The anomalous magnetic moment

Of the muon has now been measured to an exquisite precision of 0.5 ppm by the Muon (g – 2) Collaboration at Brookhaven National Laboratory. The group’s recent paper culminates a 20-year effort led by Lee Roberts (Boston University) and the late Vernon Hughes (Yale University), who died last year (see Physics Today, Physics Today 0031-9228 57 2 2004 77  https://doi.org/10.1063/1.1688080 February 2004, page 77 ). The muon’s magnetic moment is thought to be a particularly good place to look for indications of new physics beyond the standard model of particle theory. The Dirac equation yields a value of precisely 2 for g, the muon’s gyromagnetic ratio. Standard-model corrections, calculated to eight significant figures, add a predicted anomalous moment of a few parts per thousand to the Dirac g. Hughes and coworkers searched for a tiny departure from the predicted g – 2 by measuring the...