UK Physics team helps achieve landmark final result in international Muon g-2 experiment

LEXINGTON, Ky. (June 3, 2025) — A team of faculty, postdoctoral scholars and students from the University of Kentucky Department of Physics and Astronomy in the College of Arts and Sciences has contributed to the groundbreaking final result in the ongoing Muon g-2 experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory (Fermilab).

Supported by the U.S. National Science Foundation (NSF), the results, announced today, reveal the most precise measurement to date involving subatomic particles called muons. Muons are fundamental particles that are similar to electrons, but about 200 times as massive. Like electrons, muons are magnetic, and in the presence of a magnetic field, precess or wobble like a spinning top. The precession rate in a given magnetic field depends on the muon magnetic moment, typically represented by the symbol g. At the simplest level, the value of g is 2. However, a small (0.1%) correction to g=2 arises from so-called vacuum fluctuations, a quantum foam of virtual particles that fills space.  All particles of nature, both known and unknown, contribute to vacuum fluctuations and measuring g-2 represents an important test for theoretical models. 

Today’s long-awaited result is a tremendous achievement of precision, expected to remain the world’s most precise measurement of the muon magnetic anomaly for many years to come.

For the last five years, the UK team has played an integral role among an international collaboration of scientists exploring this uncharted territory in search of new physics. Led by UK professors Renee Fatemi and Tim Gorringe, this final result agrees with published results from 2021 and 2023 but with a much higher precision of 127 parts per billion — surpassing the original experimental design goal of 140 parts per billion.

“It is incredibly satisfying to have exceeded the ambitious goal we set for ourselves, over a decade ago, to measure the anomalous magnetic moment of the muon to better than 140 parts per billion.” Fatemi said.

Read the full press release from Fermilab at https://news.fnal.gov/2025/06/muon-g-2-most-precise-measurement-of-muon-magnetic-anomaly/.

The UK team has been instrumental in efforts related to the data acquisition system, the precession frequency analysis, the simulation development and the beam-dynamics analysis. Fatemi and Gorringe say UK’s contribution to the project could not have been possible without the support of their team of postdoctoral scholars and students.

“Their energy, enthusiasm and inventiveness were driving forces through the many years and the many ups and downs on the path to our result,” Gorringe said. “It’s both a very happy moment in achieving our goals and a rather sad moment in wrapping up our collaboration.” 

Sean Foster, Ph.D., a postdoctoral scholar in the department, studied important systematic effects related to the muon beam in the superconducting storage ring.

“Understanding the complex motion of the muons was critical for us to reach our target precision goal,” Foster said. “The important thing here is that we need to understand our experiment exquisitely well — and we have — to reach the precision we are reporting today.”

“It has been a joy to collaborate with Professor Renee Fatemi and Professor Tim Gorringe on the challenges that this measurement has sent our way,” said Foster. “To have reached this stage — the final and most precise measurement from our experiment — is bittersweet: we have reached our goal, but now we are done working together. But that’s okay. This measurement will stand as the most precise measurement of the muon magnetic anomaly for years to come. To be part of this effort has been both a humbling and rewarding experience.”

Alec Tewsley-Booth, Ph.D., also a postdoctoral fellow at UK, coordinated the analyses for determining the proton precession frequency that determines the magnetic field in the superconducting storage ring.

“This experiment has been absolutely amazing to work on,” Tewsley-Booth said. “I’ve been a part of it since the magnet first arrived at Fermilab via barge and truck, and now I've seen the final runs published. There’s something a little bittersweet about it winding down — this collaboration brought together so many fascinating people, who jointly developed an astonishing level of technical expertise.”

Joey Peck, a UK graduate student, helped determine muon precision frequency via a new, UK-developed method called the Q-Method. Instead of tracking each positron and measuring its energy, this technique looked at the total energy deposited in the detector system over time. This method comes with its own set of systematic uncertainties, providing an important independent cross-check of the traditional approach.

“Being part of the team that pushed the precision of this result to the limits of our experimental capabilities was incredibly rewarding,” Peck said. “Contributing to a result that deepens our understanding of the universe is exactly why I became an experimental physicist.”

The team also included an undergraduate, Caleb West, who is a physics and computer science senior and member of the Lewis Honors College. West worked on simulations that helped the team understand the distributions they measured in tracker detectors that reconstructed the motion of the beam.

“It was very exciting to me to be part of such a huge experiment as an undergrad.” West said. “I remember reading about g-2 when I was in high school and thinking about how neat it would be to work on a similar experiment. I had never imagined I would be able to work on the real thing. Even though I mostly worked on side projects relating to the experiment, I still had a lot of fun working with Dr. Fatemi and Sean on this and learned a lot about particle physics and simulating experiments that will be very useful later in my career.”

The Muon g-2 collaboration is made up of nearly 176 scientists from 34 institutions in seven countries. The final result was submitted to Physical Review Letters.

Research reported in this publication was supported by the U.S. National Science Foundation under Award Numbers 2110479, 2110293 and 1714014. The opinions, findings and conclusions or recommendations expressed are those of the author(s) and do not necessarily reflect the views of the U.S. National Science Foundation.

The Muon g-2 experiment is supported by the Department of Energy Office of Science under the offices of HEP, NP and ASCR (US); U.S. National Science Foundation (US); Istituto Nazionale di Fisica Nucleare (Italy); Science and Technology Facilities Council (UK); Royal Society (UK); 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).