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High tau identification performance is important for the discovery potential of
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many possible new physics signals at the Compact Muon Solenoid (CMS). The
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Standard Model background rates from true tau leptons are typically the same
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order of magnitude as the expected signal rate in many searches for new
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physics. The challenge of doing physics with taus is driven by the rate at
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which objects are incorrectly tagged as taus. In particular, quark and gluon
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jets have a significantly higher production cross-section and events where
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these objects are incorrectly identified as tau leptons can dominate the
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backgrounds of searches for new physics using taus. Efficient identification
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of hadronic tau decays and low misidentification rate for quarks and gluons
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is thus essential to maximize the significance of searches for new physics at
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CMS.
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%New physics signals may be discovered through tau lepton hadronic decay channels
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%in early CMS data. The tau lepton plays a paticularly important role in
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%searches for Higgs bosons. In the Minimal Supersymmetric Model (MSSM), the
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%production cross--section is enhanced by the parameter $\tan\beta$. The
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%coupling of the MSSM Higgs to the tau lepton is also enchaced. \fixme(finish
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%this)
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%The tau plays a paticularly important role in the search for Higgs
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%boson particle. In the Standard Model (SM), the Higgs boson couplings to fermions
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%are proportional to the fermion mass, which enhances the $H \rightarrow \tau^{+}
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%\tau^{-}$ branching ratio relative to other leptonic decay modes. For SM Higgs
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%masses below the $W^{+}W^{-}$ and $ZZ$ production threshold, the SM Higgs decays
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%to tau lepton pairs approximately 10\% of the time. The significance of the tau
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%is enhanced in the Minimal Supersymmetric Model (MSSM), where the MSSM Higgs
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%coupling to the tau is enhanced by a factor of $\tan\beta$.
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Tau leptons are unique in that they are the only type of leptons which are heavy
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enough to decay to hadrons. The hadronic decays compose approximately 65\% of
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all tau decays, the remainder being split nearly evenly between $\tau^{-}
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\rightarrow \mu^{-} \bar \nu_\mu \nu_\tau$ and $\tau^{-} \rightarrow e^{-} \bar
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\nu_e \nu_\tau$. The hadronic decays are typically composed of one or three charged
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pions and zero to two neutral pions. The neutral pions decay almost
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instantaneously to pairs of photons.
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In this note, we will describe a technique to identify hadronic tau decays. Tau
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decays to electrons and muons are difficult to distinguish from electrons and
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muons produced in $pp$ collisions. Analyses that use exclusively
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non--hadronically decaying taus typically require that the leptonic ($e,\mu$)
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decays be of opposite flavor. The discrimination of hadronic tau decays from
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electrons and muons is described in~\cite{PFT08001}. With the Tau Neural
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Classifier, we aim to improve the discrimination of true hadronic tau decays
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from quark and gluon jets using a neural network approach.
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%associated with a collimated jet containing either one or three tracks
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%reconstructed in the pixel and silicon strip tracker, plus a low number of
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%neutral electromagnetic showers reconstructed in the calorimeter.
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