| 1 |
The Tau Neural classifier introduces two complimentary new techniques for tau
|
| 2 |
lepton physics at CMS: reconstruction of the hadronic tau decay mode and
|
| 3 |
discrimination from quark and gluon jets using neural networks. The decay mode
|
| 4 |
reconstruction strategy presented in section~\ref{sec:decay_mode_reco}
|
| 5 |
significantly improves the determination of the decay mode. This information has
|
| 6 |
the potential to be useful in studies of tau polarization and background
|
| 7 |
estimation.
|
| 8 |
|
| 9 |
The Tau Neural classifier tau identification algorithm significantly improves
|
| 10 |
tau discrimination performance compared to isolation--based
|
| 11 |
approaches~\cite{PFT08001} used in previous CMS analyses.
|
| 12 |
Figure~\ref{fig:finalPerfCurve} compares the performance of the ``shrinking
|
| 13 |
cone'' isolation tau--identification algorithm~\cite{PFT08001} to the
|
| 14 |
performance of the TaNC for a scan of requirements on the transformed neural
|
| 15 |
network output. The signal efficiency and QCD di--jet fake rate versus
|
| 16 |
tau--candidate transverse momentum and pseudo--rapidity for the four benchmark
|
| 17 |
points and the isolation based tau identification are show in
|
| 18 |
figure~\ref{fig:kinematicPerformance}. For tau--candidates with transverse
|
| 19 |
momentum between 20 and 50 GeV/c, the TaNC operating cut can be chosen such that
|
| 20 |
the two methods have identical signal efficiency; at this point the TaNC
|
| 21 |
algorithm reduces the background fake rate by an additional factor of 3.9. This
|
| 22 |
reduction in background will directly improve the significance of searches for
|
| 23 |
new physics using tau leptons at CMS.
|
| 24 |
|
| 25 |
\begin{figure}[thbp]
|
| 26 |
\setlength{\unitlength}{1mm}
|
| 27 |
\begin{center}
|
| 28 |
\begin{picture}(150, 150)(0,0)
|
| 29 |
\put(0.5, 0.5)
|
| 30 |
{\mbox{\includegraphics*[height=150mm]{figures/20_pt_50_perf_curve_from_5_pt_200_transform_plain_test_wrt_classic.pdf}}}
|
| 31 |
\end{picture}
|
| 32 |
\caption{
|
| 33 |
Performance curve (red) of the TaNC tau identification for various
|
| 34 |
requirements on the output transformed according to
|
| 35 |
equation~\ref{eq:tancTransform}. The horizontal axis is the efficiency for
|
| 36 |
true taus with transverse momentum between 20 and 50 GeV/c to satisfy the tau
|
| 37 |
identification requirements. The vertical axis gives the rate at which QCD
|
| 38 |
di--jets with generator--level transverse momentum between 20 and 50 GeV/c are incorrectly
|
| 39 |
identified as taus. The performance point for the same tau--candidates using
|
| 40 |
the isolation based tau--identification~\cite{PFT08001} used in many previous
|
| 41 |
CMS analyses is indicated by the black star in the figure. An additional requirement
|
| 42 |
that the signal cone contain one or three charged hadrons (typical
|
| 43 |
in a final physics analysis) has been
|
| 44 |
applied to the isolation based tau--identification to ensure a conservative
|
| 45 |
comparison.
|
| 46 |
}
|
| 47 |
\label{fig:finalPerfCurve}
|
| 48 |
\end{center}
|
| 49 |
\end{figure}
|
| 50 |
|
| 51 |
|
| 52 |
\begin{figure}[thbp]
|
| 53 |
\setlength{\unitlength}{1mm}
|
| 54 |
\begin{center}
|
| 55 |
\begin{picture}(150, 150)(0,0)
|
| 56 |
\put(2.5, 75)
|
| 57 |
{\mbox{\includegraphics*[height=70mm]{figures/eff_background_pt.pdf}}}
|
| 58 |
\put(75, 75)
|
| 59 |
{\mbox{\includegraphics*[height=70mm]{figures/eff_background_eta.pdf}}}
|
| 60 |
\put(2.5, 0)
|
| 61 |
{\mbox{\includegraphics*[height=70mm]{figures/eff_signal_pt.pdf}}}
|
| 62 |
\put(75, 0)
|
| 63 |
{\mbox{\includegraphics*[height=70mm]{figures/eff_signal_eta.pdf}}}
|
| 64 |
\end{picture}
|
| 65 |
\caption{ Comparison of the identification efficiency for hadronic tau decays from
|
| 66 |
$Z \rightarrow \tau^{+} \tau^{-}$ decays (bottom row) and the misidentification
|
| 67 |
rate for QCD di--jets (top row) versus tau--candidate transverse momentum
|
| 68 |
(left) and pseudo-rapidity (right) for different tau identification
|
| 69 |
algorithms. The efficiency (fake--rate) in a given bin is defined as the
|
| 70 |
quotient of the number of true tau hadronic decays (generator level jets) in
|
| 71 |
that bin that are matched to a reconstructed tau--candidate that passes the
|
| 72 |
identification algorithm divided by the number of true tau hadronic decays
|
| 73 |
(generator level jets) in that bin. In the low transverse momentum region
|
| 74 |
both the number of tau--candidates in the denominator and the algorithm
|
| 75 |
acceptance vary rapidly with respect to $P_T$ for both signal and background;
|
| 76 |
a minimum transverse momentum requirement of 20 GeV/c is applied to the
|
| 77 |
pseudorapidity plots to facilitate interpretation of the plots.
|
| 78 |
}
|
| 79 |
\label{fig:kinematicPerformance}
|
| 80 |
\end{center}
|
| 81 |
\end{figure}
|
| 82 |
|
