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\section{Same-flavour Dilepton Search}
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\label{sec:HT}
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The result of Section~\ref{sec:results} is cross-checked in a similar kinematic region with an
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independent
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search relying on a different trigger path, different methods for ``physics object'' reconstruction, and a
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different background estimation method.
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This search is directed at BSM scenarios in which decay chains of a pair of new heavy particles
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produce an excess of same-flavour ($e^{+}e^{-}$ and $\mu^{+}\mu^{-}$) events over opposite-flavour ($e^{\pm}\mu^{\mp}$) events.
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For example, in the context of the CMSSM, this excess may be caused by decays of neutralinos and Z bosons to same-flavour lepton pairs.
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For the benchmark scenario LM0 (LM1), the fraction of same-flavour events in the signal region discussed
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below is 0.67 (0.86).
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The dominant background in this search is also dilepton $t\bar{t}$, for which such an excess does not exist
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because the flavours of the two leptons are uncorrelated.
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Therefore, the rate of $t\bar{t}$ decays with two same-flavour leptons
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may be estimated from the number of opposite-flavour events,
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after correcting for the
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ratio of muon to electron selection efficiencies, $r_{\mu{}e}$.
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This method actually estimates the contribution of any uncorrelated pair of leptons, including
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e.g.\ $Z\to\tau\tau$ events where the two $\tau$ leptons decay leptonically.
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This method will also subtract any BSM signal producing lepton pairs of uncorrelated flavour.
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Events with two leptons with $\pt>10\GeVc$ are selected. Because the lepton triggers are not fully
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efficient for events with two leptons of $\pt>10\GeVc$,
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the data sample for this analysis is selected with hadronic triggers based on the
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scalar sum of the transverse energies of all jets reconstructed from calorimeter signals with $\pt>20\GeVc$.
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The event is required to pass at least one of a set of hadronic triggers with transverse energy thresholds
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ranging from 100 to 150~GeV. The efficiency of this set of triggers with respect to the analysis selection is
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greater than 99\%.
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In addition to the trigger, we require $\HT>350\GeV$,
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where \HT\ in this analysis is defined as the scalar sum of
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the transverse energies of all selected jets with $\pt>30\GeVc$
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and within an increased pseudorapidity range $|\eta|<3$, in line with the trigger requirement.
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The jets, \MET, and leptons are reconstructed with the Particle Flow technique~\cite{CMS-PAS-PFT-10-002}.
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The resulting performance of the selection of leptons and jets does not differ
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significantly from the selection discussed in Section~\ref{sec:eventSel}.
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The signal region is defined by additionally requiring $\MET>150\GeV$.
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This signal region is chosen such that approximately one SM event is expected in our
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current data sample.
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The lepton selection efficiencies are measured using the $Z$
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resonance. As discussed in Section~\ref{sec:systematics}, these
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efficiencies are known with a systematic uncertainty of $2\%$. The
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selection efficiencies of isolated leptons are different in the $t\bar{t}$
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and $Z+\textrm{jets}$ samples. The ratio of muon to electron
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efficiencies $r_{\mu{}e}$, however, is found to differ by less than
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5\% in the MC simulations, and a corresponding systematic
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uncertainty is assigned to this ratio. This procedure gives $r_{\mu{}e} = 1.07 \pm
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0.06$.
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The $W+\textrm{jets}$ and QCD multijet contributions, where at least one
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of the two leptons is a secondary lepton from a heavy flavour decay or
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a jet misidentified as a lepton (non-$W/Z$ leptons) are estimated from
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a fit to the lepton isolation distribution, after relaxing
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the isolation requirement on the leptons.
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%The estimated purity is then applied to the number of observed events in the signal region to
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%infer the number of non-$W/Z$ leptons therein.
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Contributions from
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other SM backgrounds, such as DY or processes with two gauge bosons,
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are strongly suppressed by the \MET\ requirement and are expected to
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be negligible.
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We first estimate the number of SM events in a \ttbar-dominated region
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with $100 < \HT < 350\GeV$ and $\MET>80\GeV$. In order to cope with the lower \HT\ requirement,
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we use the same high-\pt lepton trigger sample as described in Section~\ref{sec:eventSel}.
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In this region we observe $26$ opposite-flavour candidates and predict $1.0\pm0.5$
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non-$W/Z$ lepton events from the fit to the lepton isolation distribution. This results in an
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estimate of $25.0 \pm 5.0$ \ttbar\ events in the $e\mu$ channel. Using the efficiency
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ratio $r_{\mu{}e}$ this estimate is then converted into a prediction for the number
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of same-flavour events in the $ee$ and $\mu\mu$ channels.
