Invisible Z′ at the CERN LHC

We study the feasibility of observing an invisibly decaying Z' at the LHC through the process pp -> ZZ'-> l(+)l(-)XX(dagger), 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 Gamma(Z)'(->(nu) over bar nu) 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 5 sigma 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.