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\section{Search in the Same-sign Dilepton Final State} |
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\label{sec:ss} |
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A wide variety of new physics processes may produce events with two same-sign (SS) leptons, which provides a very clean |
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final state due to low SM background expectations. In particular, this final state is sensitive to top |
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squarks produced in the decays of gluinos (Fig.~\ref{fig:diagrams}X) and to the pair production of bottom squarks |
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followed by the decay $\tilde{b}\to t \chi^{\pm}$. |
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\begin{figure*}[!ht] |
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\centering |
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%\begin{center} |
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\begin{tabular}{cc} |
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\subfloat[] {\includegraphics[width=0.5\textwidth]{HCPPlots/SS_B1.pdf}} & |
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\subfloat[] {\includegraphics[width=0.4\textwidth]{HCPPlots/T2bb_interpretation.pdf}} \\ |
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\end{tabular} |
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\caption{ |
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Interpretation of the results of the search in (a) the same-sign dilepton final state for |
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bottom squark pair production with $\tilde{b}\to t\chim$ depicted in Fig.~\ref{fig:diagrams}(d), and (b) |
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the all-hadronic final state for bottom squark pair production with $\tilde{b}\to b\lsp$ depicted in Fig.~\ref{fig:diagrams}(c). |
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\label{fig:ss_interpretation} |
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} |
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%\end{center} |
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\end{figure*} |
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This section presents a search in the same-sign (SS) dilepton final state, based on 10.5 fb$^{-1}$. |
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A wide variety of new physics scenarios may produce events with SS leptons, which provide a very clean |
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final state due to low SM background expectations. In particular, this final state is sensitive to |
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direct pair production of bottom squarks with $\tilde{b}\to t \chim \to t W \lsp$ depicted in Fig.~\ref{fig:diagrams}(d), |
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as well as to gluino-mediated production of top and bottom squarks. |
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We select events with two leptons (e or $\mu$) with \pt\ $>$ 20 GeV and dilepton invariant mass $m_{\ell\ell}>8$ GeV. |
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We reject events with a third lepton with \pt\ $>$ 10 GeV that forms an opposite-sign |
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This analysis is an extension of a previous search in the same-sign dilepton final state~\cite{ref:ss_inclusive}. |
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In that analysis, the background is dominated by \ttljets\ where one lepton is from the W decay and the other |
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lepton is produced in the decay of one of the b-jets. In this analysis we require the presence of at least two |
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b-tagged jets. The requirement that both b-jets are identified and well separated from the selected leptons |
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b-tagged jets (using CSVM). The requirement that both b-jets are identified and well-separated from the selected leptons |
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reduces the \ttljets\ background by an order of magnitude. |
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There are three sources of SM background. |
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The first background source is referred to as ``fake leptons'' and includes leptons from heavy-flavor decay, misidentified hadrons, |
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muons from meson decay in flight, or electrons from unidentified photon conversions. |
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This background is estimated from a sample of events with at least one lepton that passes a loose selection but fails the full analysis |
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identification and isolation requirements, using the probability for a fake lepton satisfying the loose selection to also pass the analysis |
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selection, which is determined based on studies of fake leptons in jet events. |
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The second background, estimated from MC, consists of rare SM processes and is dominated by $t\bar{t}$W and $t\bar{t}$Z. |
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The systematic uncertainty on both the fake lepton and rare backgrounds is 50\%. |
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A third, small background contributions is from ``charge flips'' and consists of events with opposite-sign leptons where one of the leptons |
