Neutrino Mixing and Leptogenesis in $mu-tau$ Symmetry

We study the consequences of the $Z_2$-symmetry behind the $mu$--$tau$ universality in neutrino mass matrix. We then implement this symmetry in the type-I seesaw mechanism and show how it can accommodate all sorts of lepton mass hierarchies and generate enough lepton asymmetry to interpret the observed baryon asymmetry in the universe. We also show how a specific form of a high-scale perturbation is kept when translated via the seesaw into the low scale domain, where it can accommodate the neutrino mixing data. We finally present a realization of the high scale perturbed texture through addition of matter and extra exact symmetries.

Neutrino Mass Textures and Partial $mu$-$tau$ Symmetry

We discuss the viability of the $mu$--$tau$ interchange symmetry imposed on the neutrino mass matrix in the flavor space. Whereas the exact symmetry is shown to lead to textures of completely degenerate spectrum which is incompatible with the neutrino oscillation data, introducing small perturbations into the preceding textures, inserted in a minimal way, lead however to four deformed textures representing an approximate $mu$--$tau$ symmetry. We motivate the form of these `minimal textures, which disentangle the effects of the perturbations, and present some concrete realizations assuming exact $mu$--$tau$ at the Lagrangian level but at the expense of adding new symmetries and matter fields. We find that all these deformed textures are capable to accommodate the experimental data, and in all types of neutrino mass hierarchies, in particular the non-vanishing value for the smallest mixing angle.