The Simplest Linear Seesaw Mechanism

Neutrinos are massless in the Standad Model (SM) but one can introduce Majorana masses effectively through a dimension-5 lepton number violating operator \(-\mathcal {L}_{\nu }^{d=5}=\frac{1}{\Lambda }(\bar{L}\Phi )(\Phi ^T L^c)+h.c.\) . The linear seesaw mechanism provides an interesting UV-completion of this operator realized in the simplest \(SU(3)\otimes SU(2)\otimes U(1)\) gauge structure. In addition to SM particles, there are three quasi-Dirac leptons, and a second, leptophilic, scalar doublet. Our proposal is consistent with electroweak precision tests, neutrino physics, rare decays, as well as restrictions from lepton flavour violation and collider experiments. Striking signatures are expected at colliders associated to the production of neutrino mass mediators.