Discrete dark matter with light Dirac neutrinos

TL;DR

This paper introduces a novel Dirac neutrino and dark matter model using an $A_4$ flavor symmetry, where neutrino masses are generated via a Dirac scoto-seesaw mechanism, and dark matter stability is ensured by residual symmetries.

Contribution

It presents the first Dirac scoto-seesaw framework combining $A_4$ flavor symmetry with dark matter stability, extending the scoto-seesaw model to Dirac neutrinos.

Findings

One light Dirac neutrino mass from type-I Dirac seesaw

Other neutrino masses from one-loop scotogenic contributions

Predicted additional relativistic degrees of freedom $ riangle N_{eff}$

Abstract

We propose a new realisation of light Dirac neutrino mass and dark matter (DM) within the framework of a non-Abelian discrete flavour symmetry based on $A_4$ group. In addition to $A_4$, we also consider a $Z_2$ and an unbroken global lepton number symmetry $U(1)_L$ to keep unwanted terms away while guaranteeing the Dirac nature of light neutrinos. The field content, their transformations and flavon vacuum alignments are chosen in such a way that the type-I Dirac seesaw generates only one light Dirac neutrino mass while the other two masses arise from scotogenic contributions at one-loop. This leads to the Dirac scoto-seesaw framework, a generalisation of the widely studied scoto-seesaw model to Dirac neutrinos. The symmetry breaking of $A_4$ leaves a remnant $\mathcal{Z}_2$ symmetry responsible for stabilising DM. Dirac nature of light neutrinos introduces additional relativistic degrees of freedom $\Delta N_{\rm eff}$ within reach of cosmic microwave background experiments.