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is an electron whose charge is misreconstructed. This background is based on the MC prediction, which is validated using a sample of Z$\to e^+e^-$ events. |
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Signal regions are defined by placing additional requirements on the jet multiplicity, b-tagged jet multiplicity, \met, and $H_T$, defined as the scalar |
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sum of the transverse momenta of selected jets. The observed data yields in these signal regions is compared to the SM background expectations in |
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Table~\ref{tab:ss}. Good agreement is observed between the data and the expected background in all signal regions. |
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The results are interpreted in the context of the model of gluino-mediated top squark production and bottom squark production indicated in |
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Fig.~\ref{fig:diagrams}b and Fig.~\ref{fig:diagrams}d, respectively, in Fig.~\ref{fig:ss_interpretation}. |
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For both of these models, the most sensitive signal region is SR6 (see Table~\ref{tab:ss}). |
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Results are presented in the plane |
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of squark mass vs. gluino mass, for a fixed choice of \lsp\ and \chip\ masses, and represent lower limits on the gluino mass of approximately 1 TeV in these scenarios. |
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Additional interpretations for other models and for different choices of \lsp\ and \chip\ masses are presented in Ref.~\cite{ref:ss}. |
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\begin{figure*} |
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\centering |
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%\begin{center} |
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\begin{tabular}{cc} |
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\includegraphics[width=0.45\textwidth]{HCPPlots/SS_A2.pdf} & |
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\includegraphics[width=0.45\textwidth]{HCPPlots/SS_B2.pdf} \\ |
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\end{tabular} |
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\caption{ |
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Interpretation of the results of the search in the same-sign dilepton final state in the model of gluino-mediated top squark production |
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of Fig.~\ref{fig:diagrams}b (left) and gluino-mediated bottom squark production of Fig.~\ref{fig:diagrams}d (right). |
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\label{fig:ss_interpretation} |
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} |
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%\end{center} |
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\end{figure*} |
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There are three sources of SM background passing the above preselection. |
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The first background source is referred to as ``fake leptons'' and includes leptons from heavy-flavor decay, |
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misidentified hadrons, muons from meson decay in flight, or electrons from unidentified photon conversions. |
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This background is estimated from a sample of events with at least one lepton that passes a loose selection |
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but fails the full analysis identification and isolation requirements. This sample is weighted by the probability |
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for a fake lepton satisfying the loose selection to also pass the analysis selection, which is determined based |
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on studies of fake |
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leptons in jet events. The second background, estimated from MC, consists of rare SM processes and is dominated |
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by $t\bar{t}$W and $t\bar{t}$Z. The systematic uncertainty on both the fake lepton and rare backgrounds is 50\%. |
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A third, small background contribution is from ``charge flips'' and consists of events with opposite-sign (OS) leptons |
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where one of the leptons is an electron whose charge is misreconstructed. |
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This background is estimated from an OS dilepton data sample, weighted by the electron charge misreconstruction probability, |
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which is extracted from studies of Z$\to$ee events in data and MC. |
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%This background is based on MC predictions, which are validated using a sample of Z$\to e^+e^-$ events. |
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Several signal regions are defined by placing additional requirements on the jet multiplicity, b-tagged jet multiplicity, |
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\met, and $H_T$, defined as the scalar sum of the transverse momenta of selected jets. |
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The observed data yields in all signal regions are in good agreement with the SM background expectations; |
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see Ref.~\cite{ref:ss} for the full quantitative results. A signal region that is sensitive to |
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bottom squark pair production with $\tilde{b}\to t \chim$ depicted in Fig.~\ref{fig:diagrams}(d) |
