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\section{Selection} |
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\label{sec:eventSelection} |
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The physics object selections (leptons, jets, and \MET) are identical to the ones |
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used in the nominal analysis~\cite{ref:osznote}, with one exception. |
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We have updated the jet energy corrections (JEC) using the official recipe. |
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The global tags GR\_R\_42\_V23 (DESIGN42\_V17) are used for data (MC). |
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We use L1FastL2L3Residual (L1FastL2L3) corrections for data (MC). |
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The nominal analysis preselection \cite{ref:osznote} consists of the following requirements: |
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The preselection sample is based on the following criteria |
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\begin{itemize} |
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\item At least 2 \pt\ $>$ 20 GeV leptons (e or $\mu$) passing the analysis identification and isolation requirements |
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\item Number of jets \njets\ $\geq$ 2 |
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\item For signal events, we require the leptons to have the same flavor (SF) (ee or $\mu\mu$) and to have an invariant |
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mass consistent with the Z mass, $81 < m_{\ell\ell} < 101$ GeV |
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\item Opposite flavor (OF) e$\mu$ events are used as a data control sample to predict the \ttbar\ background |
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\item satisfy the trigger requirement (see |
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Table.~\ref{tab:DatasetsData}) |
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\item select events with one high \pt\ electron or muon, requiring |
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\begin{itemize} |
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\item $\pt>30~\GeVc$ and $|\eta|<2.5(2.1)$ for \E(\M) |
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\item satisfy the identification and isolation requirements detailed |
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in the same-sign SUSY analysis (SUS-11-010) for electrons and the opposite-sign |
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SUSY analysis (SUS-11-011) for muons |
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\end{itemize} |
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\item require at least 4 PF jets in the event with $\pt>30~\GeV$ |
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within $|\eta|<2.5$, out of which at least 1 is b-tagged based on |
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the SSV medium working point. |
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\item require moderate $\met>50~\GeV$ |
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\end{itemize} |
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In addition, we apply the following requirements. |
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Currently, we focus on the muon channel because it is cleaner (the QCD contribution is negligible) |
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and the triggers are simpler (we use single muon triggers, as opposed to electron + 3-jet triggers). |
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We will add the electron channel, time permitting. However, since this is a systematics-dominated |
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analysis, increasing the statistics by adding the electrons is not expected to significantly improve |
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the sensitivity, especialy because the electron selection efficiency is smaller and the systematic |
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uncertainty associated with the QCD background is larger. |
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A benchmark signal region is selected by tightening the \met\ and |
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adding an \mt\ requirement |
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\begin{itemize} |
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\item We veto events containg any b jets which pass the \pt\ threshold for N jet counting (30 GeV) using the track-counting high efficiency algorithm~\cite{BTV11003}. |
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In order to reject as many real b jets as possible, we use the loose working point for jets with \pt\ $<$ 100 GeV. |
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Because the loose working point has a large mistag rate at high \pt, |
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we use the medium working point to tag jets with \pt\ $>$ 100 GeV. |
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The same b-jet veto is applied when selecting \gjets\ events for the \MET\ templates. |
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\item In order to select events containing W/Z $\rightarrow$ jets, |
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we require that the two leading jets have a dijet mass consistent |
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with W/Z decay. The window used is 70 to 110 GeV as motivated |
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by MC (see figure \ref{fig:djmass}). See also appendix |
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\ref{sec:djmass} for data-MC comparisons of this quantity. |
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\item In order to suppress background from WZ events where the W decays |
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leptonically, we veto events containing a third lepton with \pt $>$ 20 GeV passing |
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our signal lepton selection. |
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\item $\met>100~\GeV$ |
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\item $\mt>150~\GeV$ |
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\end{itemize} |
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All of the above requirements taken together are referred to as the ``preselection.'' |
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{\bf We have not looked at the data in the signal region after the first 1 fb$^{-1}$ of data.} |
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\subsection{Corrections to Jets and \met} |
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The official recommendations from the Jet/MET group are used for |
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the data and MC samples. In particular, the jet |
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energy corrections (JEC) are updated using the official recipe. |
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L1FastL2L3Residual (L1FastL2L3) corrections are applied for data (MC), |
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based on the global tags GR\_R\_42\_V23 (DESIGN42\_V17) for |
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data (MC). In addition, these jet energy corrections are propagated to |
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the \met\ calculation, following the official prescription for |
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deriving the Type I corrections. It may be noted that events with |
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anomalous ``rho'' pile-up corrections are excluded from the sample since these |
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correspond to events with unphysically large \met\ and \mt\ tail |
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signal region (see Figure~\ref{fig:mtrhocomp}). An additional correction to remove |
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the $\phi$-modulation observed in the \met\ is included, improving |
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the agreement between the data and the MC, as shown in |
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Figure~\ref{fig:metphicomp}. This correction has an effect on this analysis, |
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since the azimuthal angle enters the \mt\ distribution. |
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\clearpage |
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\begin{figure}[!ht] |
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\begin{center} |
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\includegraphics[width=0.5\linewidth]{plots/mt_rho_comp.png} |
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\caption{ \label{fig:mtrhocomp}%\protect |
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Comparison of the \mt\ distribution for events with |
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unphysical energy corrections ($\rho <0$ or $ \rho > 40$, where $\rho$ is a |
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measure of the average pileup energy density) and the |
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nominal sample. Events with large pileup corrections |
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correspond to noisy events. Since this correction is applied |
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to the jets and propagated to the \met, these events have |
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anomalously large \met\ and populate the \mt\ tail. These |
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pathological events are excluded from the analysis sample.} |
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\end{center} |
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\end{figure} |
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\begin{figure}[tbh] |
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\begin{figure}[!hb] |
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\begin{center} |
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\includegraphics[width=0.8\linewidth]{plots/djmass.png} |
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\caption{ \label{fig:djmass}\protect |
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Dijet mass distribution in MC. |
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} |
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\includegraphics[width=0.5\linewidth]{plots/metphi.pdf}% |
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\includegraphics[width=0.5\linewidth]{plots/metphi_phicorr.pdf} |
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\caption{ \label{fig:metphicomp}%\protect |
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The PF \met\ $\phi$ distribution (left) exhibits a |
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modulation. After applying a dedicated correction, the |
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azimuthal dependence is reduced (right).} |
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\end{center} |
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\end{figure} |
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\clearpage |
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\subsection{Branching Fraction Correction} |
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The leptonic branching fraction used in some of the \ttbar\ MC samples |
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differs from the value listed in the PDG $(10.80 \pm 0.09)\%$. |
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Table.~\ref{tab:wlepbf} summarizes the branching fractions used in |
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the generation of the various \ttbar\ MC samples. |
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For \ttbar\ samples with the incorrect leptonic branching fraction, event |
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weights are applied based on the number of true leptons and the ratio |
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of the corrected and incorrect branching fractions. |
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\begin{table}[!h] |
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\begin{center} |
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\begin{tabular}{c|c} |
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\hline |
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\ttbar\ Sample - Event Generator & Leptonic Branching Fraction\\ |
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\hline |
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\hline |
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Madgraph & 0.111\\ |
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MC@NLO & 0.111\\ |
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Pythia & 0.108\\ |
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Powheg & 0.108\\ |
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\hline |
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\end{tabular} |
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\caption{Leptonic branching fractions for the various \ttbar\ samples |
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used in the analysis. The primary \ttbar\ MC sample produced with |
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Madgraph has a branching fraction that is almost $3\%$ higher than |
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the PDG value. \label{tab:wlepbf}} |
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\end{center} |
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\end{table} |
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