David Kawall, professor of physics at the University of Massachusetts Amherst, is among the researchers honored with this year’s Breakthrough Prize in Fundamental Physics for their work making measurements of the properties of the muon in a series of increasingly precise experiments, known as the Muon g-2 Collaborations, first conducted by a team at CERN (the European Organization for Nuclear Research), then Brookhaven National Laboratory, and most recently  at Fermi National Accelerator Laboratory. The Prize comes with a $3 million award to be shared among all the winners.

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This is the third time UMass Amherst’s scientists have contributed to Breakthrough Prize-winning work, and Kawall joins colleagues who were previously recognized, first in 2020 for helping to unveil the first image of a black hole and its shadow and again in 2025 for work in advanced particle physics.

“It was a wonderful surprise to have our work recognized with a Breakthrough Prize,” says Kawall. “I’ve spent more than 20 years on this measurement, first as a Yale post-doc on the g-2 experiment at Brookhaven National Lab which published its final result in 2006. We then had our first meetings to discuss how to improve on the Brookhaven result back in 2008, and it took nearly 17 years to bring those ideas to fruition in the recently completed experiment at Fermilab.” 

Over a period of more than 60 years, experiments at Fermilab, Brookhaven and CERN have pursued a quest to measure, as precisely as possible, the magnetic properties of the muon—a tiny subatomic particle that offers an opportunity to test physicists’ fundamental understanding of particles and forces.

“Congratulations to Kawall and his talented team of graduate students and postdocs for their contributions to this year’s Breakthrough Prize in Fundamental Physics,” said UMass Chancellor Javier A. Reyes. “Their collaboration with other high-impact research groups is a testament to creativity, intellectual rigor and devotion of our researchers. We should be proud of the important, high impact scientific inquiry occurring on campus.”

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The muon has been a bit of an enigma since its discovery in 1936. It shares certain characteristics with electrons, including its electrical charge and a form of internal magnetism, called a magnetic moment but is 200 times heavier. Was it just a heavy cousin of the electron, or something else? Measuring its magnetism might point to clues.

Early calculations suggested that the strength of the magnetic moment, characterized by a number called “g” should be two for both electrons and muons. But experiments in the 1940s revealed that electrons have a tiny bit of extra magnetism. Physicists expressed this “anomalous magnetic moment” as “g-2,” to represent the amount that g differs from the predicted value of two. Physicists realized that the electron’s tiny deviation from two is caused by interactions with a sea of “virtual particles” popping in and out of existence.

By measuring the g-factor of muons, physicists could see if these interactions were affecting muons, too. If g-2 was different from expectations, that discrepancy might point to a hole in their understanding of the virtual particles causing the magnetic disturbance—and possibly point to the existence of yet-to-be-discovered particles.

Kawall and members of his lab were part of the Fermilab team that made the world’s most precise measurements of g-2 of the muon, accurate to 8 significant figures.

“Though the Standard Model has been incredibly successful in describing essentially everything we see in the lab, we know that it is woefully incomplete,” says Kawall. “For instance, it doesn’t explain why there is more matter than antimatter in the universe, and it doesn’t account for dark matter, so we know there must be new physics and new particles just waiting to be discovered.”

To test the limits of the Standard Model, the Muon g-2 collaboration repeatedly sent a beam of muons into a 50-foot-diameter superconducting magnetic storage ring, where they circulated about 400 times at nearly the speed of light. Detectors lining the ring allowed scientists to determine how rapidly the muons magnetic moments were rotating. Physicists must also precisely measure the strength of the magnetic field to then determine the value of g-2.

This is where Kawall and members of his lab come in. His group worked on measuring the strength of the magnetic field through which the muons passed, as well as preparing the magnet itself, a feat requiring almost unimaginable precision. 

“One of the innovations we were responsible for,” says Kawall, “was developing a system involving 8,000 sheets of laser-cut iron foils to make the magnetic field as homogenous as possible. With our system, we were able to achieve results nearly three times better than the previous experiment.” The team also spent years developing special calibration probes of incredible fidelity to measure the magnetic field to an accuracy down to 15 parts per billion.

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“The work would not have been successful without the incredible talent of my colleagues at UMass Amherst,” says Kawall. Including Kawall, six current and former scientists from UMass Amherst are laureates and will receive part of the prize: post-docs David Flay, Jimin George and Matthew Bressler, and graduate students Alyssa Conway and David Kessler. Former UMass undergrad Scott Israel, who worked with Kawall for several years, is also a laureate.

Several UMass Amherst masters students worked on the g-2 experiment, including Phani Kandula and Winnie Wang, as well as many undergraduates, including Mor Evron (Honor thesis 2025), Arthur Alves (Honors thesis 2023), Scott Israel (2017-2020), Nate Kimmitt (Honors thesis 2015), Alysea Kim (Lee-Sip Scholar), Rigel Laundre (Honors thesis underway, Lee-Sip Scholar), Kyle O’Connell, Johnny Ayoub, Anushka Shrivastava and Nathan Hall.

The Breakthrough Prize Foundation citation recognizes the awardees’ “multi-decade, groundbreaking contributions to the measurement of the muon’s anomalous magnetic moment, pushing the boundaries of experimental precision and igniting a new era in the quest for physics beyond the Standard Model.”