The “wobble” of a tiny particle known as a muon is once again challenging our understanding of physics and could be the start of a major discovery, scientists have said.
For the third time, findings from experiments have shown this particle does not behave as predicted by the Standard Model – the rulebook physicists use to describe and understand how the universe works at the subatomic level.
Scientists said their latest results, which have been submitted to the journal Physical Review Letters, reinforce measurements of the muon’s wobble in previous experiments and are even more precise.
The findings, gathered by an international team of researchers known as the Muon g-2 collaboration, suggest there may still be undiscovered particles or forces of nature unknown to science that are affecting the results.
Professor Gavin Hesketh, of UCL Physics & Astronomy and the Muon g-2 lead at the university, said: “This is an important update from an extremely difficult experiment, and the result gives us even more confidence in what we are seeing.”
He added: “Whatever happens, this result will certainly feature in future textbooks, and it may be the start of a major discovery.”
Muons are fundamental particles similar to electrons that orbit an atom’s nucleus but significantly heavier – by more than 200 times.
They are unstable, existing for only two microseconds before decaying into other particles.
Muons also behave like magnets and wobble in the presence of a powerful magnetic field, like the axis of a spinning top.
The speed of this wobble is referred to as the particle’s “magnetic moment”.
To test muons’ magnetic moment, scientists at the US Department of Energy’s Fermi National Accelerator Laboratory in Chicago fired beams of muons into a 15-metre wide, doughnut-shaped ring with a powerful magnetic field.
Here muons move around the ring at nearly the speed of light, wobbling as they feel the force of the magnetic field.
The Standard Model predicts the magnetic moment of a muon should be a little larger than 2.
But previous experiments – conducted in the year 2000 and, more recently, in 2020 – showed these particles are more magnetic than physicists originally expected.
The latest ultra-precise measurements revealed on Thursday suggest a muon’s magnetic moment is stronger by about 0.2 parts per million, which the physicists describe as “a small but significant amount”.
The researchers said this might be caused by other subatomic particles that blink in and out of existence, altering the rate of wobble.
Brendan Casey, a senior scientist at Fermilab who has worked on the Muon g-2 experiment since 2008, said: “We’re really probing new territory.
“We’re determining the muon magnetic moment at a better precision than it has ever been seen before.”
This latest announcement adds two additional years of data to the first Fermilab results released in 2021.
It also reinforces the results from the experiments by the Brookhaven National Laboratory in New York from more than two decades ago, which was the first to hint that the muon’s behaviour disagreed with the Standard Model.
The researchers said a level of 5 sigma – the statistical milestone in physics needed for declaring a new discovery – has been achieved this time.
But a conclusive measurement of the muon’s magnetic moment will only be seen once scientists have analysed all the data spanning six years, with the final results expected in 2025.
It also means theoretical physicists will need to find a way to explain the disagreement between theory and experiment.
Prof Hesketh said: “Compared to the theory prediction from a couple of years ago, we have passed the ‘magic’ 5-sigma threshold for discovering something new.
“But there is a lot of work going on on the theory side, and it’s likely the prediction will move closer to our data.
“We’ve got more data to analyse, and our final result will stand as the most precise measurement of the muon’s magnetic moment for a long time to come.
“At the moment, it’s not easy to say what the result means.”
Dr Rebecca Chislett, also from UCL Physics & Astronomy, added: “Reaching this level of accuracy on the measurement is an incredible achievement and it is fantastic to see that all the work put in to fully understand every detail was worth it.
“I look forward to continuing to push the limits of our understanding and see how the theory and experiment evolve in the coming years leading up to the final result.”
Scientists at UCL built a key detector and developed software to analyse the Fermilab data, with funding provided by the Science and Technology Facilities Council.
Other UK institutions involved include the Universities of Manchester, Liverpool, Lancaster and the Cockcroft Accelerator Institute in Warrington.
Commenting on the result, Professor Timothy Gershon, of the University of Warwick’s Department of Physics, who was not involved, said: “I would say that this result appears to be an extremely impressive experimental achievement – it is extraordinarily challenging to reach the level of precision reported.
“The result firmly puts the ball back in the court of the theorists, who have their work cut out to make predictions for the value expected in the Standard Model with comparable precision.
“Only once that has been done, will we be able to say whether or not this measurement is discrepant with the Standard Model.
“If that turns out to be the case, it will point towards an extremely exciting future.”
