Twenty years after the discovery of $\mu-\tau$ symmetry

Abstract

It has passed 20 years after we proposed $\mu-\tau$ symmetry in light neutrino mass matrix. This model is simple but reproduced the characterestic properties of lepton sector. After that, during the experimental developments, there have appeared so many extensions but most of those phenomenological models are lacking systematic outlooks towards more fundamental theories. In this paper, we try to consider rather systematic model extensions and application to GUT model.