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1.1       uid12904    1: 
                      2: \newpage
                      3: \section{\label{sec:intro}\boldmath{Introduction}}
                      4: 
1.2     ! uid12904    5: \noindent Since its discovery at the Fermilab Tevatron collider in 1995, the top quark has been one 
1.1       uid12904    6: of the most important topics in High Energy Physics. The study of its production rate and
1.2     ! uid12904    7: properties allows us to perform precision tests of the Standard Model (SM) predictions. 
1.1       uid12904    8: Amongst all subsequent top decays, the process $t \rightarrow Wb \rightarrow \tau \nu_{\tau} b$
                      9: represents one of the most important tools for probing beyond-SM physics. For instance, 
1.2     ! uid12904   10: the MSSM (Minimal Supersymmetric Standard Model) predicts 
        !            11: the existence of the decay $t \rightarrow H^{\pm}b$ if $m_{H^{\pm}} < m_{t} - m_{b}$ \cite{Charged Higgs Theory}.
1.1       uid12904   12: As the Higgs-fermion coupling is proportional to the latter's 
                     13: mass, the subsequent decay of a charged Higgs boson into a $\tau$ lepton
                     14: is much more favored than its decays into $e$'s and $\mu$'s.
1.2     ! uid12904   15: Therefore, for high values of $\tan\beta$ (the ratio of the vacuum expectation values
1.1       uid12904   16: of the two Higgs doublets) the charged Higgs preferentially decays to $\tau \nu_{\tau}$,
                     17: which increases the branching ratio (BR) of $t \rightarrow \tau \nu_{\tau} b$ relative
                     18: to the SM prediction. Thus, any non-standard flavor- and
1.2     ! uid12904   19: mass-dependent could produce a significant effect on the $\tau$ production channel. As this
        !            20: analyis is limited to SM only in consists of an important test of the SM predictions as well as
1.1       uid12904   21: one step further on the investigation of non-SM processes.
                     22: 
                     23: 
                     24: In this analysis we study the process when the $W$ boson from one of the top quarks decays into a 
                     25: $\tau$ lepton and its associated neutrino, while the other $W$ boson decays into 
                     26: a quark-antiquark pair. The $\tau$ is the heaviest lepton and its prompt decay into other
                     27: particles and the probability of being faked by electrons, muons and jets
1.2     ! uid12904   28: makes its reconstruction and identification  more difficult than other leptonic decays of the top.
1.1       uid12904   29: 
                     30: Here we focus on events where the $\tau$ decays hadronically, 
1.2     ! uid12904   31: meaning to one or more charged hadrons, zero or more neutral hadrons and a tau neutrino. Thus
        !            32: this analysis is sensitive to 65\% of all $\tau$ decays.
        !            33: This particular $\tau$ decay mode plus 2 $b$ jets produce a final state with four or more jets. 
        !            34: Therefore we look for signal events with ate least 4 high-$p_{T}$ jets including 2 $b$ jets
        !            35: at least one $\tau$ and large $\not\!\! E_{T}$.
1.1       uid12904   36: Figures \ref{fig:feynman} and \ref{fig:pie} show respectively the Feynman diagram that decribes
                     37: the process $t\bar{t} \rightarrow \tau + jets$ and the pie chart of top decay.
1.2     ! uid12904   38: In Section \ref{sec:dataset} we discuss the signal and main backgrounds.
1.1       uid12904   39: 
                     40: \begin{figure}[h]
                     41: \includegraphics[scale=0.50]{plots/feynman}
                     42: \caption{Feynman diagram for $t\bar{t} \rightarrow \tau + jets$ .}
                     43: \label{fig:feynman}
                     44: \end{figure}
                     45: 
                     46: \newpage
                     47: 
                     48: \begin{figure}[t]
1.2     ! uid12904   49: \includegraphics[scale=0.40]{plots/top_pair_branching_frac}
1.1       uid12904   50: \caption{Top quark decay pie chart.}
                     51: \label{fig:pie}
                     52: \end{figure}
                     53: 
                     54: %\clearpage
                     55: 
                     56: The present work is the third measurement of the $t\bar{t}$ cross section in the $\tau + jets$ 
1.2     ! uid12904   57: channel performed with the D0 detector. Previous results using p14
        !            58: and p17 data \cite{p14_note,p17_note} summarized in Table 1 (only statistical uncertainties are shown).
1.1       uid12904   59: 
                     60: \begin{table}[htbp]
                     61: \begin{center}
                     62: \begin{tabular}{|c|r|} \hline
1.2     ! uid12904   63: $\int \mathcal{L}\mbox{d}t$ ($\mbox{pb}^{-1}$)  & cross section ($\mbox{pb}$)        \\ \hline
1.1       uid12904   64: 
                     65: \hline
                     66: 
                     67: p14 (349.0) & \multicolumn{1}{c|}{$5.05\;\;_{-3.46}^{+4.31}$}\\ \hline
                     68: 
                     69: p17 (974.2) & \multicolumn{1}{c|}{$6.90\;\;_{-1.20}^{+1.20}$}\\ \hline
                     70: 
                     71: \end{tabular}
                     72: \caption{Previous $t\bar{t}$ cross section measurements in the $\tau + jets$ channel}
                     73: \end{center}
                     74: \label{previous} 
                     75: \end{table}
                     76: 
                     77:  
1.2     ! uid12904   78: The main improvements over the previous p17 analysis are listed below:
1.1       uid12904   79: 
                     80: \begin{itemize}
                     81: \item 5 times more data (RunIIb1, RunIIb2 and RunIIb3). 
1.2     ! uid12904   82: \item We use a new set of multijet triggers that represents a gain of ~ 10\% in the final efficiency.
1.1       uid12904   83: \item Use of vertex confirmed jets.
                     84: \item Tau energy scale added to the analysis.
1.2     ! uid12904   85: \item Improved event neural net (NN) optimization.
        !            86: \item New set of p20 b-tag tag-rate functions (TRF's).
1.1       uid12904   87: \end{itemize}