Annotation of ttbar/p20_taujets_note/Objective.tex, revision 1.1
1.1 ! uid12904 1:
! 2: \section{Objective}
! 3:
! 4:
! 5: \subsection{Standard Model}
! 6:
! 7: The presented analysis involves measurement of the cross-section of
! 8: top quark pair production using the $\tau+jets$ final state (Figure
! 9: \ref{diagram}).%
! 10: \begin{figure}
! 11: \includegraphics[scale=0.7]{plots/feynman}
! 12:
! 13:
! 14: \caption{The dominant Feynman diagram for the $t\bar{t}\rightarrow\tau+jets$
! 15: process.}
! 16:
! 17: \begin{centering}\label{diagram}\par\end{centering}
! 18: \end{figure}
! 19:
! 20:
! 21: Theoretical computation of $\sigma(p\bar{p}\rightarrow t\bar{t})$
! 22: is constantly improving. The latest published NNLO cross section is
! 23: 6.8$\pm$0.4 pb \cite{NNLO}. The decay mode to $\tau+jets$ has branching
! 24: fraction of 0.15 (Figure \ref{pie}), the sane as $e+jets$ and $\mu+jets$
! 25: channels.
! 26:
! 27: %
! 28: \begin{figure}
! 29: \includegraphics[bb=15bp 26bp 597bp 777bp,clip,scale=0.7]{plots/pie}
! 30:
! 31:
! 32: \caption{\char`\"{}Pie chart\char`\"{}, displaying the branching fractions
! 33: of different final states of top quark pair decay.}
! 34:
! 35: \begin{centering}\label{pie}\par\end{centering}
! 36: \end{figure}
! 37:
! 38:
! 39: No cross section measurement in the $\tau$ + X channels has yet been
! 40: performed. This is largely due to the challenges of $\tau$ reconstruction.
! 41: Unlike electrons and muons, $\tau$ leptons decay before reaching
! 42: the detector volume and need to be reconstructed from their decay
! 43: products. Also, unlike other $l+jets$ channels, where the only source
! 44: of $\not\!\! E_{T}$ is $W$ decay, $\tau$ lepton produces neutrinos
! 45: in its own subsequent decay.
! 46:
! 47: The D0 $\tau$ - ID algorithm only reconstructs $\tau$ which undergo
! 48: hadronic decay, which happens only 65 \% of the time. This leads to
! 49: an additional efficiency hit, compared to $e$ and $\mu$ channels.
! 50:
! 51: Still, as can be seen from Figure \ref{pie}, $\tau+X$ constitute
! 52: 24\% of the total $t\bar{t}$ decay width. Thus, a large fraction
! 53: of $t\bar{t}$ events would be missed totally if we ignore the taus.
! 54:
! 55: Furthermore, in order to thoroughly test the Standard Model it is
! 56: important to study top physics in all possible decay modes. If there
! 57: are any flavor or mass dependent coupling (beyond SM) present in $t\bar{t}$
! 58: decay, it may show up preferentially in the $\tau+X$ final states
! 59: (due to the relatively large mass of $\tau$).
! 60:
! 61:
! 62: \subsection{Beyond the Standard Model}
! 63:
! 64: In fact - $\tau$ lepton modes become especially useful to look for
! 65: signs of new phenomena. Many theoretical models predict violation
! 66: of SM flavor universality. If such processes exist they can very well
! 67: favor $\tau$ over other leptons, enhancing the branching fraction
! 68: of our channel.
! 69:
! 70: An interesting example is the charged Higgs boson, which appears in
! 71: extensions of the SM Higgs sector to 2HDMs (Two-Higgs Doublet Models)
! 72: and is required in MSSM \cite{Charged Higgs Theory}. Since Higgs
! 73: coupling is proportional to mass it favor sheavy $\tau$ to light
! 74: $e$ and $\mu.$ This prompts us to search for $H^{+}$to $\tau$s.
! 75: D0 and CDF had performed such search in Run I (\cite{CDF Charged Higgs,D0 Charged Higgs}).
! 76:
! 77: Table 1 shows all possible decay modes of $t\bar{t}\rightarrow\tau+X$
! 78: available if $H^{+}$ exists. D0 \cite{D0 Charged Higgs} had optimized
! 79: their selection criteria for the states 2, 4 and 5 ($\tau+jets$ channel).
! 80: CDF \cite{CDF Charged Higgs} had chosen 1, 3 and 5 ($\tau+e$ and
! 81: $\tau+\mu$ channels). Both analysis had to take into account the
! 82: ditau channel. The measurement described here establishes the foundation
! 83: for undertaking such search at Run II.
! 84:
! 85: %
! 86: \begin{table}
! 87: \begin{tabular}{|c|c|c|c|}
! 88: \hline
! 89: {\small Final state}&
! 90: {\small First decay}&
! 91: {\small Secondary decays}&
! 92: {\small B for secondary decays at large $\tan\beta$}\tabularnewline
! 93: \hline
! 94: \hline
! 95: {\small 1}&
! 96: {\small $t\overline{t}$$\rightarrow W^{\mp}W^{\pm}b\bar{b}$}&
! 97: {\small $W^{\mp}\rightarrow$$\tau^{\mp}\nu$, $W^{\pm}\rightarrow l\nu$}&
! 98: {\small 0.012}\tabularnewline
! 99: \hline
! 100: {\small 2}&
! 101: {\small $t\overline{t}$$\rightarrow W^{\mp}W^{\pm}b\bar{b}$}&
! 102: {\small $W^{\mp}\rightarrow$$\tau^{\mp}\nu$, $W^{\pm}\rightarrow jets$}&
! 103: {\small 0.074}\tabularnewline
! 104: \hline
! 105: {\small 3}&
! 106: {\small $t\overline{t}$$\rightarrow W^{\mp}H^{\pm}b\bar{b}$}&
! 107: {\small $W^{\mp}\rightarrow$$l\nu$, $H^{\pm}\rightarrow\tau^{\pm}\nu$}&
! 108: {\small 0.11}\tabularnewline
! 109: \hline
! 110: {\small 4}&
! 111: {\small $t\overline{t}$$\rightarrow W^{\mp}H^{\pm}b\bar{b}$}&
! 112: {\small $W^{\mp}\rightarrow$$jets$, $H^{\pm}\rightarrow\tau^{\pm}\nu$}&
! 113: {\small 0.64}\tabularnewline
! 114: \hline
! 115: {\small 5}&
! 116: {\small $t\overline{t}$$\rightarrow H^{\mp}H^{\pm}b\bar{b}$}&
! 117: {\small $H^{\mp}\rightarrow$$\tau^{\mp}\nu$, $H^{\pm}\rightarrow\tau^{\pm}\nu$}&
! 118: {\small 0.92}\tabularnewline
! 119: \hline
! 120: \end{tabular}
! 121:
! 122:
! 123: \caption{Decay modes and their branching ratios, for $\tau+jets$, assuming
! 124: large $\tan\beta$. The $l$ refers to any single lepton channel}
! 125: \end{table}
! 126:
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