$A_4$ Flavour Model for Dirac Neutrinos: Type I and Inverse Seesaw

TL;DR

This paper develops two $A_4$ flavor symmetry-based models, type I and inverse seesaw, to explain light Dirac neutrinos, predicting specific mass hierarchies, mixing angles, and CP phases with implications for neutrino mass bounds.

Contribution

It introduces novel $A_4$ flavor models for Dirac neutrinos using type I and inverse seesaw mechanisms, detailing their distinct predictions and parameter correlations.

Findings

Both models predict normal hierarchy and lower octant atmospheric mixing angle.

The models establish correlations between neutrino parameters and model parameters.

Inverse seesaw model can saturate cosmological neutrino mass bounds.

Abstract

We propose two different seesaw models namely, type I and inverse seesaw to realise light Dirac neutrinos within the framework of $A_4$ discrete flavour symmetry. The additional fields and their transformations under the flavour symmetries are chosen in such a way that naturally predicts the hierarchies of different elements of the seesaw mass matrices in these two types of seesaw mechanisms. For generic choices of flavon alignments, both the models predict normal hierarchical light neutrino masses with the atmospheric mixing angle in the lower octant. Apart from predicting interesting correlations between different neutrino parameters as well as between neutrino and model parameters, the model also predicts the leptonic Dirac CP phase to lie in a specific range -$\pi/3$ to $\pi/3$. While the type I seesaw model predicts smaller values of absolute neutrino mass, the inverse seesaw predictions for the absolute neutrino masses can saturate the cosmological upper bound on sum of absolute neutrino masses for certain choices of model parameters.