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The tau identification strategies used in previously published CMS analyses are
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fully described in~\ref{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~\ref{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 subdetectors 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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\fixme{Talk about lead track}
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Both methods use an isolation criteria to reject quark and gluon jet background.
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Quark and gluon jets are less collimated and have a higher constituent
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multiplicity and softer constituent $p_T$ spectrum than a kinematically
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comparable hadronic tau decays. Isolation algorithms exploit this feature by
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defining an isolation annulus about the kinematic center of the jet and
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requiring no detector activity about a threshold in that annulus. This approach
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rejects QCD backgrounds by a factor of $\mathcal O(100)$ while a maintaing hadronic
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tau identification of approximately 50\%.
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