Dark-Portal Leptogenesis in a Non-Holomorphic Modular Scoto-Seesaw Model

Abstract: This work explores the neutrino phenomenology of the scotoseesaw model under non-holomorphic $A_4$ modular flavor symmetry providing a non-SUSY framework for realization of the modular symmetry. To prevent mixing between the beyond standard model fields associated with the tree and loop-level neutrino mass contributions, we assign even and odd modular weights to these sectors, respectively. The physical allowed ranges of oscillation parameters are used to identify the viable region of modulus parameter $τ$ in its fundamental domain. With the complex modulus $τ$ serving as the unique source of CP violation (all other parameters are real) the framework realizes successful low-scale leptogenesis through CP-violating decays of the lightest right-handed neutrino into Standard Model leptons and the Higgs boson. The requisite CP asymmetry arises from one-loop diagrams involving dark-sector states, obviating the need for degenerate mass spectra and thereby circumventing the usual resonant leptogenesis mechanism. The observation of a long-lived charged particle ($η^{\pm}$) in collider experiments would offer compelling evidence for the inert scalar sector of the model and provide a crucial experimental hint on the dark-sector assisted generation of neutrino masses and leptogenesis.