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December 5, 2002

"A Precision Measurement of the Anomalous Magnetic Moment of the Muon: A Search for Physics Beyond the Standard Model"

Professor Paul T. Debevec

Department of Physics
University of Illinois at Urbana-Champaign

The g-factors of the electron and the muon are not exactly 2. They are slightly larger. The deviation from the 2, the g-factor anomaly or g-2, arises from the coupling to virtual fields and particles. For the electron only virtual photons, and electron-positron pairs are important. Theory (quantum electrodynamics) and experiment agree to an astounding precision. For the muon with 207 times the electron mass, virtual fields and particles from both the strong and the weak interaction are also important. These contributions can be calculated within the framework of the Standard Model. A deviation of experiment from theory would then be evidence for physics beyond the Standard Model.

CERN was able to achieve a precision of 7 ppm in a classic experiment completed 25 years ago. The Brookhaven National Laboratory g-2 experiment, E821, has been operating since 1997. The precision of E821 has steadily improved, and the most recent result of September, 2002 has a precision of 0.7 ppm. As each new result from E821 has been released, the Standard Model calculation has been subjected to greater scrutiny. The lastest experimental result may be showing a deviation from the Standard Model. This conclusion is quite controversial, but many theorists have suggested that supersymmetry, a natural extension to the Standard Model, can account for the current difference between experiment and theory.

I will describe how we measure g-2, why the comparison with theory is controversial, and how matters may be resolved.