Muon g-2

Muon g − 2 (pronounced "gee minus two") was a particle physics experiment at Fermilab to measure the anomalous magnetic dipole moment of a muon to a precision of 0.14 ppm, which is a sensitive test of the Standard Model. It might also provide evidence of the existence of new particles.

The muon, like its lighter sibling the electron, acts like a tiny magnet. The parameter known as the "g factor" indicates how strong the magnet is and the rate of its gyration in an externally applied magnetic field. It is this rate of gyration that is indirectly measured in the Muon g − 2 experiment.

The value of g is slightly larger than 2, hence the name of the experiment. This difference from 2 (the "anomalous" part) is caused by higher-order contributions from quantum field theory. In measuring g − 2 with high precision and comparing its value to the theoretical prediction, physicists will discover whether the experiment agrees with theory. Any deviation would point to as yet undiscovered subatomic particles that exist in nature.

On July 9, 2023 the Fermilab collaboration concluded the experiment after six years of data collection. The initial results (based on data from the first year of the experiment's operation) were released on April 7, 2021. The results from the first three years of data-taking were announced in August 2023. The final results, based on the full six years of data-taking, were published on June 3, 2025.

The first muon g − 2 experiments began at CERN in 1959 at the initiative of Leon M. Lederman. A group of six physicists formed the first experiment, using the Synchrocyclotron at CERN. The first results were published in 1961, with a 2% precision with respect to the theoretical value, and then the second ones with this time a 0.4% precision, hence validating the quantum electrodynamics theory.

A second experiment started in 1966 with a new group, working this time with the Proton Synchrotron, also at CERN. The results were then 25 times more precise than the previous ones and showed a quantitative discrepancy between the experimental values and the theoretical ones, and thus required the physicists to recalculate their theoretical model.

The third experiment, which started in 1969, published its final results in 1979, confirming the theory with a precision of 0.0007%.

The United States took over the g − 2 experiment in 1984.