\section{\label{sub:xsect}Cross section} Having presented the preselection yelds on Section \ref{sub:Preselection} we now show the results of the efficiencies for $\tau$ ID, b-tagging and trigger for all $t\bar{t}$ channels (only statistical uncertainties are shown). \begin{table}[h] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 22.20 \pm 0.24 $ & $ 22.20 \pm 0.24 $ \\ Trigger & $ 84.54 \pm 0.55 \ $ & $ 18.77 \pm 0.22\ $ \\ b-tagging & $ 61.82 \pm 0.55 \ $ & $ 11.61 \pm 0.16\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow\tau+jets$ cut flow for taus of Types 1 and 2.} %\end{center} \label{taujets_final12} \end{table} \begin{table}[h] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 12.37 \pm 0.21 $ & $ 12.37 \pm 0.21 $ \\ Trigger & $ 84.79 \pm 0.75 \ $ & $ 10.49 \pm 0.19\ $ \\ b-tagging & $ 59.63 \pm 0.75 \ $ & $ 6.26 \pm 0.13\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow\tau+jets$ cut flow for taus of Type 3} %\end{center} \label{taujets_final3} \end{table} \begin{table}[h] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 10.81 \pm 0.20 $ & $ 10.81 \pm 0.20 $ \\ Trigger & $ 83.40 \pm 0.81 \ $ & $ 9.02 \pm 0.18\ $ \\ b-tagging & $ 61.30 \pm 0.82 \ $ & $ 5.52 \pm 0.12\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow e+jets$ cut flow for taus of Types 1 and 2} %\end{center} \label{elecjets_final12} \end{table} \begin{table}[b] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 2.25 \pm 0.11 $ & $ 2.25 \pm 0.11 $ \\ Trigger & $ 83.62 \pm 1.77 \ $ & $ 1.88 \pm 0.09 \ $ \\ b-tagging & $ 58.26 \pm 1.76 \ $ & $ 1.10 \pm 0.06\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow e+jets$ cut flow for taus of Type 3} %\end{center} \label{elecjets_final3} \end{table} %\newpage \begin{table}[b] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 3.38 \pm 0.19 $ & $ 3.38 \pm 0.19 $ \\ Trigger & $ 84.44 \pm 2.13 \ $ & $ 2.86 \pm 0.17\ $ \\ b-tagging & $ 61.25 \pm 2.16 \ $ & $ 1.75 \pm 0.11\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow \mu +jets$ cut flow for taus of Types 1 and 2.} %\end{center} \label{muonjets_final12} \end{table} \begin{table}[b] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 3.88 \pm 0.21 $ & $ 3.88 \pm 0.21 $ \\ Trigger & $ 82.79 \pm 2.04 \ $ & $ 3.21 \pm 0.18\ $ \\ b-tagging & $ 58.11 \pm 2.05 \ $ & $ 1.87 \pm 0.11\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow \mu +jets$ cut flow for taus of Type 3.} %\end{center} \label{muonjets_final3} \end{table} \begin{table}[b] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 21.18 \pm 0.37 $ & $ 21.18 \pm 0.37 $ \\ Trigger & $ 79.56 \pm 0.90 \ $ & $ 16.85 \pm 0.34 \ $ \\ b-tagging & $ 62.83 \pm 0.92 \ $ & $ 10.59 \pm 0.25\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow dilepton$ cut flow for taus of Types 1 and 2.} %\end{center} \label{dilep_final12} \end{table} \clearpage \begin{table}[t] %\begin{center} \begin{tabular}{ccccc} \hline Selection & Relative(\%) & Cumulative(\%) \\ \hline $\tau$ ID & $ 14.73 \pm 0.34 $ & $ 14.73 \pm 0.34 $ \\ Trigger & $ 78.78 \pm 1.08 \ $ & $ 11.60 \pm 0.30\ $ \\ b-tagging & $ 63.62 \pm 1.11 \ $ & $ 7.38 \pm 0.22\ $ \\ \hline \end{tabular} \caption{$t\overline{t}\rightarrow dilepton$ cut flow for taus of Type 3.