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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 astrophysical 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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$\mathrm{SumJetPt}>300$~GeV and $\met/\sqrt{\rm SumJetPt} > 8.5$~GeV$^{1/2}$.
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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. As shown in Table~\ref{tab:sigyield}, the expected total SM yield in 35 pb$^{-1}$ is 1.4 events,
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while the expectations from the LMO and LM1 SUSY benchmark points are 6.5 and
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2.6 events, respectively.
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We cut on \met$/\sqrt{\rm SumJetPt}$ rather than \met
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because the variables SumJetPt 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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\begin{table}[hbt]
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\begin{center}
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\caption{\label{tab:sigyield} MC expected yields in the signal region for 35~pb$^{-1}$.
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The errors are statistical only.}
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\begin{tabular}{lcccc}
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\hline
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Sample & $ee$ & $\mu\mu$ & $e\mu$ & tot \\
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\hline
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$t\bar{t}\rightarrow \ell^{+}\ell^{-}$ & 0.27 $\pm$ 0.03 & 0.22 $\pm$ 0.03 & 0.56 $\pm$ 0.05 & 1.05 $\pm$ 0.06 \\
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$t\bar{t}\rightarrow \mathrm{other}$ & 0.01 $\pm$ 0.01 & 0.00 $\pm$ 0.00 & 0.01 $\pm$ 0.01 & 0.02 $\pm$ 0.01 \\
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$Z^0 \rightarrow \ell^{+}\ell^{-}$ & 0.05 $\pm$ 0.05 & 0.05 $\pm$ 0.05 & 0.05 $\pm$ 0.05 & 0.16 $\pm$ 0.09 \\
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$W^{\pm}$ + jets & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 \\
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$W^+W^-$ & 0.00 $\pm$ 0.00 & 0.01 $\pm$ 0.00 & 0.02 $\pm$ 0.01 & 0.03 $\pm$ 0.01 \\
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$W^{\pm}Z^0$ & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 \\
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$Z^0Z^0$ & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 \\
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single top & 0.00 $\pm$ 0.00 & 0.00 $\pm$ 0.00 & 0.01 $\pm$ 0.00 & 0.01 $\pm$ 0.00 \\
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\hline
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total SM MC & 0.35 $\pm$ 0.06 & 0.29 $\pm$ 0.06 & 0.65 $\pm$ 0.07 & 1.29 $\pm$ 0.11 \\
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\hline
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data & 0 & 0 & 1 & 1 \\
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\hline
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LM0 & 1.75 $\pm$ 0.10 & 2.10 $\pm$ 0.11 & 2.42 $\pm$ 0.12 & 6.28 $\pm$ 0.20 \\
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LM1 & 0.90 $\pm$ 0.03 & 1.10 $\pm$ 0.03 & 0.57 $\pm$ 0.02 & 2.57 $\pm$ 0.04 \\
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\hline
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\end{tabular}
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\end{center}
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\end{table} |
