UCL Mathematical & Physical Sciences


New measurement of particle wobble hints at new physics

10 August 2023

A new, ultraprecise measurement of the subatomic muon particle’s anomalous magnetic moment, conducted at US-based Fermilab and involving researchers from UCL, reinforces a discrepancy between theory and experiment that physicists can’t explain, potentially hinting at new physics.

The Muon g-2 ring sits in its detector hall amidst electronics racks, the muon beamline, and other equipment

The latest results, submitted to Physical Review Letters, reinforce previous measurements of the muon’s magnetic moment conducted by the Muon g-2 collaboration, the international research team operating the experiment. However, the measured strength differs from theoretical predictions of it, calculated from the Standard Model of particle physics, leading to hope that there may still be undiscovered new particles of forces affecting the results.

Dr Rebecca Chislett (UCL Physics & Astronomy), who led building and running the data acquisition system for the experiment, said: “These new, exciting results further reinforce our team’s previous precise measurements of the muon’s anomalous magnetic moment, reaching unprecedented accuracy in testing the Standard Model and probing deeper into the subatomic world.”    

Muons are negatively charged fundamental subatomic particles, similar to electrons but about 200 times as massive. Importantly, muons are also magnetic, and wobble as they spin in the presence of a powerful magnetic field. Their magnetic moment describes how strong their inherent magnets are, and how much a surrounding magnetic field causes the particles to wobble, or “precess.”

The muon’s magnetic moment as a function of the particle’s spin is represented by the letter g, and according to theory should be a little larger than 2. However, the newly announced measurements, which agree with previous experiments by the team, found the magnetic moment is stronger by about .2 parts per million, a small but significant amount.

To test the particles’ magnetic moment, researchers at the U.S. Department of Energy’s Fermi National Accelerator Laboratory fired beams of muons into a 15-metre-diameter, donut-shaped superconducting magnetic storage ring. As the muons circulate around the ring at nearly the speed of light, they interact with other subatomic particles that blink in and out of existence and alter their rate of precession.

The difference between the calculated and observed precession rate hints that there might be some as-yet undiscovered particle or force affecting the muons.

This most recent experimental run builds on previous work carried out by the team over the course of several years. This latest announcement adds two additional years of data to their initial year of data collection, released in 2021, more than quadrupling the total amount of data analysed. This extra data decreases the uncertainty of the initial measurement, ensuring it’s not just a statistical fluctuation. The team is still working to integrate three more years of data together for a conclusive and definitive measurement of the muon’s magnetic moment. The most recent results are about twice as precise of a measurement of the magnetic moment as the 2021 release.

Professor Gavin Hesketh (UCL Physics & Astronomy), g-2 lead at UCL, said: “This is an important update from an extremely difficult experiment, and the result gives us even more confidence in what we are seeing. Researchers from UCL have been essential in getting us this far, and we now focus on analysing the full dataset.”

In 2020, the Muon g-2 Theory Initiative released their calculations for what the magnetic moment should be based on the available data at the time. This most recent experimental result deviates from that theoretical prediction with a confidence level of 5 sigma, an important statistical milestone in physics that is usually the threshold for declaring a new discovery. However the researchers caution that work by the Theory Initiative is ongoing to incorporate new techniques, and likely when that is included it will result in a smaller and less significant discrepancy.

Professor Mark Lancaster of the University of Manchester, UK lead for the g-2 experiment and an ex-co-spokesperson of the experiment, said: “The precision of this measurement is an incredible achievement, made possible by the talent and ingenuity of many physicists and engineers, and particularly the young researchers.”

The international Muon g-2 collaboration brings together 181 collaborators from 33 institutions across seven countries, including the UK. Their effort replicates and improves upon a previous experiment at Brookhaven National Laboratory in New York, whose 2006 results were the first to suggest the muon’s behaviour differed from the Standard Model. The subsequent measurements at Fermilab reinforced this result with more certainty.

The collaboration expects to release their final, most precise result incorporating all of their collected data in 2025.

Scientists in the UK, funded by the Science and Technology Facilities Council, played a central 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.



  • The Muon g-2 ring sits in its detector hall amidst electronics racks, the muon beamline, and other equipment. The experiment operates at negative 450 degrees Fahrenheit and studies the precession (or wobble) of muons as they travel through the magnetic field. 
  • Image credit: Reidar Hahn / Fermilab

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Mike Lucibella

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