} %\end{center} \label{dilep_final3} \end{table} %\newpage After having computed all efficiencies it is worthy to summarize all of them (in \%) for the different tau types: \begin{table}[h] %\begin{center} \begin{tabular}{ccccc} \hline Channel &Preselection & $\tau$ ID & Trigger & b-tag \\ \hline $t\overline{t}\rightarrow\tau+jets$ & $ 3.70 \pm 0.02 $ & $ 22.20 \pm 0.24 $ & $ 18.77 \pm 0.22 $ & $ 11.61 \pm 0.16 $\\ $t\overline{t}\rightarrow e+jets$ & $ 3.54 \pm 0.02 $ & $ 10.80 \pm 0.20 $ & $ 9.02 \pm 0.18 $ & $ 5.53 \pm 0.12 $\\ $t\overline{t}\rightarrow \mu +jets$ & $ 1.67 \pm 0.01 $ & $ 3.38 \pm 0.19 $ & $ 2.86 \pm 0.17 $ & $ 1.75 \pm 0.11 $\\ $t\overline{t}\rightarrow dilepton$ & $ 1.36 \pm 0.01 $ & $ 21.18 \pm 0.37 $ & $ 16.85 \pm 0.34 $ & $ 10.59 \pm 0.25 $\\ \hline \end{tabular} \caption{Summary of all selections for taus of Types 1 and 2.} %\end{center} \label{summary12} \end{table} \begin{table}[h] %\begin{center} \begin{tabular}{ccccc} \hline Channel &Preselection & $\tau$ ID & Trigger & b-tag \\ \hline $t\overline{t}\rightarrow\tau+jets$ & $ 3.70 \pm 0.02 $ & $ 12.37 \pm 0.21 $ & $ 10.49 \pm 0.19 $ & $ 6.26 \pm 0.13 $\\ $t\overline{t}\rightarrow e+jets$ & $ 3.54 \pm 0.02 $ & $ 2.25 \pm 0.11 $ & $ 1.88 \pm 0.09 $ & $ 1.10 \pm 0.06 $\\ $t\overline{t}\rightarrow \mu +jets$ & $ 1.67 \pm 0.01 $ & $ 3.88 \pm 0.21 $ & $ 3.21 \pm 0.18 $ & $ 1.87 \pm 0.11 $\\ $t\overline{t}\rightarrow dilepton$ & $ 1.36 \pm 0.01 $ & $ 14.73 \pm 0.34 $ & $ 11.60 \pm 0.30 $ & $ 7.38 \pm 0.22 $\\ \hline \end{tabular} \caption{Summary of all selections for taus of Type 3.} %\end{center} \label{summary3} \end{table} %\clearpage Table \ref{event yeild summary1} summarizes the number of events in each channel after the final selection. \begin{table}[h] \caption{Final number of events in the two analysis channels.} %\begin{ruledtabular} \begin{tabular}{cccccc} \hline &$\tau$ type I,II &$\tau$ type I,II (fitted) &$\tau$ type III &$\tau$ type III (fitted)&\\ \hline data& 386 & & 459 & &\\ $t\overline{t}\rightarrow\tau+jets$& 72.04 $\pm$ 0.53& & 38.82 $\pm$ 0.39&\\ $t\overline{t}\rightarrow e+jets$& 38.35 $\pm$ 0.36& & 6.52 $\pm$ 0.16& &\\ $t\overline{t}\rightarrow\mu+jets$& 4.81 $\pm$ 0.14& & 5.14 $\pm$ 0.14& &\\ $t\overline{t}\rightarrow l+l$& 6.02 $\pm$ 0.07& & 4.20 $\pm$ 0.06& &\\ $t\overline{t}$ total MC& & 121.22 $\pm$ 0.43& & 54.68 $\pm$ 0.20&\\ $t\overline{t}$ total fitted& & 133.04 $\pm$ 17.09& & 33.12 $\pm$ 15.04&\\ $W$+jets& 17.82 $\pm$ 0.33& & 11.26 $\pm$ 0.23& &\\ $Z$+jets& 2.78 $\pm$ 0.14& & 2.39 $\pm$ 0.12& &\\ QCD& & 232.35 $\pm$ 17.09& & 412.22 $\pm$ 15.04\\ Signal significance& & 6.77& & 1.54 &\\ S/B ratio& & 0.52& & 0.08\\ \end{tabular} %\end{ruledtabular} \label{event yeild summary1} \end{table} % \clearpage %The cross section is defined as %$\sigma=\frac{Number\, of\, signal\, events}{\varepsilon(t\bar{t})\cdot BR(t\bar{t}\rightarrow \tau+jets)\cdot Luminosity}$. %However, we are not simply doing a `counting experiment`, but want to utilize the entire range of NN output. The cross section is measured by minimizing the sum of the negative log-likelihood functions for each bin of both the Types 1 and 2 channel and the Type 3 $\tau$ channel. These are functions used by MINUIT to perform fits shown in Figs \ref{fig:nnout_type2} and \ref{fig:nnout_type3} in Section \ref{sub:NN-variables}. But there $L$ was function of $f(QCD)$ and now we want to use it to measure the cross section, so we must express it in terms of $\sigma(\ttbar)$: \begin{center} \begin{equation} L(\sigma, \tilde{N}_{i}, N^{obs}_{i}) \equiv -\ln(\prod_{i} \frac{\tilde{N}^{N^{obs}_{i}}_{i}}{N^{obs}_{i}!