mu-tau symmetry in Zee-Babu model

TL;DR

This paper investigates mu-tau symmetry in the Zee-Babu model, finding it likely in the normal hierarchy and proposing a Froggatt-Nielsen-like symmetry to realize it naturally.

Contribution

It identifies conditions for mu-tau symmetry in the Zee-Babu model and introduces a specific $Z_5$ symmetry model for the normal hierarchy case.

Findings

Mu-tau symmetry probable in normal hierarchy

Possible absence of symmetry in inverted hierarchy

Proposed a $Z_5$ symmetry model for natural realization

Abstract

We study the Zee-Babu two-loop neutrino mass generation model and look for a possible flavor symmetry behind the tri-bimaximal neutrino mixing. We find that there probably exists the mu-tau symmetry in the case of the normal neutrino mass hierarchy, whereas there may not be in the inverted hierarchy case. We also propose a specific model based on a Froggatt-Nielsen-like $Z_5$ symmetry to naturally accomplish the mu-tau symmetry on the neutrino mass matrix for the normal hierarchy case.