Lepton number violation in TeV scale see-saw extensions of the standard model

The low-energy neutrino physics constraints on the TeV scale type I see-saw scenarios of neutrino mass generation are revisited. It is shown that lepton charge (L) violation, associated to the production and decays of heavy Majorana neutrinos N_j having masses in the range of M_j ∼ (100 ÷ 1000) GeV and present in such scenarios, is hardly to be observed at ongoing and future particle accelerator experiments, LHC included, because of very strong constraints on the parameters and couplings responsible for the corresponding |ΔL| = 2 processes. If the heavy Majorana neutrinos N _j are observed and they are associated only with the type I mechanism, they will behave effectively like pseudo-Dirac fermions. Conversely, the observation of effects proving the Majorana nature of N_j would imply that these heavy neutrinos have additional relatively strong couplings to the Standard Model particles or that light neutrino masses compatible with the observations are generated by a mechanism other than see-saw (e.g., radiatively at one or two loop level) in which the heavy Majorana neutrinos N_j are nevertheless involved.