  We investigate the LHC discovery potential for electroweak scale heavy
neutrino singlets (seesaw I), scalar triplets (seesaw II) and fermion triplets
(seesaw III). For seesaw I we consider a heavy Majorana neutrino coupling to
the electron or muon. For seesaw II we concentrate on the likely scenario where
the new scalars decay to two leptons. For seesaw III we restrict ourselves to
heavy Majorana fermion triplets decaying to light leptons plus gauge or Higgs
bosons, which are dominant except for unnaturally small mixings. The possible
signals are classified in terms of the charged lepton multiplicity, studying
nine different final states ranging from one to six charged leptons. Using a
fast detector simulation of signals and backgrounds, it is found that the
trilepton channel l+- l+- l-+ is by far the best one for scalar triplet
discovery, and for fermion triplets it is as good as the like-sign dilepton
channel l+- l+-. For heavy neutrinos with a mass O(100) GeV, this trilepton
channel is also better than the usually studied like-sign dilepton mode. In
addition to evaluating the discovery potential, we make special emphasis on the
discrimination among seesaw models if a positive signal is observed. This could
be accomplished not only by searching for signals in different final states,
but also by reconstructing the mass and determining the charge of the new
resonances, which is possible in several cases. For high luminosities, further
evidence is provided by the analysis of the production angular distributions in
the cleanest channels with three or four leptons.
