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                      2: \newpage
                      3: \section{\label{sec:intro}\boldmath{Introduction}}
                      4: 
                      5: \noindent Since its discovery at the Fermilab TEVATRON collider in 1995, the top quark has been one 
                      6: of the most important topics in High Energy Physics. The study of its production rate and
                      7: properties allows us to perform precision tests of standard model (SM) predictions as well as
                      8: represents a chance of observing possible deviations from such predictions. 
                      9: Amongst all subsequent top decays, the process $t \rightarrow Wb \rightarrow \tau \nu_{\tau} b$
                     10: represents one of the most important tools for probing beyond-SM physics. For instance, 
                     11: the MSSM (Minimal Supersymmetric Standard Model) \cite{Charged Higgs Theory} predicts 
                     12: the existence of the decay $t \rightarrow H^{+}b$ if $m_{H_{-}^{+}} < m_{t} - m_{b}$.
                     13: As the Higgs-fermion coupling is proportional to the latter's 
                     14: mass, the subsequent decay of a charged Higgs boson into a $\tau$ lepton
                     15: is much more favored than its decays into $e$'s and $\mu$'s.
                     16: Therefore, for high values of tan $\beta$ (the ratio of the vacuum expectation values
                     17: of the two Higgs doublets) the charged Higgs preferentially decays to $\tau \nu_{\tau}$,
                     18: which increases the branching ratio (BR) of $t \rightarrow \tau \nu_{\tau} b$ relative
                     19: to the SM prediction. Thus, any non-standard flavor- and
                     20: mass-dependent could produce a significant effect on the $\tau$ production channel. In this
                     21: respect, the work presented here represents an important test of the SM predictions as well as
                     22: one step further on the investigation of non-SM processes.
                     23: 
                     24: 
                     25: In this analysis we study the process when the $W$ boson from one of the top quarks decays into a 
                     26: $\tau$ lepton and its associated neutrino, while the other $W$ boson decays into 
                     27: a quark-antiquark pair. The $\tau$ is the heaviest lepton and its prompt decay into other
                     28: particles and the probability of being faked by electrons, muons and jets
                     29: makes its reconstruction and identification much more difficult than other top decays.
                     30: 
                     31: Here we focus on events where the $\tau$ decays hadronically, 
                     32: meaning to one or three charged hadrons, zero or more neutral hadrons and a tau neutrino.
                     33: This implies that our signal consists of a final state with four or more jets. 
                     34: Figures \ref{fig:feynman} and \ref{fig:pie} show respectively the Feynman diagram that decribes
                     35: the process $t\bar{t} \rightarrow \tau + jets$ and the pie chart of top decay.
                     36: In Section \ref{sec:dataset} we discuss or signal and main backgrounds.
                     37: 
                     38: \begin{figure}[h]
                     39: \includegraphics[scale=0.50]{plots/feynman}
                     40: \caption{Feynman diagram for $t\bar{t} \rightarrow \tau + jets$ .}
                     41: \label{fig:feynman}
                     42: \end{figure}
                     43: 
                     44: \newpage
                     45: 
                     46: \begin{figure}[t]
                     47: \includegraphics[scale=0.40]{plots/pie}
                     48: \caption{Top quark decay pie chart.}
                     49: \label{fig:pie}
                     50: \end{figure}
                     51: 
                     52: %\clearpage
                     53: 
                     54: The present work is the third measurement of the $t\bar{t}$ cross section in the $\tau + jets$ 
                     55: channel performed with the D\O\ detector. Previous results \cite{p14_note,p17_note} used using p14 RunI
                     56: and p17 RunIIa Data and are summarized in Table \ref{previous} (only statistical uncertainties are shown).
                     57: 
                     58: \begin{table}[htbp]
                     59: \begin{center}
                     60: \begin{tabular}{|c|r|} \hline
                     61: Data set ($pb^{-1}$)  & cross section ($pb$)        \\ \hline
                     62: 
                     63: \hline
                     64: 
                     65: p14 (349.0) & \multicolumn{1}{c|}{$5.05\;\;_{-3.46}^{+4.31}$}\\ \hline
                     66: 
                     67: p17 (974.2) & \multicolumn{1}{c|}{$6.90\;\;_{-1.20}^{+1.20}$}\\ \hline
                     68: 
                     69: \end{tabular}
                     70: \caption{Previous $t\bar{t}$ cross section measurements in the $\tau + jets$ channel}
                     71: \end{center}
                     72: \label{previous} 
                     73: \end{table}
                     74: 
                     75:  
                     76: The main improvements upon previous p17 analysis are listed below:
                     77: 
                     78: \begin{itemize}
                     79: \item 5 times more data (RunIIb1, RunIIb2 and RunIIb3). 
                     80: \item Trigger used: we use a new set of multijet triggers that represents a gaim of ~ 10\% in the final efficiency.
                     81: \item Use of vertex confirmed jets.
                     82: \item Tau energy scale added to the analysis.
                     83: \item Improved neural net (NN) optimization.
                     84: \item New set of p20 b-tag TRF's.
                     85: \end{itemize}