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

a r X i v : 0 8 0 2 .3 2 4 1 v 3 [ h e p - p h ] 3 J u n 2 0 0 8 Lepton flavor violating muon decays in a model of electroweak-scale right-handed neutrinos Jian-Ping Bu, Yi Liao 1 , Ji-Yuan Liu Department of Physics, Nankai University, Tianjin 300071, China 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 re- cently 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 in- teractions. 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 → eγ and μ → eee? due to the gauge interactions. Al- though the former is generally smaller than the latter by three orders of magnitude, parameter regions exist in which μ → eγ is reachable in the next generation of experiments even if the cur- rent stringent bound on μ → eee? is taken into account. If light neutrinos dominate for μ → 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 → eee?. PACS: 13.35.Bv, 14.60.St, 12.60.-i, 12.15.-y Keywords: lepton flavor violation, neutrino, rare muon decay 1 liaoy@ 1 Neutrino oscillation has provided the first evidence of physics beyond the Standard Model (SM) that the neutrinos are massive, non-degenerate and mix. The large or even maximal mixing angles measured in solar and atmospheric neutrinos should in principle allow the d