Muon g-2: Beyond or within the Standard Model?
Measurements of the magnetic moments of the electron and muon were intertwined with the development of the modern physics of the 20th century. The measurements are expressed in terms of the -value, the proportionality constant between the magnetic moment and the spin. The Stern-Gerlach experiment and atomic spectroscopy told us that for the electron, which was subsequently predicted by Dirac theory. Later, experiments showed that for the electron and it was necessary to add an anomalous piece, g =2(1+ a).
For point-like particles, the anomaly, arises from radiative corrections. The simplest correction was first obtained by Schwinger, who found that , and by doing so, carried out what we now call the very first loop calculation in quantum electrodynamics. This remarkable result was also found to also describe the muon’s magnetic moment, which indicated that in a magnetic field the muon behaved like a heavy electron. In principle, loops containing all virtual particles that interact with the muon can contribute.
I will trace the development of ideas and experiments that culminated with the measurement of the muon anomaly to a relative precision of 0.54 parts per million by E821 at Brookhaven Lab, along with theoretical calculations which match this precision. At present there appears to be a 3.2 to 3.6 standard deviation difference between the Standard Model and experimental values of , which might be a harbinger of New Physics contributing to the muon anomaly. This difference between experiment and theory has motivated a new experiment that is being prepared at Fermilab.