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In order to search for a possible signal from stop decays giving rise to a signature of \ttbar\ with additional \met\ |
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from the LSPs, it is necessary to determine the composition of the SM backgrouns in the signal region. |
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from the LSPs, it is necessary to determine the composition of the SM backgrounds in the signal region. |
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This section details the methods pursued to estimate the background in the signal sample and describes the |
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procedure to estimate the systematic uncertainties. The general strategy is to use the MC prediction for the |
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backgrounds after applying corrections derived from data. |
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The most important background to a stop signal arises from SM \ttbar. The \ttbar\ background may be |
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separated into contributions containing a single lepton \ttlj\ and two leptons \ttll. As described in this section, |
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the \ttll\ background is the dominant process satisfying the event selection, contributing $\sim 80\%$ of the |
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signal sample defined with the benchmark selection of $\met\>100~\GeV$ and $\mt\>150~\GeV$. This |
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background has large true \met\ and consequently larger \mt\ due to the presence of two neutrinos. |
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the \ttll\ background is the dominant process in our signal region (\met\ $>$ 100~\GeV and \mt\ $>$ 150~\GeV, $\ge 1$ b-tags, isolated track veto), |
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contributing $\sim 80\%$ of the background yield. |
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% |
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This background has large true \met\ and consequently larger \mt\ due to the presence of two neutrinos. |
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Additional contributions to the single lepton sample arise from \wjets\ and single top. The combination of |
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all single lepton backgrounds, \ttlj, \wjets\ and single top, comprises $\sim 15\%$ of the signal sample. |
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Finally, other background sources such as dibosons, \dy\ + jets, in addition to rarer processes such as \ttbar\ |
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produced in association with a vector boson and tribosons, provide a combined contribution to the signal sample |
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at the level of $\sim 5\%$. |
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Finally, the QCD background contribution is small, particularly in the signal sample, with a large \met requirement. |
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produced in association with a vector boson, dibosons and tribosons, |
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provide a combined contribution to the signal |
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sample at the level of $\sim 5\%$. |
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Finally, the QCD background contribution is small, particularly in the signal sample, with a large \met\ requirement. |
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The total bkg in the signal region is estimated according to: |
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$$ N_{bkg} = N_{1 lep} + N_{2 lep} + N_{rare} $$ |
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$$ N_{1 lep} = N_{1 lep}^{MC} |
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\times |
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{(1- \epsilon_{fake})^{data} \over (1 - \epsilon_{fake})^{MC}} |
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\times |
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{N_{peak}^{data} \over N_{peak}^{MC}} |
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$$ |
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|
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$$ N_{2 lep} = N_{2 lep}^{MC} |
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\times |
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{(1- \epsilon_{iso\ trk})^{data} \over (1 - \epsilon_{iso\ trk})^{MC}} |
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\times |
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{N_{peak}^{data} \over N_{peak}^{MC}} |
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$$ |
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All of these terms will be defined clearly in this section, including |
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their corrections and sources of systematic errors. |
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%We then define the following subsamples within this preselection sample: |
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%\begin{itemize} |
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%\item $N_{b-tag} = 0$, i.e. b-veto region $\to$ used to validate the lepton + jets bkg estimation method (see Section~\ref{sec:bkg_singlelep}). |
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%For raw yields prior to any corrections see Tables~\ref{tab:bvetoyieldpeak} and ~\ref{tab:bvetoyieldtail}. |
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%% |
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%\item $N_{b-tag} \ge 1 $, i.e. b-tagged region |
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%For raw yields prior to any corrections see Table~\ref{tab:btagpreselection}. |
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%% |
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%\begin{itemize} |
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%\item with $\met > 100 \GeV$, $ 60 < \mt < 100 \GeV$, and without an additional isolated track veto $\to$ used to normalize top bkg (see Section~\ref{sec:topnorm}). |
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%For raw yields prior to any corrections see Table~\ref{tab:btagpeakregionnotrkiso}. |
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%% |
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%\item with an additional isolated track veto, $\met > 100 \GeV$, $\mt > 150 \GeV$ $\to$ used as signal region |
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%For raw yields prior to any corrections see Table~\ref{tab:btagtailaftertrkiso}. As this is our signal region, only MC is shown at this point. |
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%\end{itemize} |
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%\end{itemize} |
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