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\begin{table}[hbt]
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\begin{center}
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\caption{\label{tab:CRresults}
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Number of predicted and observed $ee$ and $\mu\mu$
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events in the control region, defined as $100 < \HT < 350\GeV$ and $\MET > 80\GeV$.
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``SM MC'' indicates the sum of all MC samples ($t\bar{t}$, DY, $W+\textrm{jets}$, and $WW/WZ/ZZ$)
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and includes statistical uncertainties only.
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}
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\vspace{2 mm}
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\begin{tabular}{l|cc}
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\hline
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& \multicolumn{2}{c}{Control region} \\
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\hline
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Process & $ee$ & $\mu\mu$ \\
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\hline
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$t\bar{t}$ predicted from $e\mu$ & $11.7\pm 2.4$ & $13.4\pm 2.8$ \\
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Non-$W/Z$ leptons & $0.5\pm 0.3$ & $0.4\pm0.2$ \\
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\hline
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Total predicted & $12.2\pm 2.4$ & $13.8 \pm 2.8$ \\
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\hline\hline
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Total observed & $10$ & $15$ \\
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\hline \hline
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SM MC & $8.4\pm 0.2$ & $10.5 \pm 0.3$ \\
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%LM0 & $3.7\pm0.2$ & $4.2\pm0.2$ \\
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%LM1 & $0.5\pm0.1$ & $0.7\pm0.1$ \\
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\hline
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\end{tabular}
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\end{center}
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\end{table}
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Table~\ref{tab:CRresults} shows the number of expected SM background same-flavour events in the control region for the MC,
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as well as the prediction from the background estimation techniques based on data. There are a total of 25 same-flavour
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events, in good agreement with the prediction of $25.9 \pm 5.2$ events.
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We thus proceed to the signal region selection.
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%It is worth noting that in this control region, we expect $7.9 \pm 1.4$ and $1.2 \pm 0.2$ from LM0 and LM1 respectively.
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The SM background predictions in the signal region from the opposite-flavour and non-$W/Z$ lepton methods
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are summarized in Table~\ref{tab:results}.
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We find one event in the signal region in the $e\mu$ channel with a prediction of non-$W/Z$ leptons
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of $0.1\pm0.1$, and thus predict $0.9 {}_{-0.8}^{+2.2}$ same-flavour events using Poisson statistical uncertainties.
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In the data we find no same-flavour events, in agreement with the prediction, in contrast with $7.3\pm1.6$
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and $3.6\pm0.7$ expected events for the benchmark points LM0 and LM1, respectively.
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%With zero events observed, the non-$W/Z$ lepton prediction is also zero.
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The predicted background from non-$W/Z$ leptons is negligible.
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\begin{table}[hbt]
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\begin{center}
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\caption{\label{tab:results}
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Number of predicted and observed events in the signal region, defined as $\HT > 350\GeV$ and $\MET> 150\GeV$.
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``SM MC'' indicates the sum of all MC samples ($t\bar{t}$, DY, $W+\textrm{jets}$, and $WW/WZ/ZZ$)
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and includes statistical uncertainties only.
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}
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\vspace{2 mm}
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\begin{tabular}{l|cc}
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\hline
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& \multicolumn{2}{c}{Signal region} \\
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\hline
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Process & $ee$ & $\mu\mu$ \\
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\hline
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$t\bar{t}$ predicted from $e\mu$ & $0.4 {}_{-0.4}^{+1.0}$ & $0.5 {}_{-0.4}^{+1.2}$ \\
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Non-$W/Z$ & 0 & 0 \\
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\hline
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Total predicted & $0.4 {}_{-0.4}^{+1.0}$ & $0.5 {}_{-0.4}^{+1.2}$ \\
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\hline\hline
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Total observed & $0$ & $0$ \\
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\hline \hline
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SM MC & $0.38\pm 0.08$ & $0.56 \pm 0.07$ \\
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LM0 & $3.4\pm0.2$ & $3.9\pm0.2$ \\
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LM1 & $1.6\pm0.1$ & $2.0\pm0.1$ \\
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\hline
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\end{tabular}
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\end{center}
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\end{table}
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Table~\ref{tab:results} demonstrates the sensitivity of this approach.
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We observe comparable yields of the same benchmark points as for the high-\pt\
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lepton trigger search, where 35--60\% of the events are common to both
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searches for LM0 and LM1.
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Either approach would have given an excess in the presence of a signal.
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