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has at least four jets, \met\ $>$ 120 GeV and |
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$H_T$ $>$ 200 GeV. In this region we observe 1 event in data and predict 2.22 $\pm$ 0.96 events. |
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The results in all signal regions are used to exclude a region of the model parameter space in Fig.~\ref{fig:ss_interpretation}(a), |
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which demonstrates that our search probes bottom squarks with masses up to 450 GeV. |
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Naturalness considerations favor a bottom squark with mass not exceeding 500--700 GeV, similar to the constraint |
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on the top squark mass. |
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Several additional interpretations for models with gluino-mediated top and |
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bottom squark production are presented in Ref.~\cite{ref:ss}. |
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|
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%\input{ss_table.tex} |
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%compared to the SM background expectations in |
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%Table~\ref{tab:ss}. Good agreement is observed between the data and the expected background in all signal regions. |
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%\begin{table} |
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%\centering |
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%\vspace*{5cm} % with the correct table height |
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%\end{table} |
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\begin{table*} |
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\centering |
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\caption{\label{tab:ss} Summary of the results of the search in the same-sign dilepton final state. |
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Several signal regions (SR) are indicated, including the kinematic requirements, the prediction of the three background (BG) contributions, |
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the total background, and the observed yield in data. The jet multiplicity requirement in the first row includes both b-tagged and untagged jets.} |
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\tabcolsep 2.7pt |
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\begin{scriptsize} |
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\begin{tabular}{l|c|c|c|c|c|c|c|c|c} |
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\hline |
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\hline |
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& SR0 & SR1 & SR2 & SR3 & SR4 & SR5 & SR6 & SR7 & SR8 \\ |
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\hline |
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No. of jets & $\geq 2$ & $\geq 2$ & $\geq 2$ & $\geq 4$ & $\geq 4$ & $\geq 4$ & $\geq 4$ & $\geq 3$ & $\geq 4$ \\ |
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No. of btags & $\geq 2$ & $\geq 2$ & $\geq 2$ & $\geq 2$ & $\geq 2$ & $\geq 2$ & $\geq 2$ & $\geq 3$ & $\geq 2$ \\ |
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Lepton charges & $++/--$ & $++/--$ & $++$ & $++/--$ & $++/--$ & $++/--$ & $++/--$ & $++/--$ & $++/--$ \\ |
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\met & $> 0$ GeV & $> 30$ GeV & $> 30$ GeV & $> 120$ GeV & $> 50$ GeV & $> 50$ GeV & $> 120$ GeV & $> 50$ GeV & $> 0$ GeV \\ |
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$H_T$ & $> 80$ GeV & $> 80$ GeV & $> 80$ GeV & $> 200$ GeV & $> 200$ GeV & $> 320$ GeV & $> 320$ GeV & $> 200$ GeV & $> 320$ GeV \\ |
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\hline |
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Charge-flip BG & $3.35 \pm 0.67$ & $2.70 \pm 0.54$ & $1.35 \pm 0.27$ & $0.04 \pm 0.01$ & $0.21 \pm 0.05$ & $0.14 \pm 0.03$ & $0.04 \pm 0.01$ & $0.03 \pm 0.01$ & $0.21 \pm 0.05$\\ |
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Fake BG & $24.77 \pm 12.62$ & $19.18 \pm 9.83$ & $9.59 \pm 5.02$ & $0.99 \pm 0.69$ & $4.51 \pm 2.85$ & $2.88 \pm 1.69$ & $0.67 \pm 0.48$ & $0.71 \pm 0.47$ & $4.39 \pm 2.64$ \\ |
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Rare SM BG & $11.75 \pm 5.89$ & $10.46 \pm 5.25$ & $6.73 \pm 3.39$ & $1.18 \pm 0.67$ & $3.35 \pm 1.84$ & $2.66 \pm 1.47$ & $1.02 \pm 0.60$ & $0.44 \pm 0.39$ & $3.50 \pm 1.92$ \\ |
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\hline |
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Total BG & $39.87 \pm 13.94$ & $32.34 \pm 11.16$ & $17.67 \pm 6.06$ & $2.22 \pm 0.96$ & $8.07 \pm 3.39$ & $5.67 \pm 2.24$ & $1.73 \pm 0.77$ & $1.18 \pm 0.61$ & $8.11 \pm 3.26$ \\ |
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Event yield & 43 & 38 & 14 & 1 & 10 & 7 & 1 & 1 & 9 \\ |
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\hline |
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% $N_{{UL}}$ (13\% unc.) & 27.2 &26.0 &9.9 &3.6 &10.8 &8.6 &3.6 &3.7 &9.6 \\ |
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% $N_{{UL}}$ (20\% unc.) & 28.2 &27.2 &10.2 &3.6 &11.2 &8.9 &3.7 &3.8 &9.9 \\ |
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% $N_{{UL}}$ (30\% unc.) & 30.4 &29.6 &10.7 &3.8 &12.0 &9.6 &3.9 &4.0 &10.5 \\ |
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\hline |
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\end{tabular} |
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\end{scriptsize} |
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\end{table*} |
