Colloquia

The gyromagnetic ratio g for an elementary point-particle differs from the Dirac value of g = 2 by the “magnetic anomaly” a = (g − 2)/2 ≈ 10⁻³, due to couplings to virtual particles in the vacuum. Muon, through its greater mass, probes significantly deeper into the high-mass excitations of the vacuum than does the better studied electron. Hence, efforts to measure the muon magnetic anomaly a_µ have persisted for decades, culminating in Fermilab E989, the Muon g−2 experiment. In July 2023, the E989 collaboration concluded data taking in Run-6, its last run cycle, and a few weeks later unblinded and published results of Run-2 and -3 data analysis, adding to the Run-1 results of 2021. The Run-2/3 results bring about a two-fold improvement in the precision of the world average of a_µ. In parallel, recent theoretical and experimental developments regarding the hadronic vacuum polarization (HVP) have led to a reexamination of the context for the interpretation of the experimental value of a_µ. This talk will examine the experimental method, uncertainties, and results of E989. We will place the new a_µ result in the context of a changing HVP landscape, and discuss the future prospects for the field.