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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 mass difference between the top squark and the LSP, |
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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\ 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, followed be either of these decays, leads to events with 2 b-jets, 2 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. |
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The presence of two W bosons leads to a significant branching fraction to the single lepton final state, |
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which we focus on here because it has smaller SM backgrounds than the all-hadronic final state. |
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The analysis strategy is thus to 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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\subsection{Event Selection} |
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We require the presence of exactly one well-identified and isolated lepton (e or $\mu$) with transverse |
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momentum \pt\ $>$ 30 GeV and $|\eta|<1.44$ ($|\eta|<2.1$) for electrons (muons). |
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We select events with at least four jets with \pt\ $>$ 30 GeV and $|\eta|<2.5$, |
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which must be separated by $\Delta R \equiv \sqrt{(\Delta\eta)^2+(\Delta\phi)^2} < 0.4$ from the selected leptons. |
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At least one of these jets is required to be consistent with coming from the decay of heavy flavor hadrons, as |
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identified by the Combined Secondary Vertex Medium Point (CSVM) b-tagging algorithm~\cite{ref:btag}. |
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The (b-)jet requirements suppress SM backgrounds from W bosons produced in association with jets from initial state |
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to have a large branching fraction. If there is a light chargino \chipm, the decay |
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$\tilde{t}\to b\chip\to b W \lsp$ may also 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 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 here 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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|
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%\subsection{Event Selection} |
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|
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We require the presence of exactly one well-identified and isolated electron (e) or muon ($\mu$) with transverse |
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momentum \pt\ $>$ 30 GeV. |
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We select events with at least four jets with \pt\ $>$ 30 GeV, |
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which must be well-separated from the selected leptons. |
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At least one of these jets is required to be consistent with coming from the decay of a heavy flavor hadron, as |
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identified by the Combined Secondary Vertex medium working point (CSVM) b-tagging algorithm~\cite{ref:btag}. |
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The jet requirements suppress SM backgrounds from W bosons produced in association with jets from initial state |
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radiation (ISR), referred to as the \wjets\ background. |
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The \met\ is required to exceed 50 GeV, suppressing the background from QCD multijet production. |
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\subsection{Backgrounds and Estimation Strategy} |
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%\subsection{Backgrounds and Estimation Strategy} |
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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) elec- |
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troweak 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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We define signal regions by requiring the events to have large transverse mass, defined as |
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To define signal regions, we require the events to have large transverse mass, defined as: |
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\begin{equation} |
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M_T = \sqrt{ 2 p_{T}^{\ell} \met ( 1-cos(\Delta\phi))}, |
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\end{equation} |
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where $p_{T}^{\ell}$ is the lepton transverse momentum and $\Delta\phi$ is the difference in azimuthal angles between the lepton |
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and \met. This requirement strongly suppresses the background from \ttljets\ and \wjets. For both of these background sources, the lepton |
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and neutrino which gives rise to the \met\ are produced together in the 2-body decay of the W, leading to the kinematic |
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endpoint \mt\ $<$ $M_W$. For signal events, as well as the \ttll\ background, the presence of more than one invisible |
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particle in the final state leads to events with \mt\ $>$ $M_W$. In addition to the \mt\ requirement, we make several |
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and \met. This requirement strongly suppresses the background from \ttljets\ and \wjets, which have a kinematic endpoint |
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at \mt\ $\approx$ $M_W$ since the lepton and neutrino (which produces the \met) are produced together in the decay of the W. |
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For signal events, as well as for the \ttll\ background, the presence of more than one invisible |
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particle in the final state leads to events with \mt\ $>>$ $M_W$. |
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In addition to the \mt\ requirement, we make several |
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\met\ requirements to achieve sensitivity to signals with different mass spectra. |
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Signal regions with a large \met\ requirement are more sensitive to signals with large $\Delta m$. |
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Signal regions with large (small) \met\ requirements are more sensitive to signals with large (small) values of $\Delta m$. |
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The dominant background in our signal regions is \ttll, which may produce events with large \met\ and \mt\ due to the presence of |
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two invisible neutrinos. In order for \ttll\ events to pass the signal region selection, one of the two W leptons must not be identified, |
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which occurs if one of the leptons is outside the acceptance, is a hadronic $\tau$ decaying to three charged particles (3-prong decay), |
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more than one invisible neutrino. In order for \ttll\ events to pass the signal region selection, one of the two W leptons must not be identified, |
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which occurs if it is outside the acceptance, is a hadronic $\tau$ decaying to three charged particles (3-prong decay), |
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is a hadronic $\tau$ decaying to a single charged particle (1-prong decay), or is an electron or muon that fails the lepton identification |
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requirements. The latter two categories are suppressed by vetoing events that contain, in addition to the selected lepton, a charged particle |
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with \pt\ $>$ 10 GeV that is isolated in space from other energetic charged particles. Furthermore, additional jets from initial state or |
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final state radiation (ISR/FSR) are required to satisfy the jet multiplicity requirement $n_{jets}\geq4$. To validate and correct the MC modeling |
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of jets from radiation, the MC is compared to data in a control dominated by \ttll, obtained by selecting events with two reconstructed leptons, |
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moderate \met, and at least one b-jet. The MC distribution of $n_{jets}$ is reweighted to match the corresponding data distribution, resulting |
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in correction factors of a few \%. |
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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 dilepton control region dominated by \ttll. |
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The MC distribution of $n_{jets}$ is reweighted to match the corresponding data distribution, resulting in small corrections of a few \%. |
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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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(if necessary) corrected using comparisons to data in control regions. The MC expectation is normalized to data in the \mt\ peak region, |
