Discovering hidden sectors with monophoton Z′ searches

In many theories of physics beyond the standard model, from extra dimensions to Hidden Valleys and models of dark matter, Z′ bosons mediate between standard model particles and hidden sector states. We study the feasibility of observing such hidden states through an invisibly decaying Z′ at the LHC. We focus on the process pp→γZ′→ γXX†, where X is any neutral, (quasi-) stable particle, whether a standard model neutrino or a new state. This complements a previous study using pp→ZZ′→ℓ+ℓ-XX†. Only the Z′ mass and two effective charges are needed to describe this process. If the Z′ decays invisibly only to standard model neutrinos, then these charges are predicted by observation of the Z′ through the Drell-Yan process, allowing discrimination between Z′ decays to standard model ν's and invisible decays to new states. We carefully discuss all backgrounds and systematic errors that affect this search. We find that hidden sector decays of a 1 TeV Z′ can be observed at 5σ significance with 50fb-1 at the LHC. Observation of a 1.5 TeV state requires super-LHC statistics of 1ab-1. Control of the systematic errors, in particular, the parton distribution function uncertainty of the dominant Zγ background, is crucial to maximize the LHC search reach.