Annotation of ttbar/p20_taujets_note/Introduction.tex, revision 1.2
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}
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