Standard Model Predictions for the Muon (g − 2)/2

where e, m and g are the charge, mass and gyromagnetic factor of the particle. In the Dirac theory of a charged pointlike spin-1/2 particle, g = 2, and QED effects slightly increase the g value. Conventionally, a quantity a ≡ (g−2)/2 is referred to as the anomalous magnetic moment. The electron and muon anomalous magnetic moments have been measured with a very high relative accuracy of 0.24 ppb [ 1] and 0.54 ppm [ 2], respectively. The theoretical prediction for ae is only mildly affected by strong and weak interactions providing a test of QED and giving the most precise value of the fine-structure constant α. In contrast, aμ allows to test all sectors of the Standard Model since all of them contribute significantly to the total. Although the electron anomalous magnetic moments is known much more precisely, aμ is much more sensitive to new physics effects: the gain is usually ∼ (mμ/me) ≈ 4.3 · 10. The τ lepton magnetic anomaly has even better potential, but because of the small lifetime of the τ , it has not yet been measured with the best limits coming