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\section{Definition of the signal region}
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\label{sec:sigregion}
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We define a signal region to look for possible
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new physics contributions in the opposite sign isolated
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dilepton sample. The choice of signal region is driven by
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three observations:
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\begin{enumerate}
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\item astrophisical evidence for dark matter suggests that
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we concentrate on the region of high \met;
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\item new physics signals should have high $\sqrt{\hat{s}}$;
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\item observable high cross section new physics signals
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are likely to be produced strongly; thus, we expect significant
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hadronic activity in conjunction with the two leptons.
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\end{enumerate}
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Following these observations, we add the following two requirements
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to the preselection of Section~\ref{sec:eventSel}:
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\begin{center}
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SumJetPt$>$300 GeV and $\met/\sqrt{\rm SumJetPt} > 8.5$.
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\end{center}
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\noindent This selection preserves about 1\% of the $t\bar{t}$
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signal. We cut on \met$/\sqrt{\rm SumJetPt}$ rather than \met
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because the variables \met and \met$/\sqrt{\rm SumJetPt}$ are
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largely uncorrelated for the dominant $t\bar{t}$ background.
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This allows us to use a data driven ABCD method to estimate the
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background (see Section~\ref{sec:abcd}).
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