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The tau identification strategies used in previously published CMS analyses are
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fully described in~\cite{PFT08001}. A summary of the basic methods and
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strategies is given here. There are two primary methods for selecting objects
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used to reconstruct tau leptons. The CaloTau algorithm uses tracks
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reconstructed by the tracker and clusters of hits in the electromagnetic and
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hadronic calorimeter. The other method (PFTau) uses objects reconstructed by
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the CMS particle flow algorithm, which is described in~\cite{PFT09}. The
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particle flow algorithm provides a global and unique description of every
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particle (charged hadron, photon, electron, etc.) in the event; measurements
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from sub--detectors are combined according to their measured resolutions to
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improve energy and angular resolution and reduce double counting. The
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strategies described in this paper use the particle flow objects.
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Both methods typically use an ``leading object'' and an isolation requirement to reject
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quark and gluon jet background. Quark and gluon jets are less collimated and
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have a higher constituent multiplicity and softer constituent $p_T$ spectrum
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than a hadronic tau decay of the same transverse momentum. The ``leading
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track'' requirement is applied by requiring a relatively high momentum object
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near the center of the jet; typically a charged track with transverse momentum
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greater than 5 GeV/c within $\Delta R < 0.1$ about the center of the jet axis.
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The isolation requirement exploits the collimation of true taus by defining an
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isolation annulus about the kinematic center of the jet and requiring no
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detector activity about a threshold in that annulus. This approach yields a
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misidentification rate of approximately 1\% for QCD backgrounds and a hadronic
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tau identification efficiency of approximately 50\%~\cite{PFT08001}.
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