The Muon Anomalous Magnetic Moment
W. Marciano · 1997
The Dirac equation predicts a muon magnetic moment, M = gµ e 2mµ S, with gyromagnetic ratio gµ = 2. Quantum loop effects lead to a small calculable deviation from gµ = 2, parameterized by the anomalous magnetic moment aµ ≡ gµ − 2 2. (1) That quantity can be accurately measured and, within the Standard Model (SM) framework, precisely predicted. Hence, comparison of experiment and theory tests the SM at its quan-tum loop level. A deviation in aexpµ from the SM expectation would signal effects of new physics, with current sensitivity reaching up to mass scales of O(TeV) [1,2]. For a recent and very thorough muon g − 2 review, see Ref. 3. The E821 experiment at Brookhaven National Lab (BNL) studied the precession of µ+ and µ − in a constant external magnetic field as they circulated in a confining storage ring. It found [4] aexpµ+ = 11 659 203(6)(5) × 10−10, aexpµ − = 11 659 214(8)(3) × 10−10, (2) where the first errors are statistical and the second systematic. Assuming CPT invariance and taking into account correlations between systematic errors, one finds for their average [4] aexpµ = 11 659 208.0(5.4)(3.3) × 10−10. (3) These results represent about a factor of 14 improvement over the classic CERN experiments of the 1970’s [5]. The SM prediction for aSMµ is generally divided into three parts (see Fig. 1 for representative Feynman diagrams) aSMµ = a QED µ + a