Lepton Masses and Mixing in a Left-Right Symmetric Model with a TeV-scale Gravity

Abstract

We construct a left-right symmetric (LRS) model in five dimensions which accounts naturally for the lepton flavor parameters. The fifth dimension is described by an orbifold, S_1/Z_2 times Z'_2, with a typical size of order TeV^{-1}. The fundamental scale is of order 25 TeV which implies that the gauge hierarchy problem is ameliorated. In addition the LRS breaking scale is of order few TeV which implies that interactions beyond those of the standard model are accessible to near future experiments. Leptons of different representations are localized around different orbifold fixed points. This explains, through the Arkani-Hamed-Schmaltz mechanism, the smallness of the tau mass compared to the electroweak breaking scale. An additional U(1) horizontal symmetry, broken by small parameters, yields the hierarchy in the charged lepton masses, strong suppression of the light neutrino masses and accounts for the mixing parameters. The model yields several unique predictions. In particular, the branching ratio for the lepton flavor violating process mu^- --> e^+ e^- e^- is comparable with its present experimental sensitivity.