Long-lived heavy particles in neutrino mass models

Abstract

All extensions of the standard model that generate Majorana neutrino masses at the electro-weak scale introduce some "heavy" mediators, either fermions and/or scalars, weakly coupled to leptons. Here, by "heavy" we understand implicitly the mass range between a few 100 GeV up to, say, roughly 2 TeV, such that these particles can be searched for at the LHC. We study decay widths of these mediators for several different tree-level neutrino mass models. The models we consider range from the simplest $d=5$ seesaw up to $d=11$ neutrino mass models. For each of the models we identify the most interesting parts of the parameter space, where the heavy mediator fields are particularly long-lived and can decay with experimentally measurable decay lengths. One has to distinguish two different scenarios, depending on whether fermions or scalars are the lighter of the heavy particles. For fermions we find that the decay lengths correlate with the inverse of the overall neutrino mass scale. Thus, since no lower limit on the lightest neutrino mass exists, nearly arbitrarily long decay lengths can be obtained for the case where fermions are the lighter of the heavy particles. For charged scalars, on the other hand, there exists a maximum value for the decay length. This maximum value depends on the model and on the electric charge of the scalar under consideration, but can at most be of the order of a few millimeters. Interestingly, independent of the model, this maximum occurs always in a region of parameter space, where leptonic and gauge boson final states have similar branching ratios, i.e. where the observation of lepton number violating final states from scalar decays is possible.