Majoron emission in muon and tau decays revisited

In models where the breaking of lepton number is spontaneous a massless Goldstone boson, the Majoron (J), appears. We calculate the theoretically allowed range for the branching ratios of Majoron-emitting charged lepton decays, such as Br(μ→eJ) and Br(μ→eJγ), in a supersymmetric model with spontaneous breaking of R-parity. Br(μ→eJ) is maximal in the same region of parameter space for which the lightest neutralino decays mainly invisibly. A measurement of Br(μ→eJ) thus potentially provides information on R-parity violation complementary to accelerator searches. We also briefly discuss existing bounds and prospects for future improvements on the Majoron coupling to charged leptons.