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in order to remove systematic uncertainties from integrated luminosity and $t\bar{t}$ cross section, and then extrapolated to the large \mt\ region. |
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Correction factors and corresponding systematic uncertainties on the MC extrapolation factors are evaluated by comparing MC to data in dedicated |
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control regions dominated by \wjets\ (obtained by vetoing events with b-jets), \ttll\ (obtained by requiring two selected leptons), |
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and a mixture of \ttll\ and \ttljets\ (obtained by requiring a selected lepton and an isolated track). |
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(where necessary) corrected using comparisons to data in control regions. The MC expectation is normalized to data in the \mt\ peak region, |
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in order to remove systematic uncertainties from integrated luminosity and $t\bar{t}$ cross section, and then extrapolated to the |
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large \mt\ region. Correction factors and corresponding systematic uncertainties on the MC extrapolation factors are evaluated by |
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comparing MC to data in dedicated control regions dominated by \wjets\ (obtained by vetoing events with b-jets), \ttll\ |
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(obtained by requiring two selected leptons), and a mixture of \ttll\ and \ttljets\ (obtained by requiring a selected lepton and |
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an isolated track). The dominant systematic uncertainty in the background prediction is due to the limited statistical precision in |
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the data control samples used for these tests. |
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\input{results_table.tex} |
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\subsection{Results} |
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%\subsection{Results} |
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The results of the search are summarized in Table~\ref{tab:stop}, which displays the SM background expectations and the observed data yields |
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in the signal regions. The distribution of \met\ after the requirement \mt\ $>$ 120 GeV for the SM background expectations is compared to |
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data in Fig.~\ref{fig:stop}. Good agreement between the data and the expected background is observed, and we thus do not find evidence |
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for the production of top squarks. |
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in the signal regions. The distribution of \met\ after the requirement \mt\ $>$ 120 GeV is displayed in Fig.~\ref{fig:stop}. |
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Good agreement between the data and the expected background is observed. We find no evidence |
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for the pair production of top squarks. |
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\begin{figure} |
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% Use the relevant command for your figure-insertion program |
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% to insert the figure file. |
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\centering |
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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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\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\ distribution in data, compared to the sum of expected backgrounds, for the top squark pair search. |
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Two example signal models with $m_{\tilde{t}}=450$ GeV and $m_{\lsp}=50$ GeV are also indicated. For the $\tilde{t}\to b\chip$ |
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decay, the chargino mass is set by $m_{\chipm}=0.75~m_{\tilde{t}}+0.25~m_{\lsp}$.} |
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\label{fig:stop} % Give a unique label |
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\end{figure} |
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\subsection{Interpretation} |
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%\subsection{Interpretation} |
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|
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%To interpret the results of our search, we consider two signal scenarios of top squark pair production, followed by the decays |
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%$\tilde{t}\to t\lsp$ and $\tilde{t}\to b\chip\to b W \lsp$. In the first scenario, the only SUSY particles which participate |
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%the chargino mass is also relevant, and we introduce a third parameter $x$, defined as $m_{\chip} = x m_{\lsp} + (1-x) m_{\tilde{t}}$. |
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%We consider $x=0.5$ and $x=0.75$ (we do not have sensitivity to the $x=0.25$ scenario). |
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|
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To interpret the results of our search, we consider top squark pair production where both top squarks decay according to $\tilde{t}\to t\lsp$ |
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(Fig.~\ref{fig:stop_interpretation}). |
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The model is parameterized by the masses of the top squark ($m_{\tilde{top}}$) and \lsp ($m_{\lsp}$. We place upper limits on the signal |
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To interpret the results of our search, we consider top squark pair production where both top squarks decay according to |
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$\tilde{t}\to t\lsp$, in Fig.~\ref{fig:stop_interpretation}. |
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The model is parameterized by the masses of the top squark and \lsp. We place upper limits on the signal |
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production cross section using, for each model point in the 2-dimensional parameter space, the signal region with the best expected |
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sensitivity. A region of the parameter space is excluded by comparing these cross section upper limits with the theoretical predictions |
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for the signal cross section, computed at next-to-leading order including the resummation of soft gluon emission at next-to-leading-logarithmic |
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accuracy (NLO+NLL)~\cite{ref:nlonll}. 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 $\tilde{t}\to b\chip\to b W \lsp$, see Ref.~\cite{ref:stop}. |
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|
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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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right-handed top quark polarization, while we take the top quark in the $\tilde{t}\to t\lsp$ decay to be unpolarized, |
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resulting in a lower signal selection efficiency in our analysis. |
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for the signal cross section. |
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%, computed at next-to-leading order including the resummation of soft gluon emission at |
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%next-to-leading-logarithmic |
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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 favors top squarks with masses not exceeding 500--700 GeV~\cite{ref:naturalsusy}. |
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We also interpret our results in the $\tilde{t}\to b\chip\to b W \lsp$ scenario |
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depicted in Fig.~\ref{fig:diagrams}(b), probing top squarks with masses up to 420 GeV~\cite{ref:stop}. |
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|
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%The ATLAS experiment has presented a similar search for top squark pairs~\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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%right-handed top quark polarization, while we take the top quark in the $\tilde{t}\to t\lsp$ decay to be unpolarized, |
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%resulting in a lower signal selection efficiency in our analysis. |
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|
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\begin{figure} |
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% Use the relevant command for your figure-insertion program |
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% to insert the figure file. |
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\centering |
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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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\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 of |
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Fig.~\ref{fig:diagrams}(a). The color scale indicates the cross section upper limits at 95\% confidence level. 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 |
