Type-II seesaw mechanism for Dirac neutrinos and its implications on Neff and lepton flavor violation in a 3-3-1 model

A bstract In this study, we implement the type-II seesaw mechanism for Dirac neutrino masses within the framework of a 3-3-1 model. To this end, we introduce a scalar sextet and impose both lepton number conservation and invariance under a discrete Z 2 symmetry in the Lagrangian. This mechanism naturally generates small Dirac neutrino masses by allowing the soft breaking of the Z 2 symmetry through a unique term in the scalar potential, while preserving lepton number. Specifically, we explore the realization of this model at low-energy scales. Regarding flavor implications, we analyze its contributions to the rare decay processes μ → eγ and μ → ē ee μ → e ¯ ee . In the cosmological context, we analyze the influence of right-handed neutrinos on the effective number of neutrino species, N eff , through interactions mediated by the Z ′ boson. Our findings establish a lower bound of m_Z m Z ′ > 4.4 TeV, which slightly exceeds the current lower limit set by the Large Hadron Collider (LHC).