Constrained Neutrino Masses and Flavour Observables from a Dual-Seesaw Framework.

Understanding the origin of neutrino masses and their mixing patterns remains a central challenge in particle physics, as it provides key insights into the limitations of the Standard Model (SM) and potential avenues for new physics. Flavour symmetries, combined with seesaw mechanisms, offer a promising framework to address these questions while yielding testable predictions in current and future experiments. Motivated by these considerations, we present a novel framework for neutrino masses and mixing based on the interplay of Type-I and Type-II seesaw mechanisms under non-Abelian $A_4$ discrete flavour symmetry. The framework is consistent with the normal hierarchy (NH) of neutrino masses and is governed by three real parameters that span the entire model and its predictions. It determines the neutrino mass eigenvalues and Majorana phases. The model further constrains the effective Majorana mass ($m_{\beta\beta}$) relevant for neutrinoless double beta ($0\nu\beta\beta$) decay​ and the branching ratio of the dominant $\mu \rightarrow e \gamma$ charged lepton flavour violating decay. Both observables are sensitive to the underlying parameters and provide indirect tests of the model. Overall, this minimal $A_4$ symmetric framework demonstrates that a small set of parameters, guided by symmetry and seesaw dynamics, can lead to predictive outcomes. The interplay between theoretical predictions and low-energy observables highlights the model’s relevance and accessibility to future experimental investigations.