Trimaximal neutrino mixing from vacuum alignment in A4 and S4 models

TL;DR

This paper develops models based on A4 and S4 symmetries to explain neutrino mixing patterns, predicting trimaximal mixing with a large reactor angle and a testable sum rule, while addressing recent experimental results.

Contribution

It introduces a novel vacuum alignment mechanism in A4 and S4 models that naturally yields trimaximal mixing and a new testable sum rule for neutrino oscillations.

Findings

Predicts accurate trimaximal mixing with approximate tri-bimaximal mixing.

Proposes a renormalisable S4 model unifying flavons to control reactor angle.

Predicts a new mixing sum rule testable in future experiments.

Abstract

Recent T2K results indicate a sizeable reactor angle theta_13 which would rule out exact tri-bimaximal lepton mixing. We study the vacuum alignment of the Altarelli-Feruglio A4 family symmetry model including additional flavons in the 1' and 1" representations and show that it leads to trimaximal mixing in which the second column of the lepton mixing matrix consists of the column vector (1,1,1)^T/sqrt{3}, with a potentially large reactor angle. In order to limit the reactor angle and control the higher order corrections, we propose a renormalisable S4 model in which the 1' and 1" flavons of A4 are unified into a doublet of S4 which is spontaneously broken to A4 by a flavon which enters the neutrino sector at higher order. We study the vacuum alignment in the S4 model and show that it predicts accurate trimaximal mixing with approximate tri-bimaximal mixing, leading to a new mixing sum rule testable in future neutrino experiments. Both A4 and S4 models preserve form dominance and hence predict zero leptogenesis, up to renormalisation group corrections.