Calculation of the μ → eγ muon decay in the small-neutrino-mass limit

Abstract

The μ → eγ decay is strictly forbidden in the Standard Model, where the lepton flavour is conserved as an accidental symmetry, but in extensions of the Standard Model the lepton flavour is generally no longer conserved, so that decay can take place. In this paper we calculate the amplitude, in an Rξ gauge, of muon decay in the small-neutrino-mass limit. The result, in which ξ-dependence is cancelled, is different from the amplitude and the technical points obtained in previous work.