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\section{\label{sec:intro}\boldmath{Introduction}} |
\section{\label{sec:intro}\boldmath{Introduction}} |
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\noindent Since its discovery at the Fermilab TEVATRON collider in 1995, the top quark has been one |
\noindent Since its discovery at the Fermilab Tevatron collider in 1995, the top quark has been one |
of the most important topics in High Energy Physics. The study of its production rate and |
of the most important topics in High Energy Physics. The study of its production rate and |
properties allows us to perform precision tests of standard model (SM) predictions as well as |
properties allows us to perform precision tests of the Standard Model (SM) predictions. |
represents a chance of observing possible deviations from such predictions. |
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Amongst all subsequent top decays, the process $t \rightarrow Wb \rightarrow \tau \nu_{\tau} b$ |
Amongst all subsequent top decays, the process $t \rightarrow Wb \rightarrow \tau \nu_{\tau} b$ |
represents one of the most important tools for probing beyond-SM physics. For instance, |
represents one of the most important tools for probing beyond-SM physics. For instance, |
the MSSM (Minimal Supersymmetric Standard Model) \cite{Charged Higgs Theory} predicts |
the MSSM (Minimal Supersymmetric Standard Model) predicts |
the existence of the decay $t \rightarrow H^{+}b$ if $m_{H_{-}^{+}} < m_{t} - m_{b}$. |
the existence of the decay $t \rightarrow H^{\pm}b$ if $m_{H^{\pm}} < m_{t} - m_{b}$ \cite{Charged Higgs Theory}. |
As the Higgs-fermion coupling is proportional to the latter's |
As the Higgs-fermion coupling is proportional to the latter's |
mass, the subsequent decay of a charged Higgs boson into a $\tau$ lepton |
mass, the subsequent decay of a charged Higgs boson into a $\tau$ lepton |
is much more favored than its decays into $e$'s and $\mu$'s. |
is much more favored than its decays into $e$'s and $\mu$'s. |
Therefore, for high values of tan $\beta$ (the ratio of the vacuum expectation values |
Therefore, for high values of $\tan\beta$ (the ratio of the vacuum expectation values |
of the two Higgs doublets) the charged Higgs preferentially decays to $\tau \nu_{\tau}$, |
of the two Higgs doublets) the charged Higgs preferentially decays to $\tau \nu_{\tau}$, |
which increases the branching ratio (BR) of $t \rightarrow \tau \nu_{\tau} b$ relative |
which increases the branching ratio (BR) of $t \rightarrow \tau \nu_{\tau} b$ relative |
to the SM prediction. Thus, any non-standard flavor- and |
to the SM prediction. Thus, any non-standard flavor- and |
mass-dependent could produce a significant effect on the $\tau$ production channel. In this |
mass-dependent could produce a significant effect on the $\tau$ production channel. As this |
respect, the work presented here represents an important test of the SM predictions as well as |
analyis is limited to SM only in consists of an important test of the SM predictions as well as |
one step further on the investigation of non-SM processes. |
one step further on the investigation of non-SM processes. |
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Line 26 In this analysis we study the process wh
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Line 25 In this analysis we study the process wh
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$\tau$ lepton and its associated neutrino, while the other $W$ boson decays into |
$\tau$ lepton and its associated neutrino, while the other $W$ boson decays into |
a quark-antiquark pair. The $\tau$ is the heaviest lepton and its prompt decay into other |
a quark-antiquark pair. The $\tau$ is the heaviest lepton and its prompt decay into other |
particles and the probability of being faked by electrons, muons and jets |
particles and the probability of being faked by electrons, muons and jets |
makes its reconstruction and identification much more difficult than other top decays. |
makes its reconstruction and identification more difficult than other leptonic decays of the top. |
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Here we focus on events where the $\tau$ decays hadronically, |
Here we focus on events where the $\tau$ decays hadronically, |
