# Littlest mu-tau seesaw

## TL;DR

This paper introduces a $$-LSS model that enforces $$ reflection symmetry in neutrino masses, resulting in a simple low-energy neutrino mass matrix with predictive power despite a complex high-energy framework.

## Contribution

The paper develops a $$-LSS model that fixes mass parameters via symmetry, leading to a simple, predictive neutrino mass matrix after considering RG corrections.

## Key findings

- Neutrino mass matrix satisfies $$ reflection symmetry.
- Model predicts lepton mixing angles and CP phases.
- Low energy neutrino predictions are robust despite high energy complexity.

## Abstract

We propose a $\mu-\tau$ reflection symmetric Littlest Seesaw ($\mu\tau$-LSS) model. In this model the two mass parameters of the LSS model are fixed to be in a special ratio by symmetry, so that the resulting neutrino mass matrix in the flavour basis (after the seesaw mechanism has been applied) satisfies $\mu-\tau$ reflection symmetry and has only one free adjustable parameter, namely an overall free mass scale. However the physical low energy predictions of the neutrino masses and lepton mixing angles and CP phases are subject to renormalisation group (RG) corrections, which introduces further parameters. Although the high energy model is rather complicated, involving $(S_4\times U(1))^2$ and supersymmetry, with many flavons and driving fields, the low energy neutrino mass matrix has ultimate simplicity.