Annotation of ttbar/p20_taujets_note/Introduction.tex, revision 1.1.1.1
1.1 uid12904 1:
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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