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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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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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$$ N_{2 lep} = N_{2 lep}^{MC} |
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\times |
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{(1- \epsilon_{iso\ trk})^{data} \over (1 - \epsilon_{iso\ trk})^{MC}} |
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\times |
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{N_{peak}^{data} \over N_{peak}^{MC}} |
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$$ |
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All of these terms will be defined clearly in this section, including their corrections and sources of systematic errors. |
