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\section{Introduction}
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\label{ref:intro}
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This note presents two searches for beyond-the-standard model (BSM) physics in events
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containing a leptonically-decaying Z boson, jets, and missing transverse energy. This
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is an update of previous searches performed with 2011 data~\cite{ref:Zpaper,ref:EWKPAS}.
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The search is based on a data sample of pp collisions collected at $\sqrt{s}=8$ TeV in 2012,
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corresponding to an integrated luminosity of \lumi.
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The production of Z bosons is expected in many BSM scenarios, for example supersymmetric (SUSY)
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models. For example, Z bosons may be produced in the decays $\chi^0_2\to Z \chi^0_1$
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$\chi^0_1\to Z \tilde{G}$, where $\chi^0_2$ is the second lightest neutralino, $\chi^0_1$ is
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the lightest neutralino, and $\tilde{G}$ is the gravitino. Such decays may occur in the cascade
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decays of the strongly-produced squarks and gluinos, or via direct production of the electroweak
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charginos and neutralino. Examples of such processes (see Fig.~\ref{fig:diagrams}) are:
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\begin{itemize}
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\item strong production: $pp\to\tilde{g}\tilde{g}\to (q\bar{q}\chi^0_2) (q\bar{q}\chi^0_2)\to(q\bar{q}Z\chi^0_1) (q\bar{q}Z\chi^0_1)\to$ ZZ + 4 jets + \MET
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\item electroweak production: $pp\to\chi^\pm_1\chi^0_2\to (W \chi^0_1)(Z \chi^0_1) \to$ WZ + \MET
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\end{itemize}
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1.2 |
\begin{figure}[!h]
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\begin{center}
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\begin{tabular}{cc}
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\includegraphics[width=0.4\textwidth]{plots/T5zz.pdf} &
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\includegraphics[width=0.4\textwidth]{plots/TChiwz.pdf} \\
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\end{tabular}
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\caption{
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Examples of BSM physics signatures targeted in this search. In the left diagram, Z bosons are produced
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in the cascade decays of the strongly-interacting gluinos. In the right diagram, a Z boson is produced
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via direct production of the weakly-coupled charginos and neutralinos.
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\label{fig:diagrams}
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}
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\end{center}
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\end{figure}
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We thus pursue two strategies. The first is an inclusive strategy which selects events with a Z$\to\ell\ell$ candidate,
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at least two jets, and large \MET. This strategy is useful for targeting, e.g., the production of Z bosons in the
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cascades decays of strongly-interacting particles as depicted in Fig.~\ref{fig:diagrams} (left). In the second strategy,
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we impose additional requirements which strongly suppress the backgrounds while retaining high efficiency for events
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with Z bosons produced via direct production of the weakly-coupled charginos and neutralinos. These two strategies
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are referred to as the ``inclusive search'' and the ``targeted search,'' respectively.
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After selecting events with jets and a $\Z\to\ell^+\ell^-$ ($\ell=e,\mu$) candidate,
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the dominant background consists of SM \Z production accompanied by jets from initial-state radiation (\zjets).
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The \MET\ in \zjets\ events arises primarily when jet energies are mismeasured.
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The \zjets\ cross section is several orders of magnitude larger
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than our signal, and the artificial \MET\ is not necessarily well reproduced in simulation.
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Therefore, the critical prerequisite to a discovery of BSM physics in the $Z+\rm{jets}+\MET$ final state is
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to establish that a potential excess is not due to SM \zjets\ production accompanied by artificial
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\MET\ from jet mismeasurements. In this note, the \zjets\ background is estimated with the \MET templates technique,
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in which the artificial \MET in \zjets events is modeled using a \gjets control sample.
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The second background category consists of processes which produce leptons with uncorrelated flavor.
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These ``flavor-symmetric'' (FS) backgrounds, which are dominated by \ttbar but also contain WW, DY$\to\tau\tau$
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and single top processes, are estimated using a data control sample of e$\mu$ events.
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Additional backgrounds from WZ and ZZ production are estimated from MC, after validation of the MC modeling
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of these processes using 3-lepton and 4-lepton data control samples.
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