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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}