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\section{Backgrounds}
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\label{sec:bg}
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There are three sources of SM backgrouds to the event selection:
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\begin{itemize}
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\item ``fake leptons'', {\it i.e.}, leptons fron heavy flavor decay,
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misidentified hadrons, muons from meson decay in flight, or electrons
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from unidentified conversions;
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\item ``charge flips'', {\it i.e.} events with opposite sign isolated
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leptons where one of the leptons is an electron and its charge is
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misreconstructed due to severe brehmstralung in the tracker material;
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\item rare SM processes that yield same-sign high $P_T$ leptons
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and b-jets, mostly $pp \to t\bar{t}W$ and $pp \to t\bar{t}Z$.
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\end{itemize}
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These backgrounds are estimated using the techniques described in
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Ref.~\cite{Chatrchyan:2012qka}. The systematic uncertainties
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are xx\% for charge-flips, and 50\% for both fakes and rare SM.
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The $pp \to t\bar{t}W$ and $pp \to t\bar{t}Z$ cross-sections
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used to normalize the Monte Carlo predictions are
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232 fb~\cite{Campbell:2012dh}
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and 208 fb~\cite{ttzNLO,Garzelli:2011is}
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respectively. |