meaning to one or three charged hadrons, zero or more neutral hadrons and a tau neutrino. |
meaning to one or more charged hadrons, zero or more neutral hadrons and a tau neutrino. Thus |
This implies that our signal consists of a final state with four or more jets. |
this analysis is sensitive to 65\% of all $\tau$ decays. |
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This particular $\tau$ decay mode plus 2 $b$ jets produce a final state with four or more jets. |
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Therefore we look for signal events with ate least 4 high-$p_{T}$ jets including 2 $b$ jets |
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at least one $\tau$ and large $\not\!\! E_{T}$. |
Figures \ref{fig:feynman} and \ref{fig:pie} show respectively the Feynman diagram that decribes |
Figures \ref{fig:feynman} and \ref{fig:pie} show respectively the Feynman diagram that decribes |
the process $t\bar{t} \rightarrow \tau + jets$ and the pie chart of top decay. |
the process $t\bar{t} \rightarrow \tau + jets$ and the pie chart of top decay. |
In Section \ref{sec:dataset} we discuss or signal and main backgrounds. |
In Section \ref{sec:dataset} we discuss the signal and main backgrounds. |
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\begin{figure}[h] |
\begin{figure}[h] |
\includegraphics[scale=0.50]{plots/feynman} |
\includegraphics[scale=0.50]{plots/feynman} |
Line 44 In Section \ref{sec:dataset} we discuss
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Line 46 In Section \ref{sec:dataset} we discuss
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\begin{figure}[t] |
\begin{figure}[t] |
\includegraphics[scale=0.40]{plots/pie} |
\includegraphics[scale=0.40]{plots/top_pair_branching_frac} |
\caption{Top quark decay pie chart.} |
\caption{Top quark decay pie chart.} |
\label{fig:pie} |
\label{fig:pie} |
\end{figure} |
\end{figure} |
Line 52 In Section \ref{sec:dataset} we discuss
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Line 54 In Section \ref{sec:dataset} we discuss
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The present work is the third measurement of the $t\bar{t}$ cross section in the $\tau + jets$ |
The present work is the third measurement of the $t\bar{t}$ cross section in the $\tau + jets$ |
channel performed with the D\O\ detector. Previous results \cite{p14_note,p17_note} used using p14 RunI |
channel performed with the D0 detector. Previous results using p14 |
and p17 RunIIa Data and are summarized in Table \ref{previous} (only statistical uncertainties are shown). |
and p17 data \cite{p14_note,p17_note} summarized in Table 1 (only statistical uncertainties are shown). |
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\begin{table}[htbp] |
\begin{table}[htbp] |
\begin{center} |
\begin{center} |
\begin{tabular}{|c|r|} \hline |
\begin{tabular}{|c|r|} \hline |
Data set ($pb^{-1}$) & cross section ($pb$) \\ \hline |
$\int \mathcal{L}\mbox{d}t$ ($\mbox{pb}^{-1}$) & cross section ($\mbox{pb}$) \\ \hline |
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\hline |
\hline |
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Line 73 p17 (974.2) & \multicolumn{1}{c|}{$6.90\
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Line 75 p17 (974.2) & \multicolumn{1}{c|}{$6.90\
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\end{table} |
\end{table} |
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The main improvements upon previous p17 analysis are listed below: |
The main improvements over the previous p17 analysis are listed below: |
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\begin{itemize} |
\begin{itemize} |
\item 5 times more data (RunIIb1, RunIIb2 and RunIIb3). |
\item 5 times more data (RunIIb1, RunIIb2 and RunIIb3). |
\item Trigger used: we use a new set of multijet triggers that represents a gaim of ~ 10\% in the final efficiency. |
\item We use a new set of multijet triggers that represents a gain of ~ 10\% in the final efficiency. |
\item Use of vertex confirmed jets. |
\item Use of vertex confirmed jets. |
\item Tau energy scale added to the analysis. |
\item Tau energy scale added to the analysis. |
\item Improved neural net (NN) optimization. |
\item Improved event neural net (NN) optimization. |
\item New set of p20 b-tag TRF's. |
\item New set of p20 b-tag tag-rate functions (TRF's). |
\end{itemize} |
\end{itemize} |