Heavy Neutral Leptons via Mixing and Transition Dipole Moments

A well-motivated extension to the Standard Model (SM) that can generate the observed light neutrino masses is the SM gauge-singlet fermion; the minimum number of singlet fermions required to produce the observed neutrino oscillation data is two. Unless explicitly forbidden by an additional symmetry beyond the SM, it is possible to write a Majorana mass term for these singlet states that violates lepton number by two units. The scale associated with such a term is unrelated to the electroweak symmetry breaking of the SM and can in principle lie anywhere from an eV to the Planck scale. This opens up a rich phenomenology that crucially depends on how the singlet states are coupled to the SM; two possibilities are the mass-mixing and dipole portals. Here, we briefly review the theory, phenomenology, and corresponding constraints on the two portals.