Ameliorating the popular lepton mixings with A4 symmetry: A see-saw model for realistic neutrino masses and mixing

TL;DR

This paper proposes a neutrino mass and mixing model using A4 symmetry and a combined see-saw mechanism, explaining observed neutrino oscillation data and predicting testable CP-violation effects.

Contribution

It introduces a novel A4-symmetric see-saw model with dominant Type-II contribution and adjustable solar mixing angle, aligning with experimental data and allowing for large CP violation.

Findings

Model accommodates atmospheric mass splitting and maximal mixing.

Predicts neutrino mass ordering and CP-violation phases.

Lightest neutrino mass estimated at 0.05 eV or more.

Abstract

A model for neutrino masses and mixing is devised appointing the see-saw mechanism. The proffered model is fabricated with a combination of Type -I and Type-II see-saw contributions of which the latter dominates. The scalars and the leptons in the model are assigned $A4$ charges conducive to obtain the mass matrices viable for the scheme. The Type -II see-saw mass matrix accommodates atmospheric mass splitting and maximal mixing in the atmospheric sector ($\theta_{23}=\pi/4$). It is characterized by vanishing solar mass splitting and $\theta_{13}$ whereas the third neutrino mixing angle is free to acquire any value of $\theta_{12}^0$. Particular alternatives of $\theta_{12}^0$ corresponding to the popular lepton mixings viz. $\theta_{12}^0=35.3^\circ$ (tribimaximal), $45.0^\circ$ (bimaximal), $31.7^\circ$ (golden ratio) are accounted for. Another choice of $\theta_{12}^0=0^\circ$ (no solar mixing) is reckoned. The subdominant Type-I see-saw constituent of the model propels all the neutrino oscillation parameters into the ranges allowed by the data which in its turn get interrelated owing to their common origin. This makes the model testable in the light of future experimental data. As an example, $\theta_{23}$ emerges in the first (second) octant for normal (inverted) ordering. CP-violation is governed by phases present in the right-handed Majorana neutrino mass matrix, $M_{\nu R}$. Only normal ordering is allowed if these phases are absent. If $M_{\nu R}$ is complex the Dirac CP-violating phase $\delta$, is capable of being large, i.e., $\sim \pm \pi/2$, and inverted ordering of neutrino masses is also permitted. T2K and NOVA preliminary data favouring normal ordering and $\delta \sim -\pi/2$ predicts lightest neutrino mass to be 0.05 eV or more within the framework of this model.