Particle’s ‘wobble’ hints at new physics
The “wobble”, or rate of precession, of the muon particle in a magnetic field is different from what our best theoretical model of the subatomic world would predict, according to an experiment involving UCL researchers that strengthens evidence for new, unknown physics. The Muon g-2 experiment, carried out at the Fermi National Accelerator Laboratory in the United States, measured with unprecedented precision the rate at which the muon “wobbled” (precessed) as it circulated a 15-metre magnetic ring at nearly the speed of light.
When placed in a magnetic field, the particle acts like a tiny magnetic compass and, like the axis of a spinning top, it precesses, or rotates, as its spins. This rotation was faster than is predicted by the Standard Model of particle physics. The muon is 200 times more massive than its cousin, the electron. It is unstable and decays in a few millionths of a second. As muons circulate in the Muon g-2 experiment, they interact with subatomic particles popping in and out of existence. Interactions with these short-lived particles cause the muons’ precession to speed up or slow down very slightly. A different rate of precession from what the Standard Model predicts suggests additional forces or particles that the model does not account for.
The international Muon g-2 experiment, involving dozens of labs and universities in seven countries including the UK, aimed to replicate a previous experiment at the US’s Brookhaven National Laboratory, whose results, published in 2006, suggested the muon’s behaviour deviated from the Standard Model. The latest measurement confirmed this result with more certainty. Combined with other developments including the LHCb result last week, it could herald a new era for physics.
Scientists in the UK, funded by the Science and Technology Facilities Council, played a vital role in the experiment, with teams at UCL building a key detector and developing software to analyse the data. Other UK institutions involved include the Universities of Manchester, Liverpool, Lancaster and the Cockcroft Accelerator Institute.