Electroweak scale seesaw and heavy Dirac neutrino signals at LHC

Models of type I seesaw can be implemented at the electroweak scale in a natural way provided that the heavy neutrino singlets are quasi-Dirac particles. In such case, their contribution to light neutrino masses has the suppression of a small lepton number violating parameter, so that light neutrino masses can arise naturally even if the seesaw scale is low and the heavy neutrino mixing is large. We implement the same mechanism with fermionic triplets in type III seesaw, deriving the interactions of the new quasi-Dirac neutrinos and heavy charged leptons with the SM fermions. We then study the observability of heavy Dirac neutrino singlets (seesaw I) and triplets (seesaw III) at LHC. Contrarily to common wisdom, we find that heavy Dirac neutrino singlets; with a mass around 100 GeV are observable at the 5 sigma level with a luminosity of 13 fb(-1). Indeed, in the final state with three charged leptons l(+/-)l(+/-)l(+/-), not previously considered, Dirac neutrino signals can be relatively large and backgrounds are small. In the triplet case, heavy neutrinos can be discovered with a luminosity of 1.5 fb(-1) for a mass of 300 GeV in the same channel. (C) 2009 Elsevier B.V. All rights reserved.