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1.1 |
\section{Event Selection}
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\label{sec:eventSel}
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Samples of MC events are used to guide the design of the analysis.
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These events are generated using either the
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\PYTHIA6.4.22~\cite{Pythia} or \MADGRAPH4.4.12~\cite{Madgraph} event
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generators. They are then simulated using a GEANT4-based
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model~\cite{Geant} of the CMS detector, and finally reconstructed and
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analyzed using the same software as is used to process collision data.
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We apply a preselection based on that of the $t\bar{t}$ cross section
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measurement in the dilepton channel~\cite{ref:top}. Events
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with two opposite-sign, isolated leptons ($e^+e^-$,
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$e^{\pm}\mu^{\mp}$, or $\mu^+\mu^-$) are selected. At least one of the leptons must
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have $\pt > 20\GeVc$ and both must have $\pt > 10\GeVc$, and the
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electrons (muons) must have $|\eta| < 2.5$ ($|\eta| < 2.4$). In events
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with more than two such leptons, the two leptons with the
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highest \pt are selected. Events with an $e^+e^-$ or $\mu^+\mu^-$ pair
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with invariant mass between 76\GeVcc and 106\GeVcc or below
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1.2 |
12\GeVcc are removed, in order to suppress Drell--Yan (DY)
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1.1 |
$Z/\gamma^{*}\to\ell\ell$ events, as well as low mass dilepton
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resonances.
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Events are required to pass at least one of a set of $ee$, $e\mu$ or $\mu\mu$
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double-lepton triggers. The efficiency for events containing two
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leptons passing the analysis selection to pass at least one of these
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triggers is measured to be approximately 100\%, 95\%, and 90\%
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for $ee$, $e\mu$ or $\mu\mu$ double-lepton triggers, respectively.
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In the following, the MC yields are weighted by these trigger efficiencies.
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Because leptons produced in the decays of low-mass particles, such as
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hadrons containing $b$ and $c$ quarks, are nearly always inside jets, they can be
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suppressed by requiring the leptons to be isolated in space from other
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particles that carry a substantial amount of transverse momentum. The
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details of the lepton isolation measurement are given in
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Ref.~\cite{ref:top}. In brief, a cone is constructed of size
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$\Delta{}R\equiv\sqrt{(\Delta\eta)^2+(\Delta\phi)^2}=0.3$ around the
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lepton momentum direction. The lepton relative isolation is then
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quantified by summing the transverse energy (as measured in the
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calorimeters) and the transverse momentum (as measured in the silicon
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tracker) of all objects within this cone, excluding the lepton, and
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dividing by the lepton transverse momentum. The resulting quantity
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is required to be less than 0.15, rejecting
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the large background arising from QCD production of jets.
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We require the presence of at least two jets with $\pt > 30\GeVc$ and $|\eta| < 3.0$,
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separated by $\Delta R >$ 0.4 from leptons passing the analysis
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selection with $\pt > 10\GeVc$. The anti-$k_T$ clustering
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algorithm~\cite{antikt} with $\Delta{}R = 0.5$ is used for jet
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clustering. The jets and \MET are reconstructed with the Particle Flow
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technique~\cite{CMS-PAS-PFT-10-002}.
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The event is required to satisfy $\HT > 100\GeV$, where \HT\ is defined as
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the scalar sum of the transverse energies of the selected jets. In
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addition, the \MET\ in the event is required to exceed 50\GeV.
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