Lepton Flavor Violating Muon Decays in a Model of Electroweak-Scale Right-Handed Neutrinos

Abstract

The small neutrino mass observed in neutrino oscillations is nicely explained by the seesaw mechanism. Rich phenomenology is generally expected if the heavy neutrinos are not much heavier than the electroweak scale. A model with this feature built in has been suggested recently by Hung. The model keeps the standard gauge group but introduces chirality-flipped partners for the fermions. In particular, a right-handed neutrino forms a weak doublet with a charged heavy lepton, and is thus active. We analyze the lepton flavor structure in gauge interactions. The mixing matrices in charged currents (CC) are generally non-unitary, and their deviation from unitarity induces flavor changing neutral currents (FCNC). We calculate the branching ratios for the rare decays μ\to eγand μ\to ee\bar e due to the gauge interactions. Although the former is generally smaller than the latter by three orders of magnitude, parameter regions exist in which μ\to eγis reachable in the next generation of experiments even if the current stringent bound on μ\to ee\bar e is taken into account. If light neutrinos dominate for μ\to eγ, the latter cannot set a meaningful bound on unitarity violation in the mixing matrix of light leptons due to significant cancelation between CC and FCNC contributions. Instead, the role is taken over by the decay μ\to ee\bar e.