Unusual muon behaviour hints at mysterious new particles and forces

Strange muon behaviour hints at mysterious new particles and forces

The Muon g−2 experimental tools at Fermilab in Illinois

Reidar Hahn

The unusual behaviour of a basic particle referred to as a muon might trace on the existence of unique particles and forces past the usual mannequin of physics. We now have had indicators of this anomaly earlier than, however a brand new set of measurements has elevated the chance that it’s actual.

Muons are electrically charged particles, so when they’re positioned in a magnetic discipline, they begin to spin. Physicists can measure the frequency of that spin due to a phenomenon referred to as precession, by which the spin axis of the particle wobbles barely, permitting them to make what they name a wiggle plot.


The frequency at which a muon rotates when uncovered to a magnetic discipline is set by its interactions with different particles and forces, represented by a quantity referred to as the g issue. Utilizing the commonplace mannequin of particle physics, researchers can predict what this quantity must be with excessive precision.

However in 2006, experimental outcomes from Brookhaven Nationwide Laboratory in New York began to diverge from these theoretical predictions – the muons have been spinning barely sooner than they must. The outcomes weren’t statistically important sufficient to show that the usual mannequin was incorrect, however they have been a trigger for concern.

Now, a brand new set of experiments at Fermilab in Illinois has corroborated the issues delivered to gentle by these previous outcomes. “We might have made an error at Brookhaven, however then Fermilab, which has a way more refined set-up, might have gotten a unique reply – they usually didn’t,” says William Morse at Brookhaven Nationwide Laboratory.

This anomaly in all probability arises from a quantum mechanical phenomenon referred to as digital particles. These are pairs consisting of 1 particle and its antimatter counterpart that pop into existence as a result of quantum fluctuations, earlier than vanishing once more moments later. Whereas they briefly exist, they’ll have an effect on the behaviour of actual particles, like muons.

As a result of these digital pairs are random and are available from space-time itself, they are often any sort of particle. Some is perhaps ones that we already know of – for example, an electron and its antimatter associate, a positron – however some is perhaps one thing extra unique. “It’s not simply the identified particles that pop out and in of existence, but additionally those which have but to be found,” says Joe Worth on the College of Liverpool, UK, a part of the Fermilab workforce.

The fashions we use to foretell the muon’s g issue solely embody the consequences anticipated from identified digital particles, although – so if our experiments battle with these fashions, it factors to the potential for different particles past the usual mannequin, and unusual forces to manipulate these particles as effectively.

The Fermilab outcomes come on the heels of an announcement that physicists on the CERN particle physics laboratory’s Giant Hadron Collider close to Geneva, Switzerland, have discovered one thing unusual happening with the way in which that muons decay. Worth says the 2 could also be associated. “Perhaps it’s the identical physics from a unique angle, or possibly it’s completely different physics.”

Just like the CERN measurements, there isn’t fairly sufficient information to show that there should be new particles and forces past the usual mannequin. Nevertheless, the Fermilab researchers have solely evaluated about one tenth of the information from their experiments thus far they usually proceed to gather extra, so Worth says they need to be capable to inform quickly if this anomaly is absolutely brought on by unique particles or is simply an artefact of statistical uncertainty. These extra measurements can also assist us slender down what kinds of unique particles might exist.

Journal reference: Bodily Overview Letters, DOI: 10.1103/PhysRevLett.126.141801

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