Annotation of ttbar/p20_taujets_note/Introduction.tex, revision 1.1
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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}
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