} e^{-\tilde{N}_{i}}) \label{log_xsec} \end{equation} \end{center} \noindent where \(\tilde{N}_{i} = \sigma \times BR \times \mathcal{L} \times \epsilon(t\bar{t})_{i} + N_{bkg}\) is number events predicted in bin $i$ of the data NN distribution and \(N^{obs}_{i}\) is the actual count observed in that bin. The minimum value of the graph of the function in Eq \ref{log_xsec} is the cross section. But, as stressed out in Section \ref{sub:Results-of-the}, we have to take into account both signal ($\ttbar$) and electroweak contamination in the loose-tight sample we use to model QCD in the high NN region used for the measurement. The electroweak component is small and therefore it is kept fixed during the fit and subtracted from the loose-tight sample. However, as dicussed before, the numbers for signal contamination are 5.4\% and 3.0\% for taus types 1 and 2 and type 3 respectively when we assumed a $t\bar{t}$ cross section of 7.46 pb. This means that 5.4\% (12.55 events) of 232.35 QCD events for taus types 1 and 2 are actually $\ttbar$ events and 3.0\% (12.37 events) of 412.22 QCD events for taus type 3 are actually $\ttbar$ events. 12.55 and 12.37 events represent increases of 9.43\% and 37.35\% on the number of signal events for types 1 and 2 and type 3 respectively. However this is not the final measurement yet since the cross section measurement only makes sense if the cross section we measure in the and is the same as the one we have assumed to normalize $t\bar{t}$ MC samples. This means that we had to iterate back by normalizing the signal samples until we found a convergence of the cross section. Table 33 summarizes the iteration process. \begin{table}[htbp] \label{est} \begin{center} \begin{tabular}{|c|r|r|r|} \hline Assumed $\sigma(\ttbar)$ (pb) & signal contamination for types 1 \& 2 (\%) & signal contamination for type 3 (\%) & measured $\sigma(\ttbar)$ (pb) \\ \hline \hline \multicolumn{1}{|c|}{7.46} & \multicolumn{1}{c|}{5.4} & \multicolumn{1}{c|}{3.0} & \multicolumn{1}{c|}{8.37} \\ \hline %$t\bar{t} \rightarrow \mbox{dilepton}$ & \multicolumn{1}{c|}{1.4} \\ \hline \multicolumn{1}{|c|}{8.37} & \multicolumn{1}{c|}{6.1} & \multicolumn{1}{c|}{3.3} & \multicolumn{1}{c|}{8.42} \\ \hline \multicolumn{1}{|c|}{8.42} & \multicolumn{1}{c|}{6.2} & \multicolumn{1}{c|}{3.4} & \multicolumn{1}{c|}{8.46} \\ \hline \multicolumn{1}{|c|}{8.46} & \multicolumn{1}{c|}{6.2} & \multicolumn{1}{c|}{3.4} & \multicolumn{1}{c|}{8.46} \\ \hline \end{tabular} \caption{Cross section iteration process.} \end{center} \label{iteration1} \end{table} Table \ref{iteration1} shows that when we assumed a cross section of 8.46 pb we measured the exact same value, which means that we had to take into account signal contaminations of 6.2\% (14.40 events) and 3.4\% (14.02 events) for taus types 1 and 2 and 3 respectively. This represents an increase in the number of signal events of 10.82\% for types 1 and 2 and 42.33\% for type 3. By considering such events as part of the signal $\ttbar$ sample we measure for the cross sections: %\newpage \begin{center}$\tau$+jets types 1 and 2 cross section: \[\sigma (t\overline{t}) = 8.83\;\;_{-1.12}^{+1.14}\;\;({\textrm{stat}})\;\;_{-0.79}^{+0.84}\;\;({\textrm{syst}})\;\;\pm 0.3\;\;({\textrm{lumi}})\;\; \rm{pb,}\] \par\end{center} \begin{center}$\tau$+jets type 3 cross section: \[\sigma (t\overline{t}) = 6.06\;\;_{-2.62}^{+2.77}\;\;({\textrm{stat}})\;\;_{-0.82}^{+0.88}\;\;({\textrm{syst}})\;\;\pm 0.3\;\;({\textrm{lumi}})\;\; \rm{pb,}\] \par\end{center} \begin{center}Combined cross section: \[\sigma (t\overline{t}) = 8.46\;\;_{-1.04}^{+1.06}\;\;({\textrm{stat}})\;\;_{-0.78}^{+0.84}\;\;({\textrm{syst}})\;\;\pm 0.3\;\;({\textrm{lumi}})\;\; \rm{pb.