Enhanced pair production of heavy Majorana neutrinos at the LHC

Abstract

Towards experimental confirmations of the type-I seesaw mechanism, we explore a prospect of discovering the heavy Majorana right-handed neutrinos (RHNs) from a resonant production of a new massive gauge boson (Z 0 ) and its subsequent decay into a pair of RHNs (Z 0 → NN) at the future high luminosity runs at the Large Hadron Collider (LHC). Recent simulation studies have shown that the discovery of the RHNs through this process is promising in the future. However, the current LHC data very severely constrains the production cross section of the Z 0 boson into a dilepton final states,. Extrapolating the current bound to the future, we find that a significant enhancement of the branching ratio BRðZ 0 → NNÞ over BRðZ 0 → l þ l − Þ is necessary for the future discovery of RHNs. As a well-motivated simple extension of the standard model (SM) to incorporate the Z 0 boson and the type-I seesaw mechanism, we consider the minimal Uð1Þ X model, which is a generalization of the well-known minimal B − L model without extending the particle content. We point out that this model can yield a significant enhancement up to. This is in sharp contrast with the minimal B − L model, a benchmark scenario commonly used in simulation studies, which predicts BRðZWith such an enhancement and a realistic model-parameter choice to reproduce the neutrino oscillation data, we conclude that the possibility of discovering RHNs with, for example, a 300 fb −1 luminosity implies that the Z 0 boson will be discovered with a luminosity of 170.5 fb −1 (125 fb −1 ) for the normal (inverted) hierarchy of the light neutrino mass pattern. DOI: 10.1103/PhysRevD.97.115023 Although neutrinos are massless particles in the standard model (SM), the experimental evidence of the neutrino oscillation [1] indicate that neutrinos have tiny but nonzero masses and flavor mixings. Hence, we need to extend the SM to incorporate the nonzero neutrino masses and flavor mixings. From a perspective of low energy effective theory, one can do so by introducing a dimension-5 operator [2] involving the Higgs and lepton doublets, which violates the lepton number by ΔL ¼ 2 units. After the electroweak (EW) symmetry breaking, the neutrinos acquire tiny Majorana masses suppressed by the scale of the dimension-5 operator. In the context of a renormalizable theory, the dimension-5 operator is naturally generated by introducing heavy Majorana right-handed neutrinos (RHNs), which are singlet under the SM gauge group, and integrating them out. This is the so-called type-I seesaw mechanism [3][4][5][6][7].If the RHNs have masses around 1 TeV or smaller, they can be produced at the Large Hadron Collider (LHC) with a smoking-gun signature of a same-sign dilepton in the final state, which indicates a violation of the lepton number. Since the RHNs are singlet under the SM gauge group, they can be produced only through their mixings with the SM neutrinos. To reproduce the observed light neutrino mass scale, m ν ¼ Oð0.1Þ eV, through the type-I seesaw mechanism with heavy neutrino masses at...

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