Abstract
Collider signals of heavy Majorana neutrino mass origin are studied in the minimal Left-Right symmetric model, where their mass is generated spontaneously together with the breaking of lepton number. The right-handed triplet Higgs boson Δ, responsible for such breaking, can be copiously produced at the LHC through the Higgs portal in the gluon fusion and less so in gauge mediated channels. At Δ masses below the opening of the V V decay channel, the two observable modes are pair-production of heavy neutrinos via the triplet gluon fusion gg → Δ → NN and pair production of triplets from the Higgs h → ΔΔ → 4N decay. The latter features tri- and quad same-sign lepton final states that break lepton number by four units and have no significant background. In both cases up to four displaced vertices may be present and their displacement may serve as a discriminating variable. The backgrounds at the LHC, including the jet fake rate, are estimated and the resulting sensitivity to the Left-Right breaking scale extends well beyond 10 TeV. In addition, sub-dominant radiative modes are surveyed: the γγ, Zγ and lepton flavour violating ones. Finally, prospects for Δ signals at future e+e− colliders are presented.
Highlights
1Here we focused on the dominant mode mediated by ∆+R+; the lepton flavor violating (LFV)-γγ interplay and the conclusions are the same if WR loops are taken into account
Even though this cross-section is small, it is interesting that ∆ strahlung by WR gives rise to final states with ∆L = 2 when WR decays to jets, and to ∆L = 4 when WR decays to N, as in the Keung and Senjanovic (KS) process
The study has been carried out in the context of the Left-Right symmetric model, which serves as a complete model of neutrino mass origin, linked to the spontaneous breaking of parity
Summary
SU(2)L × SU(2)R × U(1)B−L, with an additional discrete symmetry that may be generalized parity P or charge conjugation C. Because in general ∆ receives its mass from vR, the relevant couplings ρ1 and α1 turn out to be small in the range of masses relevant for this work, i.e. m∆ 200 GeV; see appendix A for complete details In this particular limit the h − ∆ tri-linear couplings are unambiguously determined by the masses of h and ∆, the mixing angle sθ and the LR scale. Such a formulation is especially convenient for phenomenological studies because there is no need to worry about inter-dependencies of LRSM potential parameters responsible for the production and decay rates. The expressions are collected in the appendix A including the formulae for small non-vanishing ∆ − H mixing
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