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 ($Z^{\prime}$) and its subsequent decay into a pair of RHNs ($Z^{\prime}\to NN$) at the future 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 $Z^{\prime}$ boson into a dilepton final states, $pp \to Z^{\prime}\to \ell^{+}\ell^{-} $ ($\ell=e$ or $\mu$). Extrapolating the current bound to the future, we find that a significant enhancement of the branching ratio ${\rm BR}(Z^{\prime}\to NN$) over ${\rm BR}(Z^{\prime}\to \ell^{+}\ell^{-}$) is necessary for the future discovery of RHNs. As a well-motivated simple extension of the Standard Model (SM) to incorporate the $Z^\prime$ boson and the type-I seesaw mechanism, we consider the minimal U(1)$_X$ model. We point out that this model can yield a significant enhancement up to ${\rm BR}(Z^{\prime}\to NN)/{\rm BR}(Z^{\prime}\to \ell^{+}\ell^{-}) \simeq 5$ (per generation). This is in sharp contrast with the minimal $B-L$ model, a benchmark scenario commonly used in simulation studies, which predicts ${\rm BR}(Z^{\prime}\to NN)/{\rm BR}(Z^{\prime}\to \ell^{+}\ell^{-}) \simeq 0.5$ (per generation). With such an enhancement and a realistic model-parameter choice to reproduce the neutrino oscillation data, we conclude that the possibility of discovering RHNs with a $300 \; {\rm fb}^{-1}$ luminosity implies that the $Z^\prime$ boson will be discovered with a luminosity of $170.5 \;{\rm fb}^{-1}$ ($125 \; {\rm fb}^{-1}$) for the normal (inverted) hierarchy of the light neutrino mass pattern.
Highlights
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
If the right-handed neutrinos (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 B − L gauge boson (Z0) couples with both the SM fermions and the RHNs, once the Z0 boson is resonantly produced at the LHC, its subsequent decay produces a pair of RHNs
Summary
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 (Z0) and its subsequent decay into a pair of RHNs (Z0 → NN) at the future high luminosity runs at the Large Hadron Collider (LHC). Extrapolating the current bound to the future, we find that a significant enhancement of the branching ratio BRðZ0 → NNÞ over BRðZ0 → lþl−Þ is necessary for the future discovery of RHNs. As a well-motivated simple extension of the standard model (SM) to incorporate the Z0 boson and the type-I seesaw mechanism, we consider the minimal Uð1ÞX model, which is a generalization of the well-known minimal B − L model without extending the particle content. This is in sharp contrast with the minimal B − L model, a benchmark scenario commonly used in simulation studies, which predicts BRðZ0 → NNÞ=BRðZ0 → lþl−Þ ≃ 0.5 (per generation) With 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 Z0 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. The production of RHNs at the LHC with an observable rate is unlikely.
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