A Gauge Model for Quasi-Dirac Neutrinos

In a model with $L_{\mu} - L_{\tau}$ Abelian gauge symmetry, anomaly-free chiral fermions, which are Standard Model singlets, are introduced as the origin of right-handed neutrinos. This $\mathsf{U} (1)$ symmetry keeps the right-handed neutrinos Majorana massless. Tiny nonvanishing Dirac masses of neutrinos are due to higher-dimensional operators with natural coupling constants. After gauge symmetry breaking, a quasi-Dirac neutrino scenario naturally appears, and realistic neutrino physics can be produced. Phenomenological and cosmological aspects of the model are discussed.