The anomalous magnetic moment
DOI: 10.1063/1.4796445
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, 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 very small difference between the muon’s cyclotron and precessional frequencies in the group’s muon storage ring. At this point, the measured g – 2 differs from the standard-model calculation by a tantalizing but inconclusive 2.7 standard deviations. In coming months, new empirical inputs from electron–positron collider data are expected to sharpen the standard-model prediction, and the Muon (g – 2) Collaboration is looking for funding to continue its experiment. (
