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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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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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1.1 |
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friis |
1.2 |
\begin{figure}[thbp]
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friis |
1.1 |
\setlength{\unitlength}{1mm}
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\begin{center}
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\begin{picture}(150, 150)(0,0)
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\put(0.5, 0.5)
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{\mbox{\includegraphics*[height=150mm]{figures/20_pt_50_perf_curve_from_5_pt_200_transform_plain_test_wrt_classic.pdf}}}
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\end{picture}
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\caption{
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Performance curve (red) of the TaNC tau identification for various
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requirements on the output transformed according to
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equation~\ref{eq:tancTransform}. The horizontal axis is the efficiency for
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true taus with transverse momentum between 20 and 50 GeV/c to satisfy the tau
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identification requirements. The vertical axis gives the rate at which QCD
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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. An additional requirement
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that the signal cone of the tau 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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1.1 |
}
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\label{fig:finalPerfCurve}
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\end{center}
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\end{figure}
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1.2 |
\begin{figure}[thbp]
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1.1 |
\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, 75)
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{\mbox{\includegraphics*[height=70mm]{figures/eff_background_pt.pdf}}}
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\put(75, 75)
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{\mbox{\includegraphics*[height=70mm]{figures/eff_background_eta.pdf}}}
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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_eta.pdf}}}
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1.1 |
\end{picture}
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1.2 |
\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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1.1 |
}
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\label{fig:kinematicPerformance}
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\end{center}
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\end{figure}
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