Quasi-degenerate Neutrinos and Leptogenesis from L Μ − L Τ

We provide a framework for quasi-degenerate neutrinos consistent with a successful leptogenesis, based on the Lµ − Lτ flavor symmetry and its breaking pattern. In this scheme, a fine-tuning is needed to arrange the small solar neutrino mass splitting. Once it is ensured, the atmospheric neutrino mass splitting and the deviation from the maximal atmospheric mixing angle ∆θ23 are driven by the same symmetry breaking parameter λ ∼ 0.1, and the reactor angle θ13 is predicted to be slightly smaller than λ while the Dirac CP phase is generically of order one. Given that the pseudo-Dirac nature of right-handed neutrinos is protected from the flavor symmetry breaking, a small mass splitting can be generated radiatively. For moderate values of tan β ∼ 10, this allows for low-scale supersymmetric leptogenesis, overcoming a strong wash-out effect of the quasi-degenerate light neutrinos and evading the gravitino overproduction.