Centre d'études et d'expertise sur les risques, l'environnement, la mobilité et l'aménagement

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The Majorana nature of neutrinos is strongly motivated from the theoretical and phenomenological point of view. A plethora of neutrino mass models, known collectively as Seesaw models, exist that could generate both a viable neutrino mass spectrum and mixing pattern. They can also lead to rich, new phenomenology, including lepton number non-conservation as well as new particles, that may be observable at collider experiments. It is therefore vital to search for such new phenomena and the mass scale associated with neutrino mass generation at high energy colliders. In this review, we consider a number of representative Seesaw scenarios as phenomenological benchmarks, including the characteristic Type I, II, and III Seesaw mechanisms, their extensions and hybridizations, as well as radiative constructions. We present new and updated predictions for analyses featuring lepton number violation and expected coverage in the theory parameter space at current and future colliders. We emphasize new production and decay channels, their phenomenological relevance and treatment across different facilities in e + e − , e − p, and pp collisions, as well as the available Monte Carlo tools available for studying Seesaw partners in collider environments.

Vector boson fusion processes become increasingly more important at higher collider energies and for probing larger mass scales due to collinear logarithmic enhancements of the cross section. In this context, we revisit the production of a hypothetic heavy Majorana neutrino (N ) at hadron colliders. Particular attention is paid to the fusion process W γ → N ℓ ± . We systematically categorize the contributions from a photon initial state in the elastic, inelastic, and deeply inelastic channels. Comparing with the leading channel via the Drell-Yan production qq ′ → W * → N ℓ ± at NNLO in QCD, we find that the W γ fusion process becomes relatively more important at higher scales, surpassing the DY mechanism at m N ∼ 1 TeV (770 GeV), at the 14 TeV LHC (100 TeV VLHC). We investigate the inclusive heavy Majorana neutrino signal, including QCD corrections, and quantify the Standard Model backgrounds at future hadron colliders. We conclude that, with the currently allowed mixing |V µN | 2 < 6 × 10 −3 , a 5σ discovery can be made via the same-sign dimuon channel for m N = 530 (1070) GeV at the 14 TeV LHC (100 TeV VLHC) after 1 ab −1 . Reversely, for m N = 500 GeV and the same integrated luminosity, a mixing |V µN | 2 of the order 1.1 × 10 −3 (2.5 × 10 −4 ) may be probed.