Experiment measuring particle wobble issues final result

The measurement is in perfect agreement with the experiment’s previous results, but unlike with earlier results, is not seen as contradicting the Standard Model of physics, the best model of matter we have, because of new theoretical predictions that reduce the discrepancy between the experimental results and theory.

However, the long-awaited value will be the world’s most precise measurement of the muon magnetic anomaly for many years to come.

Dr Rebecca Chislett, based at UCL’s Department of Physics & Astronomy, who led on building and running the data acquisition system for the experiment, said: “This result is an incredible achievement as it surpasses the precision the experiment aimed to achieve - something that is very difficult to do and involved the hard work of many different people across a wide range of expertise.

“The value itself is a stringent test of our best theory, the Standard Model, and has a rich history of furthering our understanding of the world around us. We look forward to future updates from our theoretical colleagues to compare to this amazing experimental result.” 

Professor Gavin Hesketh, based at UCL’s Department of Physics & Astronomy and the g-2 lead at UCL, said: "This result is hugely exciting - the most precise measurement ever made with a particle accelerator! Based on previous results it looked like we had uncovered a problem in the Standard Model, which would have been a major discovery, but this sparked a huge amount of work on the theory prediction, and the latest calculation now seems to agree with our measurement.

“So, perhaps not the end of the Standard Model just yet, but this measurement is still a landmark in particle physics, and as the final word from this experiment on g-2, it is a result that will not be beaten for many years to come. The work required to reach this level of precision is mind-blowing, and a testament to everyone involved with the experiment.” 

Muons are electrically 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.”

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 precession speed in a magnetic field depends on properties of the muon described by a number called the g-factor. Theoretical physicists calculate the g-factor based on the current knowledge of how the universe works at a fundamental level, which is contained in the Standard Model of particle physics.

Nearly 100 years ago, the value of g was predicted to be 2. But experimental measurements soon showed g to be slightly different from 2 by a quantity known as the magnetic anomaly of the muon.

Previous measurements taken at Brookhaven National Laboratory in the late 1990s and early 2000s showed a possible discrepancy with the theoretical calculation at that time.

When experiment doesn’t align with theory, it could indicate new physics. Specifically, physicists wondered if this discrepancy could be caused by as-yet undiscovered particles pulling at the muon’s precession.

So physicists decided to upgrade the Muon g-2 experiment to make a more precise measurement. In 2013, Brookhaven’s magnetic storage ring was transported from Long Island, New York, to Fermilab in Batavia, Illinois. After years of significant upgrades and improvements, the Fermilab Muon g-2 experiment started up on May 31, 2017.

In parallel, an international collaboration of theorists formed the Muon g-2 Theory Initiative to improve the theoretical calculation. In 2020, the Theory Initiative published an updated, more precise Standard Model value based on a technique that uses input data from other experiments.

The discrepancy with the result from that technique continued to grow in 2021 when Fermilab announced its first experimental result, confirming the Brookhaven result with a slightly improved precision. At the same time, a new theoretical prediction came out based on a second technique that heavily relies on computational power. This new number was closer to the experimental measurement, narrowing the discrepancy.

Recently, the Theory Initiative published a new prediction combining the results of several groups that used the new computational technique. This result remains closer to the experimental measurement, dampening the possibility of new physics. However, the theoretical effort will continue to work to understand the discrepancy between the data-driven and computational approaches.

The Muon g-2 collaboration describes the result in a paper that they submitted to Physical Review Letters.

A future experiment at the Japan Proton Accelerator Research Complex will likely make another measurement of the muon magnetic anomaly in the early 2030s, but, initially, they won’t achieve the same precision as Fermilab.

Meanwhile, the Theory Initiative will continue working to resolve the inconsistency between their two theoretical predictions.

The Muon g-2 collaboration is made up of nearly 176 scientists from 34 institutions in seven countries.

Unlike other high-energy physics experiments, Muon g-2 needed more than just high-energy physicists; the collaboration is also composed of accelerator physicists, atomic physicists and nuclear physicists.

While the experiment’s main analysis has come to an end, there is more to be mined from the six years of Muon g-2 data. In the future, the collaboration will produce measurements of a property of the muon called the electric dipole moment  as well as tests of a fundamental property of physical laws known as charge, parity, and time-reversal symmetry.

The Muon g-2 experiment is supported by the US's Department of Energy Office of Science under the offices of HEP, NP, and ASCR (US); National Science Foundation (US); Science and Technology Facilities Council (UK); Royal Society (UK); Istituto Nazionale di Fisica Nucleare (Italy); European Union’s Horizon 2020; National Natural Science Foundation of China; MSIP, NRF and IBS-R017-D1 (Republic of Korea); German Research Foundation (DFG); and Leverhulme Trust (UK).

Links

• The submitted paper 
• Professor Gavin Hesketh’s academic profile
• Dr Rebecca Chislett’s academic profile
• UCL Physics & Astronomy
• UCL Mathematical & Physical Sciences
• Fermilab

Image

• 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

Media contact

Mark Greaves

m.greaves [at] ucl.ac.uk

+44 (0)20 3108 9485