Probing exotic fermions from a seesaw/radiative model at the LHC

There exist tree-level generalizations of the Type-I and Type-III seesaw mechanisms that realize neutrino mass via low-energy effective operators with d > 5. However, these generalizations also give radiative masses that can dominate the seesaw masses in regions of parameter space - i.e. they are not purely seesaw models, nor are they purely radiative models, but instead they are something in between. A recent work detailed the remaining minimal models of this type. Here we study the remaining model with d = 9 and investigate the collider phenomenology of the exotic quadruplet fermions it predicts. These exotics can be pair produced at the LHC via electroweak interactions and their subsequent decays produce a host of multi-lepton signals. Furthermore, the branching fractions for events with distinct charged-leptons encode information about both the neutrino mass hierarchy and the leptonic mixing phases. In large regions of parameter-space discovery at the LHC with a 5 sigma significance is viable for masses approaching the TeV scale.