Not-that-heavy Majorana neutrino signals at the LHC

We revisit the possibility of Majorana neutrinos production at the large hadron collider (LHC) by studying the $pp\to l_i^{+}{l}_j^{+}+2jets$ ($l_j\equiv e,\mu$) process which, due to leptonic number violation, is a clear signature for intermediate Majorana neutrino contributions. The interactions between Majorana neutrinos and the Standard Model particles are obtained from an effective Lagrangian approach. Majorana neutrinos with masses of a few GeV are long-lived neutral particles, and we take advantage of its measurable decay length: in the same-sign dilepton channel, we exploit this fact imposing cuts that reject the SM background, and analyze the distribution corresponding to the angle between the final leptons, using a forward–backward like asymmetry to study the effects of the different gauge invariant operators. We also study the $pp\to l_i^{+}\nu \gamma$ process, which is dominant for low m _N masses if tensorial one-loop generated new physics leading to a magnetic moment for the heavy neutrinos is present. This channel provides a powerful signal that could be observed at the LHC with the aid of non-pointing photons observables and cuts on the displacement between the prompt lepton and the photon in the final state. (paper)