GM2

gm2 commonly refers to the muon anomalous magnetic moment, denoted a_mu, which measures the deviation of the muon's gyromagnetic ratio g from the value of 2. In the Standard Model, a_mu arises from quantum contributions of quantum electrodynamics, weak interactions, and hadronic effects. The hadronic terms are the primary source of theoretical uncertainty, making a_mu a sensitive probe for potential new physics beyond the Standard Model.

Experiments measuring gm2 have provided some of the most precise tests of the Standard Model. The Brookhaven National Laboratory experiment E821 delivered the earlier benchmark measurement. The ongoing Fermilab Muon g-2 experiment (E989) has produced results with greater precision, and when combined with the Brookhaven data, the measured a_mu differs from the SM prediction by about four standard deviations. This discrepancy has generated significant interest because it could hint at new particles or forces, but its interpretation depends on theoretical predictions of hadronic contributions, which are challenging to calculate accurately.

Theoretical status and interpretation are evolving. Several independent approaches—most notably lattice quantum chromodynamics calculations and data-driven methods for hadronic vacuum polarization and hadronic light-by-light scattering—produce slightly different SM estimates of a_mu. Until these theoretical uncertainties are resolved, the gap between experiment and theory remains an intriguing potential sign of new physics, rather than a definitive discovery. Ongoing experimental efforts and advances in theory aim to clarify whether gm2 signals new phenomena or reflects refinements in Standard Model calculations.