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

## TL;DR

This paper reviews 20 years of $- au$ symmetry in neutrino physics, emphasizing systematic model extensions and their integration into grand unified theories, amidst experimental progress.

## Contribution

It provides a systematic approach to extending $- au$ symmetry models and explores their application within grand unified theories.

## Key findings

- Systematic extensions of $- au$ symmetry models are proposed.
- Application of these models to GUT frameworks is discussed.
- The paper highlights the need for more fundamental theoretical outlooks.

## 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.