The Invisible Z ′ at the LHC

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 (SM) neutrino or a new state. The measurement of the invisible width through this process facilitates both a model independent measurement of ΓZ′→ν̄ν 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 SM 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 100fb−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.