Three generations of the Muon g-2 experiment, designed to measure the magnetic moment of the muon with ever-increasing precision, have been recognized with the 2026 Breakthrough Prize in Fundamental Physics for “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.”
The Muon g-2 experiment began at the European Organization for Nuclear Research (CERN) in the 1950s with its first publication in 1961, shifted to Brookhaven National Laboratory (BNL) in the 1980s, and concluded at Fermi National Accelerator Laboratory (Fermilab) with a final publication in 2025. Yale scientists, including Sterling Professor of Physics Vernon Hughes (1921-2003) and Satish Dhawan, senior research scientist Emeritus, played a decisive role in enabling the second-generation experiment.
The prizewinners are the living co-authors of the publications that reported the results from the measurement campaigns at CERN, BNL, and Fermilab. Dhawan received the award as part of the Brookhaven National Laboratory group .
The science
The muon is a heavy, unstable cousin of the electron, and, like the electron it can behave like a tiny magnet. Physicists are looking to capture how the muon’s magnetic strength is subtly affected by the “foam” of virtual particles constantly popping in and out of empty space around it. Measuring the muon’s magnetism and comparing it to theoretical predictions allows physicists to test whether any unknown particles or forces are hidden in this foam. In other words, muons provide a way to test the robustness of the Standard Model, our most successful theory of particles and forces, or probe for new physics beyond it.
The parameter of the muon known as the “g factor” (the gyromagnetic ratio) indicates the magnet’s strength and the rate of its gyration in an externally applied magnetic field. The theoretically expected value is slightly larger than 2 (where the excess is due to the higher-order contributions from quantum field theory: the muons interact with the virtual particles that form and disappear within the vacuum), and any deviation from the expected value could indicate that there is physics beyond the Standard Model.
The history
Generation 1 (CERN)
In the 1950s, as scientists gained a better understanding of the characteristics of subatomic particles and their interactions, they recognized that the muon’s g-2 could be a valuable tool for probing what we know about the muon and the Standard Model, which was being built from ongoing experimental and theoretical insights. A group of scientists at CERN developed a concept of storing muons in a magnetic field to measure their precessions.
Protons from the Proton Synchrotron (the first synchrotron at CERN which started operating in 1959 and is today part of the Large Hadron Collider accelerator complex) would hit a target inside the storage ring with a uniform magnetic field, creating pions that decayed into muons. As the muons circle the storage ring, they start with their spins aligned with their direction of motion and then precess as they interact with the storage ring’s magnetic field and with the virtual particles appearing from and disappearing into the vacuum. Scientists were able to measure this precession and estimate the muon g-2 value. The CERN Muon g-2 collaboration published its last result in 1979, which confirmed the Standard Model of particle physics to 0.0007% accuracy.
Generation 2 (BNL) and the Yale connection
Instead of accepting the CERN group’s experimental and theoretical agreement as the endpoint of using the muon g-2 measurement to study fundamental particles, Yale physicist Vernon Hughes (1921-2003) realized that a more precise experimental measurement could place even more rigorous limits on the validity of the Standard Model.
Hughes arrived at Yale in 1954 and continued as Sterling Professor Emeritus after his retirement in 1991. A particle physicst adept at both theory and experiment, his lifelong passion was to understand the physics of elementary particles and their interactions at the most fundamental level.
Hughes saw a new opportunity to further deepen our understanding of the fundamental interaction of particles with the upgraded Alternating Gradient Synchrotron (AGS) at BNL which can provide a higher intensity proton beam. He started a study in 1982 to assess the feasibility of a more refined measurement and organized a workshop two years later at BNL, bringing together many scientists from CERN’s Muon g-2 experiments and BNL scientists to discuss ways to improve the measurement. This workshop marks the start of the second generation of the muon g-2 experiment.
Hughes not only ensured a continuity from the first-generation experiment at CERN, but he also welcomed new collaborators and new ideas, including groups from Japan and the USSR (KEK in Japan and the Budker Institute for Nuclear Physics in Novosibirsk in the USSR). Thus, the second-generation Muon g-2 collaboration retained the first-generation experimenters’ expertise, while expanding its scientific base and collaborators.
Satish Dhawan, who has been a member of the Yale Physics Department since 1967, was one of the 1984 workshop attendees and has been actively involved in the second generation Muon g-2 from the start. Dhawan recalled that Hughes formulated at Yale the concept of carrying out the muon g-2 experiment using the AGS at Brookhaven, and Robert Adair (1924-2020), who joined Yale Physics in 1959 and served as chair of the Physics Department and director of the pision of Physical Sciences, was also involved in getting the experiment approved at Brookhaven.
On the fourth floor of Yale’s J.W. Gibbs Laboratory, Dhawan tested the magnets’ stability under temperature changes, a crucial factor to maintaining a uniform magnetic field across a large area where the muon will be stored and its precession measured. His work contributed to the collaboration’s construction of a superconducting magnet of a high uniform field.
The second-generation Muon g-2 collaboration built a 50-foot diameter superconducting magnetic storage ring at BNL to carry out their high precision measurement of muon precession. Their analyses showed a discrepancy between experimental data and Standard Model predictions. A more accurate measurement would either eliminate the discrepancy or confirm physics beyond the Standard Model.
Generation 3 (Fermilab)
Though the experimental facility was at BNL, Fermilab had the capability of producing a more intense and pure muon beam. Scientists decided to move the magnet to Fermilab. The 17-ton, 50-foot circular magnet started its 3,200 mile journey from Long Island on June 22 2013, traveling on a barge down the eastern coastline, round the tip of Florida, up the Mississippi River to reach Fermilab on July 26, 2013.
The third generation Muon g-2 experiment began data taking in 2017 and published its final result in 2025, which confirmed the experimental and theoretical discrepancy. However, this discrepancy triggered an in-depth probing of theoretical calculations, rather than generating exotic new theories. Hughes’s insight that studying muon g-2 to a higher precision would lead to a deeper understanding of fundamental interaction has proven correct.