Invisible Z′ at the CERN LHC

Abstract

We study the feasibility of observing an invisibly decaying Z′ at the LHC through the process pp→ZZ′→ℓ^+ℓ^−XX^†, where X is any neutral, (quasi-) stable particle, whether a standard model neutrino or a new state. The measurement of the invisible width through this process facilitates both a model-independent measurement of Γ_(Z′→ṽν and potentially detection of light neutral hidden states. Such particles appear in many models, where the Z′ is a messenger to a hidden sector, and also if dark matter is charged under the U(1)′ of the Z′. We find that with as few as 30 fb^(−1) of data the invisibly decaying Z′ can be observed at 5σ over standard model background for a 1 TeV Z′ with reasonable couplings. If the Z′ does not couple to leptons and therefore cannot be observed in the Drell-Yan channel, this process becomes a discovery mode. For reasonable hidden sector couplings, masses up to 2 TeV can be probed at the LHC. If the Z′ does couple to leptons, then the rate for this invisible decay is predicted by on-peak data and the presence of additional hidden states can be searched for. With 100 fb^(−1) of data, the presence of excess decays to hidden states can be excluded at 95% C.L., if they comprise 20–30% of the total invisible cross section.