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\section{Search for Direct Top Squark Pair Production in the Single Lepton Final State} |
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\section{Search for Top Squark Pair Production in the Single Lepton Final State} |
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\label{sec:stop} |
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This section presents the results of a dedicated search for the direct pair production of top squarks, based on an integrated luminosity of 9.7~fb$^{-1}$. |
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The decay of the top squark depends on the difference between its mass and that of the LSP, |
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The decay of the top squark depends on the difference between its mass and that of the \lsp\ LSP, |
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$\Delta m = m_{\tilde{t}}-m_{\lsp}$. If $\Delta m > m_{t}$, the decay $\tilde{t}\to t\lsp$ is expected |
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to have a large branching fraction. If there is a light chargino \chip, the decay |
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$\tilde{t}\to b\chip\to b W \lsp$ is expected to be significant, especially in the $\Delta m < m_{t}$ region. |
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The pair production of top squarks decaying to either of these channels leads to events with two b-jets, two W bosons, |
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and \met\ from the invisible LSPs. Our signal thus resembles SM $t\bar{t}$ production but with larger \met\ from |
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and two LSPs. Our signal thus resembles SM $t\bar{t}$ production but with larger \met\ from |
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the invisible LSPs. |
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We focus on the single lepton final state, which has a significant branching fraction due to the two W bosons, |
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We focus on the single lepton final state, which has a significant branching fraction due to the presence of two W bosons |
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in the final state, |
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and smaller SM backgrounds than the all-hadronic final state. |
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We thus select events with a single lepton and jets and discriminate between |
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signal and background using \met\ and the transverse mass \mt, discussed below. |
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The SM background satisfying the above requirements is dominated by $t\bar{t}$ production where |
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one W boson decays hadronically and the other leptonically (\ttljets), or where both W bosons decay leptonically (\ttll). |
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There is a small contribution from \wjets, as well as a variety of SM |
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processes with small cross section, including $t\bar{t}$ produced in association with a vector boson |
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($t\bar{t}W$, $t\bar{t}Z$, $t\bar{t}\gamma$), processes with two (WW, WZ, ZZ) and three (WWW, WWZ, WZZ, ZZZ) electroweak |
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vector bosons, and single top production in the tW-channel mode (these small contributions are collectively |
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referred to below as the ``rare'' background). |
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There is a small contribution from \wjets, as well as a variety of rare SM |
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processes, dominated by $t\bar{t}$ produced in association with a vector boson |
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($t\bar{t}W$ and $t\bar{t}Z$). |
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To define signal regions, we require the events to have large transverse mass, defined as: |
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requirements. The latter two categories are suppressed by vetoing events that contain, in addition to the selected lepton, |
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a charged particle with \pt\ $>$ 10 GeV that is isolated in space from other energetic charged particles. Furthermore, additional jets |
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from initial state or final state radiation (ISR/FSR) are required to satisfy the jet multiplicity requirement $n_{jets}\geq4$. |
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To validate and correct the MC modeling of jets from radiation, the MC is compared to data in a control region dominated by \ttll, obtained |
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by selecting events with two analysis leptons, moderate \met, and at least one b-jet. The MC distribution of $n_{jets}$ is reweighted to |
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To validate and correct the MC modeling of jets from radiation, the MC is compared to data in a dilepton control region dominated by \ttll. The MC distribution of $n_{jets}$ is reweighted to |
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match the corresponding data distribution, resulting in corrections of (1--7)\%. |
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The SM backgrounds are estimated from events simulated with Monte Carlo (MC) techniques, which are validated and |
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\centering |
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\includegraphics[width=0.4\textwidth]{HCPPlots/stopmet.pdf} |
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%\includegraphics[width=7cm,clip]{HCPPlots/stopmet.pdf} |
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\caption{The \met\ distributions data and expected backgrounds for the top squark pair search. The data is compared to the sum of the |
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expected backgrounds. The \met\ distributions expected in two example signal scenarios are indicated. The numbers in parentheses |
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indicate the top squark mass, the LSP mass, and the chargino mass parameter $x$ defined in the text.} |
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\caption{The \met\ distribution in data, compared to the sum of expected backgrounds, for the top squark pair search. |
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Two example signal models are also indicated.} |
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\label{fig:stop} % Give a unique label |
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\end{figure} |
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%accuracy (NLO+NLL)~\cite{ref:nlonll}. |
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Our results probe top squarks with masses up to 430 GeV. For comparison, the requirement that SUSY |
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provides a natural solution to the hierarchy problem suggests top squarks with masses not exceeding 500--700 GeV~\cite{ref:naturalsusy}. |
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We also interpret our results assuming the top squark decays according to $\tilde{t}\to b\chip\to b W \lsp$ |
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depicted in Fig.~\ref{fig:diagrams}(b)~\cite{ref:stop}. |
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We also interpret our results assuming the top squark decays according to $\tilde{t}\to b\chip\to b W \lsp$, |
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as depicted in Fig.~\ref{fig:diagrams}(b)~\cite{ref:stop}. |
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The ATLAS experiment has presented a similar search for top squark pairs in the single lepton final state~\cite{ref:atlasstop}. |
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The constraints from ATLAS on the top squark mass are more stringent than those presented here. The ATLAS model assumes large |
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\centering |
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\includegraphics[width=0.5\textwidth]{HCPPlots/stop_interpretation.pdf} |
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%\includegraphics[width=7cm,clip]{HCPPlots/stop_interpretation.pdf} |
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\caption{Interpretation of the results of the top squark pair search in the $\tilde{t}\to t\lsp$ scenario. The color scale indicates the |
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cross section upper limits. The solid black contour and dashed black contours indicate the observed excluded region and variation in this |
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\caption{Interpretation of the results of the top squark pair search in the $\tilde{t}\to t\lsp$ scenario of |
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Fig.~\ref{fig:diagrams}(a). The color scale indicates the cross section upper limits. The solid black contour |
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and dashed black contours indicate the observed excluded region and variation in this |
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excluded region due to the $\pm1\sigma$ uncertainties in the theoretical prediction of the signal cross section. The dashed blue |
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and dotted blue contours indicate the median and $\pm1\sigma$ expected excluded regions. } |
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\label{fig:stop_interpretation} % Give a unique label |
