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\section{Introduction} |
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\label{ref:intro} |
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The published analysis |
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%``A Search For New Physics in Z + Jets + MET using MET Templates'' %AN/old title |
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%arxiv title |
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``Search for physics beyond the standard model in events with a Z boson, jets, |
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and missing transverse energy in pp collisions at $\sqrt{s}$ = 7 TeV'' |
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(SUS-11-021) |
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searches for new physics in the final state of a |
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leptonically ($ee$ and $\mu\mu$) decaying Z boson, two or more jets and |
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missing transverse energy (\MET) |
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\cite{ref:oszpaper} \cite{ref:osznote} \cite{ref:oszpas}. |
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This analysis will be referred to throughout this note as the ``nominal'' |
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analysis. The basic analysis strategy is to select Z bosons and |
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use data-driven methods to predict the \MET\ distribution in the signal |
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regions. |
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The Z+Jets background is predicted using the |
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\MET\ templates method \cite{ref:templates1}\cite{ref:templates2}, the |
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\ttbar\ background is predicted using opposite flavor ($e\mu$) events, |
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and the diboson (WZ, ZZ) background is taken from Monte Carlo. |
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|
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The analysis presented in this note is a straightforward extension of |
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the nominal analysis in that the analysis strategy and methodology |
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remain unchanged. |
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The only changes with respect to the nominal analysis are the addition |
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of cuts to increase sensitivity to new physics with |
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diboson production (WZ and/or ZZ) and \MET . |
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|
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An example of one such new physics scenario is the electroweak |
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production of SUSY particles. |
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In a generic SUSY framework, the neutralinos |
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(for example, $\chi_2^0$ or $\chi_1^0$) |
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may decay to a Z boson and another neutral SUSY particle such as the LSP. |
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|
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%Although SUSY production involving strongly interacting particles |
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%(such as gluinos and squarks) is normally targeted due to its expected |
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%larger production cross section as compared with electroweak production, |
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%such searches have as of yet failed to discover new physics. |
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%Another logical search is for electroweak production, |
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%and in this case, a final state involving Z bosons is a natural place |
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%to start since leptonically decay Zs are an extremely clean signature. |
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|
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In the case in which a neutralino is pair produced, the final state |
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may be ZZ+\MET. In addition, production of a neutralino and chargino |
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may lead to a final state of WZ+\MET. |
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When the Z decays leptonically and the other boson (either |
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a W or Z) decays to jets, the final state is Z plus two jets plus \MET, |
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to which the nominal \MET\ templates analysis is particularly well suited. |
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Given that we are now searching for the specific final states WZ plus \MET\ or ZZ plus \MET , |
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rather than the more general Z plus jets plus \MET\ signature, |
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we can apply additional cuts to increase the sensitivity. |
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|
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In the nominal analysis, the search is performed in the high \MET\ tail. |
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\MET\ cuts used for signal regions are 100, 200, and 300 GeV. |
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At such high \MET\ cuts, \ttbar\ background in which (the same-flavor |
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opposite-sign) dileptons happen to fall in the Z mass window dominate. |
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Because all \ttbar\ events contain b jets, a b jet veto is |
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very effective in suppressing this background. |
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|
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Because the final state targeted involves the decay of W (Z) to jets, |
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the dijet mass peaks at the W (Z) mass. In contrast, the jets from the |
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background processes Z+jets and \ttbar\ have a very broad distribution. |
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The dijet mass is therefore a variable which can further discriminate between |
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signal and background (see section \ref{sec:eventSelection}). |
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|
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The final sub-leading backgrounds in the nominal analysis are Z plus jets |
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and dibosons (WZ and ZZ). In the case of WZ, real \MET\ is produced from |
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the leptonically decaying W. In order to suppress this background, we |
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introduce a third lepton veto. |
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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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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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$\tilde{t}\rightarrow t\chi^0_1$ and $\tilde{t}\rightarrow b \chi^+_1$ which are expected |
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to have large branching fractions if they are kinematically accessible, leading to: |
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In summary, we use the same selection as in the approved analysis SUS-11-021 and |
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place three additional requirements: |
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\begin{itemize} |
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\item Veto events containing a b-tagged jet |
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\item Require a dijet mass consistent with the hadronic decay of a W/Z boson |
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\item Veto events with a third lepton |
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\item $pp\rightarrow\tilde{t}\bar{\tilde{t}}\rightarrow t\bar{t}\chi^0_1\chi^0_1$, and |
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\item $pp\rightarrow\tilde{t}\bar{\tilde{t}}\rightarrow b\bar{b}\chi^+_1\chi^-_1 \rightarrow b\bar{b}W^+W^-\chi^0_1\chi^0_1$. |
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\end{itemize} |
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This note is organized as follows. |
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In Sec.~\ref{sec:datasets} we review the datasets used. |
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In Sec.~\ref{sec:eventSelection} we discuss the event selection. |
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In Sec.~\ref{sec:yields} we present the data and MC yields passing the event preselection. |
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In Sec.~\ref{sec:sigregion} we define the signal regions. |
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In Sec.~\ref{sec:results} we present the results. |
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In Sec.~\ref{sec:systematics} we discuss systematics on the background predictions. |
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In Sec.~\ref{sec:bsm} we provide a new physics interpretation of the results. |
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Additional material is included in the following appendices: |
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supplemental results (App. \ref{app:results}), |
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supplemental interpretation (App. \ref{sec:app_bsm}), |
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kinmatical distributions (App. \ref{sec:appkin}), |
