Title: Invisible<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>Z</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:math>at the CERN LHC
Abstract: We study the feasibility of observing an invisibly decaying ${Z}^{\ensuremath{'}}$ at the LHC through the process $pp\ensuremath{\rightarrow}Z{Z}^{\ensuremath{'}}\ensuremath{\rightarrow}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}X{X}^{\ifmmode\dagger\else\textdagger\fi{}}$, 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 ${\ensuremath{\Gamma}}_{{Z}^{\ensuremath{'}}\ensuremath{\rightarrow}\overline{\ensuremath{\nu}}\ensuremath{\nu}}$ and potentially detection of light neutral hidden states. Such particles appear in many models, where the ${Z}^{\ensuremath{'}}$ is a messenger to a hidden sector, and also if dark matter is charged under the $U(1{)}^{\ensuremath{'}}$ of the ${Z}^{\ensuremath{'}}$. We find that with as few as $30\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of data the invisibly decaying ${Z}^{\ensuremath{'}}$ can be observed at $5\ensuremath{\sigma}$ over standard model background for a 1 TeV ${Z}^{\ensuremath{'}}$ with reasonable couplings. If the ${Z}^{\ensuremath{'}}$ 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 ${Z}^{\ensuremath{'}}$ 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 $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ 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.