Strange muon behaviour hints at mysterious new particles and forces

physics experiment

The Muon g−2 experimental equipment at Fermilab in Illinois

Reidar Hahn

The strange behaviour of a fundamental particle called a muon may hint at the existence of exotic particles and forces beyond the standard model of physics. We have had signs of this anomaly before, but a new set of measurements has increased the likelihood that it is real.

Muons are electrically charged particles, so when they are placed in a magnetic field, they start to spin. Physicists can measure the frequency of that spin because of a phenomenon called precession, in which the spin axis of the particle wobbles slightly, allowing them to make what they call a wiggle plot.


The frequency at which a muon rotates when exposed to a magnetic field is determined by its interactions with other particles and forces, represented by a number called the g factor. Using the standard model of particle physics, researchers can predict what this number ought to be with extreme precision.

But in 2006, experimental results from Brookhaven National Laboratory in New York started to diverge from those theoretical predictions – the muons were spinning slightly faster than they ought to. The results weren’t statistically significant enough to prove that the standard model was wrong, but they were a cause for concern.

Now, a new set of experiments at Fermilab in Illinois has corroborated the concerns brought to light by those past results. “We could have made an error at Brookhaven, but then Fermilab, which has a much more sophisticated set-up, could have gotten a different answer – and they didn’t,” says William Morse at Brookhaven National Laboratory.

This anomaly probably arises from a quantum mechanical phenomenon called virtual particles. These are pairs consisting of one particle and its antimatter counterpart that pop into existence due to quantum fluctuations, before vanishing again moments later. While they briefly exist, they can affect the behaviour of real particles, like muons.

Because these virtual pairs are random and come from space-time itself, they can be any type of particle. Some might be the common sort that we know of – for instance, an electron and its antimatter partner, a positron – but some might be something more exotic. “It’s not just the known particles that pop in and out of existence, but also the ones that have yet to be discovered,” says Joe Price at the University of Liverpool, UK, part of the Fermilab team.

The models we use to predict the muon’s g factor only include the effects expected from known virtual particles, though – so if our experiments conflict with those models, it points to the possibility of other particles beyond the standard model, and strange forces to govern those particles as well.

The Fermilab results come on the heels of an announcement that physicists at the CERN particle physics laboratory’s Large Hadron Collider near Geneva, Switzerland, have found something strange going on with the way that muons decay. Price says the two may be related. “Maybe it’s the same physics from a different angle, or maybe it’s different physics.”

Like the CERN measurements, there isn’t quite enough data to prove that there must be new particles and forces beyond the standard model. However, the Fermilab researchers have only evaluated about one tenth of the data from their experiments so far and they continue to collect more, so Price says they should be able to tell soon if this anomaly is really caused by exotic particles or just an artefact of statistical uncertainty. Those additional measurements may also help us narrow down what sorts of exotic particles could exist.

Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.126.141801

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