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combination of interpretation results (App. \ref{app:combo}), |
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and the \MET\ templates (App. \ref{sec:appendix_templates}). |
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|
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% |
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|
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%OLD NOTE |
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|
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\begin{comment} |
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|
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In this note we describe a search for new physics in the 2011 |
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opposite sign isolated dilepton sample ($ee$, $e\mu$, and $\mu\mu$). |
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The main sources of high \pt isolated dileptons at CMS are Drell Yan and \ttbar. |
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Here we concentrate on dileptons with invariant mass consistent |
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with $Z \to ee$ and $Z \to \mu\mu$. A separate search for new physics in the non-\Z |
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sample is described in~\cite{ref:GenericOS}. |
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|
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We search for new physics in the final state of \Z plus two or more jets plus missing |
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transverse energy (MET). We reconstruct the \Z boson |
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in its decay to $e^+e^-$ or $\mu^+\mu^-$. Our search regions are defined as |
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MET $\ge$ \signalmetl~GeV (loose signal region), MET $\ge$ \signalmett~GeV |
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(medium signal region), MET $\ge$ 300~GeV (tight signal region), and two or more jets. We use data driven techniques to predict the |
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standard model background in these search regions. |
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Contributions from Drell-Yan production combined with detector mis-measurements that |
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produce fake MET are modeled via MET templates based on photon plus jets or QCD events. |
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Top pair production backgrounds, as well as other backgrounds for which the lepton |
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flavors are uncorrelated such as $W^+W^-$, DY$\rightarrow\tau\tau$, and single top, are |
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modeled via $e^\pm\mu^\mp$ subtraction. |
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|
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As leptonically decaying \Z bosons are a signature that has very little background, |
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they provide a clean final state in which to search for new physics. |
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Because new physics is expected to be connected to the Standard Model Electroweak sector, |
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it is likely that new particles will couple to W and Z bosons. |
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For example, in mSUGRA, low $M_{1/2}$ can lead to a significant branching fraction |
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for $\chi_2^0 \rightarrow Z \chi_1^0$. |
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In addition, we are motivated by the existence of dark matter to search for new physics with MET. |
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Enhanced MET is a feature of many new physics scenarios, and R-parity conserving SUSY |
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again provides a popular example. The main challenge of this search is therefore to |
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understand the tail of the fake MET distribution in \Z plus jets events. |
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|
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The basic idea of the MET template method~\cite{ref:templates1}\cite{ref:templates2} is |
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to measure the MET distribution in data in a control sample which has no true MET |
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and a similar topology to the signal events. |
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%Start the qcd vs photon discussion |
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Templates can be derived from either a QCD sample (as was done in the original implementation) |
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or a photon plus jets sample. |
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%In our case, we choose a photon sample with two or more jets as the control sample. |
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%Both the control sample and signal sample consist of a well measured object (either a |
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%photon or a leptonically decaying $Z$), which recoils against a system of hadronic jets. |
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In both cases, the instrumental MET is dominated by mismeasurements of the hadronic system, |
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and can be classified by the number of jets in the event and the scalar sum of their transverse |
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momenta. |
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The prediction is made such that the jet system in the control sample is similar to that of the |
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signal sample. |
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By using two independent control samples--QCD and photon plus jets--we are able to illustrate |
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the robustness of the MET templates method and to cross check the data driven background |
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prediction. |
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|
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This note is organized as follows. |
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In sections \ref{sec:datasets} and \ref{sec:trigSel} we descibe |
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the datasets and triggers used, followed by the detailed object definitions (electrons, muons, photons, |
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jets, MET) and event selections in sections \ref{sec:evtsel} through \ref{sec:jetsel}. |
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We define a preselection and compare data vs. MC yields passing this preselection in |
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Section~\ref{sec:yields}. |
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We then define the signal regions and show the number of observed events and MC expected |
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yields in Section~\ref{sec:sigregion}. |
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Section~\ref{sec:templates} introduces the MET template method and discusses its derivation |
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in some detail. |
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% and is followed by a demonstration in Section~\ref{sec:mc} |
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%that the method works in Monte Carlo. |
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Section~\ref{sec:topbkg} introduces the top background estimate based on opposite flavor subtraction, |
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and contributions from other backgrounds are discussed in Section~\ref{sec:othBG}. |
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Section~\ref{sec:results_combined} shows the results for applying these methods in data. |
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We analyze the systematic uncertainties in the background predictions and in signal acceptance |
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in Section~\ref{app:systematics}. |
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We then proceed to calculate upper limit on the BSM physics processes |
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in Section~\ref{sec:bsm}. |
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%Efficiencies which can be used to test specific models of new physics are given |
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%in Section \ref{sec:outreach}. |
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%Finally, in Section~\ref{sec:models} we calculate upper limits on the quantity \sta\ |
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%assuming efficiencies and uncertainties from sample benchmark SUSY processes. |
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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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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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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 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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|
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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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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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|
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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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\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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\end{center} |
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\end{figure} |
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\end{comment} |
