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\fixme{need to summarize decay mode} |
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The Tau Neural classifier introduces two complimentary new techniques for tau |
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lepton physics at CMS: reconstruction of the hadronic tau decay mode and |
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discrimination from quark and gluon jets using neural networks. The decay mode |
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reconstruction strategy presented in section~\ref{sec:decay_mode_reco} |
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significantly improves the determination of the decay mode. This information has |
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the potential to be useful in studies of tau polarization and background |
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estimation. |
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|
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The Tau Neural classifier tau identification algorithm significantly improves |
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tau discrimination performance compared to isolation--based |
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approaches~\cite{PFT08001} used in previous CMS analyses. |
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Figure~\ref{fig:finalPerfCurve} compares the performance of the ``shrinking |
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cone'' isolation tau--identification algorithm~\cite{PFT08001} to the |
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performance of the TaNC for a scan of requirements on the transformed neural |
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network output. The signal efficiency and QCD di--jet fake rate versus |
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tau--candidate transverse momentum and pseudo--rapidity for the four benchmark |
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points and the isolation based tau identification are show in |
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figure~\ref{fig:kinematicPerformance}. For tau--candidates with transverse |
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momentum between 20 and 50 GeV/c, the TaNC operating cut can be chosen such that |
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the two methods have identical signal efficiency; at this point the TaNC |
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algorithm reduces the background fake rate by an additional factor of 3.9. This |
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reduction in background will directly improve the significance of searches for |
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new physics using tau leptons at CMS. |
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|
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The Tau Neural classifier algorithm significantly improves tau identification |
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performance compared to isolation--based strategies~\cite{PFT08001} used in |
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previous CMS strategies. Figure~\ref{fig:finalPerfCurve} compares the |
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performance of the ``shrinking cone'' isolation tau--identification |
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algorithm~\cite{PFT08001} to the performance of the TaNC for a scan of |
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requirements on the transformed neural network output. For tau--candidates with |
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transverse momentum between 20 and 50 GeV/c, the TaNC operating cut can be |
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chosen such that the two methods have identical signal efficiency; at this point the TaNC |
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algorithm reduces the background fake rate by an additional factor of |
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\fixme{get exact}. |
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|
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The discriminator output of the TaNC algorithm is a continuous quantity, |
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enabling analysis specific optimization of the selection to maximize |
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sensitivity. For the convenience of the user, four operating point benchmark |
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selections are provided in addition to the continuous output. The four operating |
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points are chosen such that for tau--candidates with transverse momentum between |
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20 and 50 GeV/c, the expected QCD di--jet fake rate will be 0.1\%, 0.25\%, |
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0.50\% and 1.0\%, respectively. The signal efficiency and QCD di--jet fake rate |
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versus tau--candidate transverse momentum and pseudo--rapidity for the four |
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benchmark points and the isolation based tau identification are show in |
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figure~\ref{fig:kinematicPerformance}. |
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|
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\fixme{needs a zippy end} |
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|
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\begin{figure}[t] |
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\begin{figure}[thbp] |
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\setlength{\unitlength}{1mm} |
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\begin{center} |
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\begin{picture}(150, 150)(0,0) |
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di--jets with generator--level transverse momentum between 20 and 50 GeV/c are incorrectly |
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identified as taus. The performance point for the same tau--candidates using |
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the isolation based tau--identification~\cite{PFT08001} used in many previous |
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CMS analyses is indicated by the black star in the figure. |
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CMS analyses is indicated by the black star in the figure. An additional requirement |
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that the signal cone contain one or three charged hadrons (typical |
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in a final physics analysis) has been |
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applied to the isolation based tau--identification to ensure a conservative |
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comparison. |
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} |
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\label{fig:finalPerfCurve} |
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\end{center} |
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\end{figure} |
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|
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|
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\begin{figure}[t] |
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\begin{figure}[thbp] |
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\setlength{\unitlength}{1mm} |
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\begin{center} |
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\begin{picture}(150, 150)(0,0) |
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\put(2.5, 0) |
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{\mbox{\includegraphics*[height=70mm]{figures/eff_signal_pt.pdf}}} |
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\put(75, 0) |
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{\mbox{\includegraphics*[height=70mm]{figures/eff_signal_pt.pdf}}} |
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{\mbox{\includegraphics*[height=70mm]{figures/eff_signal_eta.pdf}}} |
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\end{picture} |
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\caption{ |
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\fixme{add caption and legend, isolation+1 or 3 prong is the black trend, |
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pt greater than 20 cut applied on eta plots} |
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\caption{ Comparison of the identification efficiency for hadronic tau decays from |
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$Z \rightarrow \tau^{+} \tau^{-}$ decays (bottom row) and the misidentification |
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rate for QCD di--jets (top row) versus tau--candidate transverse momentum |
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(left) and pseudo-rapidity (right) for different tau identification |
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algorithms. The efficiency (fake--rate) in a given bin is defined as the |
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quotient of the number of true tau hadronic decays (generator level jets) in |
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that bin that are matched to a reconstructed tau--candidate that passes the |
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identification algorithm divided by the number of true tau hadronic decays |
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(generator level jets) in that bin. In the low transverse momentum region |
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both the number of tau--candidates in the denominator and the algorithm |
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acceptance vary rapidly with respect to $P_T$ for both signal and background; |
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a minimum transverse momentum requirement of 20 GeV/c is applied to the |
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psuedorapidity plots to facilitate interpretation of the plots. |
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} |
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\label{fig:kinematicPerformance} |
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\end{center} |
