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Nobuchika Okada
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Enhanced pair production of heavy Majorana neutrinos at LHC
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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′) and
its subsequent decay into a pair of RHNs
(Z′→NN) at the future 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 Z′ boson into dilepton final
states, pp→Z′→ℓ+ℓ−
(ℓ=e or μ). Extrapolating the
current bound to the future, we find that
a significant enhancement of the branching
ratio BR(Z′→NN) over BR(Z′→ℓ+ℓ−)
is necessary for the future discovery of
RHNs. As a well-motivated simple extension
of the Standard Model (SM) to incorporate
the Z′ boson and the type-I seesaw
mechanism, we consider the minimal U(1)X
model, which is a generalization of the well-known
minimal B−L model without extension
of the particle content. We point out that
this model can yield a significant enhancement
up to BR(Z′→NN)/BR(Z′→ℓ+ℓ−)≃5
(per generation). This is in sharp contrast
with the minimal B−L model, a benchmark
model commonly used in simulation studies,
which predicts BR(Z′→NN)/BR(Z′→ℓ+ℓ−)≃0.5.
With such an enhancement and a realistic
model-parameter choice to reproduce the neutrino
oscillation data, we conclude that the possibility
of discovering RHNs in the future implies
that the LHC experiments will soon discover
the Z′ boson.

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