Discovering hidden sectors with monophotonZ′searches

In many theories of physics beyond the standard model, from extra dimensions to Hidden Valleys and models of dark matter, $Z^{\prime }$ bosons mediate between standard model particles and hidden sector states. We study the feasibility of observing such hidden states through an invisibly decaying $Z^{\prime }$ at the LHC. We focus on the process $pp\to \gamma Z^{\prime }\to \gamma X{X}^{†}$, where $X$ is any neutral, (quasi-) stable particle, whether a standard model neutrino or a new state. This complements a previous study using $pp\to Z{Z}^{\prime }\to {\ell }^{+}{\ell }^{-}X{X}^{†}$. Only the $Z^{\prime }$ mass and two effective charges are needed to describe this process. If the $Z^{\prime }$ decays invisibly only to standard model neutrinos, then these charges are predicted by observation of the $Z^{\prime }$ through the Drell-Yan process, allowing discrimination between $Z^{\prime }$ decays to standard model $\nu$’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 $Z^{\prime }$ can be observed at $5\sigma$ significance with $50{\mathrm{fb}}^{-1}$ at the LHC. Observation of a 1.5 TeV state requires super-LHC statistics of $1{\mathrm{ab}}^{-1}$. Control of the systematic errors, in particular, the parton distribution function uncertainty of the dominant $Z\gamma$ background, is crucial to maximize the LHC search reach.