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Tau lepton identification ($\tau$-ID) is a challenging but important endeavor |
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at hadron colliders. Standard Model (SM) Higgs boson searches and many new |
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physics scenarios Beyond the Standard Model (BSM) process have discovery |
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channels involving taus. In the Standard Model, the Higgs boson Yukawa |
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couplings are proportional to mass, resulting in decays to taus ten percent of |
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the time for scenearios where the higgs mass is below the diboson threshold. |
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In the Minimal Supersymmetric Model (MSSM), the coupling of members of the |
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Higgs sector doublet to the tau lepton is enhanced by a factor of $tan\beta$. |
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High tau identification performance is import for the discovery potential of |
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many possible new physics signals at the Compact Muon Solenoid (CMS). Events |
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with tau leptons are typically signal events; the Standard Model background |
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rates with true tau leptons are typically the same order of magnitude as the |
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expected signal rate in many searches. The challenge of doing physics with taus |
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is dominated by the rate at which objects are incorrectly tagged as taus. In |
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paticular, quark and gluon jets have a significantly higher production |
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cross-section and events where these objects are incorrectly identified as tau |
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leptons can dominate the backgrounds of searches for new physics using taus. |
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Efficient identification of hadronic tau decays and and low misidentification |
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rate for quarks and gluons is thus essential to maximize the significance of |
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searches for new physics at CMS. |
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The large mass of the tau makes it unique among the leptons in that can hadron |
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final states. These hadronic decays account for approximately 65\% of all tau |
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decays and have a signature of small number of collimated pions. The hadronic |
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decays are dominated by small number of collimated pions. This signature is |
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very similar QCD jet production, which in general has cross sections many |
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orders of magnitude larger than signal processes of interest. An additional |
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complication at hadron colliders is the presence of underlying event (UE), due |
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to secondary interactions in the $pp$ collision. These underlying event |
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particles are dominated by large numbers of soft pions which can overlap true |
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tau decays. |
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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 criterion for a successful $\tau$-ID is twofold: the algorithm must have |
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high tau efficiency to facilitate searches for rare new physics while |
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supressing the common backgrounds found at hadron colliders. This paper will |
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focus on novel algorithms designed to identify true hadronic tau decays and |
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reject common backgrounds. |
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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 identification in CMS is performed using objects from the ``Particle Flow'' |
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algorithm. The particle flow algorithm provides a global and unique |
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reconstruction of the event. Signals in various subdectors are linked together |
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to reconstruct physics objects at particle granularity. |
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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 typically decay to 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~\ref{PFT08001}. With the Tau Neural |
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Classifier, we aim to improve the identification of true hadronic tau decays |
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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. |
