Bilarge neutrino mixing and μ-τ permutation symmetry for two-loop radiative mechanism -: art. no. 015011

The presence of approximate electron number conservation and mu-tau permutation symmetry of S-2 is shown to naturally provide bilarge neutrino mixing. First, the bimaximal neutrino mixing together with U-e3=0 is guaranteed to appear owing to S-2, and then, the bilarge neutrino mixing together with \U-e3\much less than1 arises as a result of a tiny violation of S-2. The observed mass hierarchy of Deltam(2) much less than Deltam(atm)(2) is subject to another tiny violation of the electron number conservation. This scenario is realized in a specific model based on SU(3)(L)xU(1)(N) with two-loop radiative mechanism for neutrino masses. The radiative effects from heavy leptons contained in lepton triplets generate the bimaximal structure, and those from charged leptons, which break S-2, generate the bilarge structure together with \U-e3\much less than1. To suppress dangerous flavor-changing neutral current interactions due to Higgs exchanges especially for quarks, this S-2 symmetry is extended to a discrete Z(8) symmetry, which also ensures the absence of a one-loop radiative mechanism.