Light - by - Light Contribution to Muon g − 2 in Chiral Perturbation Theory

We compute the hadronic light-by-light scattering contributions to the muon anomalous magnetic moment, a μ (had), in chiral perturbation theory that are enhanced by large logarithms and a factor of NC . They depend on a low-energy constant entering pseudoscalar meson decay into a charged lepton pair. The uncertainty introduced by this constant is ±60 × 10, which is comparable in magnitude to the present uncertainty entering the leadingorder vacuum polarization contributions to the anomalous moment. It may be reduced to some extent through an improved measurement of the π → ee branching ratio. However, the dependence of a μ (had) on non-logarithmically enhanced effects cannot be constrained except through the measurement of the anomalous moment itself. The extraction of information on new physics would require a future experimental value for the anomalous moment differing significantly from the 2001 result reported by the E821 collaboration. Typeset using REVTEX