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\section{Introduction} |
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\label{ref:intro} |
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This note presents a search for the production of supersymmetric (SUSY) stop quark pairs in events with a |
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single isolated lepton, several jets, missing transverse energy, and large transverse mass. We use the full |
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2011 data sample, corresponding to an integrated luminosity of \lumi. |
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single isolated lepton, several jets, missing transverse energy, and large transverse mass. We use a |
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data sample corresponding to an integrated luminosity of \lumi. |
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This search is of theoretical interest because of the critical role played |
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by the stop quark in solving the hierarchy problem in SUSY models. This solution requires that the stop quark |
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be light, less than a few hundred GeV and hence within reach for direct pair production. We focus on two decay modes |
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\end{itemize} |
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Both of these signatures contain high transverse momentum (\pt) jets including two b-jets, and missing transverse |
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energy (\MET) due to the invisible $\chi^0_1$ lightest SUSY particles (LSP's). In addition, the presence of |
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energy (\met) due to the invisible $\chi^0_1$ lightest SUSY particles (LSP's). In addition, the presence of |
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two W bosons leads to a large branching fraction to the single lepton final state. Hence we require the presence |
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of exactly one isolated, high \pt electron or muon, which provides significant suppression of several backgrounds |
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of exactly one isolated, high \pt\ electron or muon, which provides significant suppression of several backgrounds |
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that are present in the all-hadronic channel. The largest backgrounds for this signature are semi-leptonic \ttbar\ |
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and \wjets. These backgrounds contain a single leptonically-decaying W boson, and the transverse mass (\mt) |
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of the lepton-neutrino system has a kinematic endpoint requiring \mt $<$ $M_W$. For signal stop quark events, |
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the presence additional LSP's in the final states allows the \mt to exceed $M_W$. Hence we search for an excess |
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of the lepton-neutrino system has a kinematic endpoint requiring \mt\ $<$ $M_W$. For signal stop quark events, |
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the presence of additional LSP's in the final states allows the \mt\ to exceed $M_W$. Hence we search for an excess |
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of events with large \mt. The dominant background in this kinematic region is dilepton \ttbar\ where one of the |
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leptons is not identified, since the presence of two neutrinos from leptonically-decaying W bosons allows the |
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\mt\ to exceed $M_W$. Backgrounds are estimated from Monte Carlo (MC) simulation, with careful validation |
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and determination of scale factors and corresponding uncertainties based on data control samples. |
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and determination of scale factors (where necessary) and corresponding uncertainties based on data control samples. |
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The expected stop quark pair production cross section (see Fig.~\ref{fig:stopxsec}) varies between O(10) pb |
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for $m_{\tilde{t}}=200$~GeV and O(0.01) pb for $m_{\tilde{t}}=500$~GeV. The critical challenge of this analysis |
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is due to the fact that for light stop quarks with a mass close to the top quark, the production cross section is |
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large but the kinematic distributions, in particular \mt, are very similar to SM \ttbar\ production, such that it becomes very |
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The expected stop quark pair production cross section (see Fig.~\ref{fig:stopxsec}) varies between 18 pb |
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for $m_{\tilde{t}}=200$~GeV and 0.09 pb for $m_{\tilde{t}}=500$~GeV~\cite{xsec0}. |
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The critical challenge of this analysis |
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is due to the fact that for light stop quarks ($m_{\tilde{t}} \approx m_t$), the production cross section is |
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large but the kinematic distributions, in particular \mt, are very similar to SM \ttbar\ production. In this regime |
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it becomes very |
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difficult to distinguish the signal and background. For large stop quark mass the kinematic distributions differ |
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from those in SM \ttbar\ production, but the cross section decreases rapidly, reducing the signal-to-background |
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ratio. |
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\begin{figure}[hbt] |
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\begin{center} |
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\includegraphics[width=0.4\linewidth]{plots/stop.pdf} |
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% \includegraphics[width=0.5\linewidth]{plots/total_scale_pdf_LHC-eps-converted-to.pdf} |
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\includegraphics[width=0.5\linewidth]{plots/stop_cs.png} |
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\caption{ |
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\label{fig:stopxsec}\protect |
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The stop quark pair production cross section in pb, as a function of the stop quark mass.} |
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The stop quark pair production cross section as a |
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function of the stop quark mass~\cite{xsec}.} |
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\end{center} |
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\end{figure} |
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