glitch, random noise or an accumulation of errors? The Muon g-2 experiment, conducted by more than 200 scientists from 35 institutions in seven countries, sends a beam of muons into a magnetised ring, where they circle at nearly the speed of light hundreds of times before they decay. Detectors lining the ring pick up the decay products, allowing scientists to precisely determine how fast the muons were precessing (i.e. spinning on their own axes) to figure out the magnetic moment.

Discrepancy With Theory

During its first run in 2018, the Fermilab experiment collected more data than all prior experiments combined, providing information on the motion of more than 8 billion muons. Bringing these results together, and factoring in all possible sources of error, the combined findings from Fermilab and Brookhaven showed a clear discrepancy with theory – a difference at a significance of 4.2 sigma. In other words, there’s a 1 in 40,000 probability that these results are just a statistical fluctuation.

But for the discrepancy to be officially acknowledged as a discovery, the chance that a random fluctuation caused the gap between theory and observation needs to drop to 1 in 3.5 million (5 sigma). Right now, Fermilab scientists are working to reach this standard. Currently analysing the second and third runs of the experiment, and with further runs ongoing, they might reach the 5 sigma level in 2023.

Solving the Riddle of the Standard Model

If they do, the result could be even more significant than the Higgs discovery. It would mean there is new physics to probe that might help explain some of the mysteries the Standard Model was never fit to solve. It would mean something is missing from our understanding of fundamental particle physics. It might even provide a path to a long-sought Theory of Everything.

For example, one overarching idea that could explain the Muon g-2 results is supersymmetry. In supersymmetric theories, each subatomic particle has a partner particle. Though attractive because their existence could explain another pernicious enigma – dark matter – no evidence has been found of supersymmetric particles yet, despite decades of searches. Other possible explanations include exotic light particles and multiple versions of the Higgs particle.

But there may be a more down-to-earth explanation for the discrepancy. Reported at the same time as the Muon g-2 results was a new discrepancy, between the established theoretical value of the g-factor and a new one. This new theoretical measurement is in line with the Fermilab muon measurements: in this interpretation of the Standard Model, theory and experiment match.

Calculating the Standard Model prediction for the g-factor is made challenging by the fact that ‘virtual particles’ constantly pop in and out of the empty space surrounding the muon, affecting its magnetic moment. Some of these virtual particles are hadrons (particles made of quarks), and it is this contribution from hadrons that is contentious.

Saving or Redrawing the Current Model

The established theoretical value of the muon’s g-factor included a hadron term translated from other particle collision experiments. The new g-factor value used supercomputers to accurately estimate the hadron term. If this new value is independently replicated by other groups to the same precision, the Standard Model of Particle Physics is likely saved. If the official value turns out to be correct, particle physicists might need to go back to the drawing board and build a new and more complete account of the fundamental inner workings of the universe.