Phenomenology of the flavor symmetric scoto-seesaw model with dark matter and TM$_1$ mixing

TL;DR

This paper introduces a flavor symmetric scoto-seesaw model that explains neutrino masses, dark matter, and predicts specific mixing angles and CP phases, with testable implications for collider and rare decay experiments.

Contribution

It develops a hybrid model combining type-I seesaw and scotogenic mechanisms under A4 symmetry, predicting neutrino properties and dark matter phenomenology with minimal parameters.

Findings

Predicts normal neutrino mass ordering with a massless lightest neutrino.

Constrains CP phases and mixing angles, favoring the upper octant for atmospheric mixing.

Provides testable predictions for neutrinoless double beta decay and rare lepton decays.

Abstract

We propose a hybrid scoto-seesaw model based on the $A_4$ non-Abelian discrete flavor symmetry. Light neutrino masses come from the tree-level type-I seesaw mechanism and from the one-loop scotogenic contribution accommodating viable dark matter candidates responsible for observed relic abundance of dark matter (DM). Respectively, both these contributions restore the atmospheric and solar neutrino mass scales. With only one right-handed neutrino, the model features specific predictions with the normal ordering of light neutrino masses, the lightest neutrino being massless, and only one relevant CP Majorana phase. The flavor symmetric setup helps us to realize the TM$_1$ mixing scheme with concrete correlations and constraints on the mixing angles and associated CP phases. The framework predicts the atmospheric mixing angle to be in the upper octant with specific ranges $0.531 (0.580) \leq \sin^2\theta_{23}\leq 0.544 (0.595)$ and the Dirac CP phase is restricted within the range $\pm(1.44-1.12)$ radian. The Majorana phase is also tightly constrained with a range of $0.82-0.95$ and $1.58-1.67$ radian, which is otherwise unconstrained from neutrino oscillations. Strict predictions on the Majorana phases also yield an accurate prediction for the effective mass parameter for neutrinoless double beta within the range of $1.61-3.85$ meV. The model offers a rich phenomenology regarding DM relic density and direct search constraints, and the fermionic DM scenario has been discussed in detail, estimating its possible connection with the neutrino sector. As an example of the model studies at colliders, the SM Higgs in the diphoton decay channel is examined. The model predicts strictly vanishing $\tau \to e\gamma$, $\tau \rightarrow 3e$ decays and testable signals by MEG-II and SINDRUM/Mu3e experiments for the $\mu \to e \gamma$ and $\mu \to 3 e$ decays, respectively.