Heidelberg 2015 – scientific programme

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SYDM: Symposium Dipole Moments - A Tool to Search for New Physics

SYDM 1: Session 1

SYDM 1.1: Invited Talk

Tuesday, March 24, 2015, 11:00–11:40, C/gHS

Searching for New Physics Effects in the Muon g -Factor — •B. Lee Roberts — Boston University

Measurements of the magnetic dipole 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 g-value, the proportionality constant between the magnetic moment and the spin, µ^→ = g (Qeℏ / 2m) s^→. The Stern-Gerlach experiment and atomic spectroscopy told us that g = 2 for the electron, which was subsequently predicted by Dirac theory. Later, experiments showed that for the electron g > 2; and it was necessary to add an anomalous piece, g = 2(1+ a). For point-like particles, the anomaly a = (g−2)/2, arises from radiative corrections. The lowest-order correction was first obtained by Schwinger, who found that a = α/(2π), 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. The electron anomaly has now been measured to 0.24 ppb at Harvard, and the muon anomaly to 0.54 ppm at Brookhaven Laboratory. When the Standard-Model value of the muon anomaly is calculated by including contributions from quantum electrodynamics, the strong interaction and the electroweak interaction, it appears to be more than three standard deviations smaller than the experimental value. To clarify whether this difference signifies contributions from New Physics or not, two new experiments are being prepared. The Fermilab experiment will use the relocated muon storage ring from Brookhaven. The J-PARC experiment will use an entirely different technique. I will review the goals and status of the future experiments.