Abstract
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 5σ significance is viable for masses approaching the TeV scale.
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McDonald, K.L. Probing exotic fermions from a seesaw/radiative model at the LHC. J. High Energ. Phys. 2013, 131 (2013). https://doi.org/10.1007/JHEP11(2013)131
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DOI: https://doi.org/10.1007/JHEP11(2013)131