  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.