}\] \par\end{center} \noindent The correspondent negative log likelihoods of these measurements are shown in Figures \ref{fig:type2_llhood}, \ref{fig:type3_llhood} and \ref{fig:type123_llhood}. Figures \ref{fig:xsec_pres2_llhood}, \ref{fig:xsec_pres3_llhood} and \ref{fig:xsec_pres123_llhood} show zoomed in graphs of the same likelihood functions described above. All associated systematics concerning this measurement can be seen in Table \ref{cap:Syst1}. %\newpage \begin{figure}[h] \includegraphics[scale=0.38]{plots2/type2_llhood.eps} \caption{The log likelihood function for type 1 and 2 $\tau$ channel} \label{fig:type2_llhood} \end{figure} %\newpage \begin{figure}[h] \includegraphics[scale=0.38]{plots2/type3_llhood.eps} \caption{The log likelihood function for type 3 $\tau$ channel} \label{fig:type3_llhood} \end{figure} \begin{figure}[b] \includegraphics[scale=0.38]{plots2/type123_llhood.eps} \caption{The log likelihood function for all three types combined} \label{fig:type123_llhood} \end{figure} \clearpage \begin{figure}[h] \includegraphics[scale=0.38]{plots2/xsec12_pres.eps} \caption{Zoom in of the log likelihood function for type 1 and 2 $\tau$ channel} \label{fig:xsec_pres2_llhood} \end{figure} %\newpage \begin{figure}[h] \includegraphics[scale=0.38]{plots2/xsec3_pres.eps} \caption{Zoom in of the log likelihood function for type 3 $\tau$ channel} \label{fig:xsec_pres3_llhood} \end{figure} \begin{figure}[b] \includegraphics[scale=0.38]{plots2/xsecall_pres.eps} \caption{Zoom in of the log likelihood function for all three types combined} \label{fig:xsec_pres123_llhood} \end{figure} \clearpage After measuring the combined cross section we observed a significant higher statistical uncertainty value if compared to the one we expected to see based on the fact that we have approximately 5 times more data than in p17, where the signal contamination was not taken into account (see Appendix \ref{app:xsec_nocont}). Further investigation showed that the cut NNelec $>$ 0.9 applied to taus type 2 only was responsible for such discrepancy. Below we show the same measurement as done above but now with no NNelec cut applied. Table \ref{event yeild summary2} summarizes the number of events in each channel after final selection. \begin{table}[h] \caption{Final number of events in the two analysis channels.} %\begin{ruledtabular} \begin{tabular}{cccccc} \hline &$\tau$ type I,II &$\tau$ type I,II (fitted) &$\tau$ type III &$\tau$ type III (fitted)&\\ \hline data& 583 & & 459 & &\\ $t\overline{t}\rightarrow\tau+jets$& 85.46 $\pm$ 0.58& & 38.82 $\pm$ 0.39&\\ $t\overline{t}\rightarrow e+jets$& 175.23 $\pm$ 0.85& & 6.52 $\pm$ 0.16& &\\ $t\overline{t}\rightarrow\mu+jets$& 8.98 $\pm$ 0.19& & 5.14 $\pm$ 0.14& &\\ $t\overline{t}\rightarrow l+l$& 12.62 $\pm$ 0.10& & 4.18 $\pm$ 0.06& &\\ $t\overline{t}$ total MC& & 282.27 $\pm$ 1.05& & 54.67 $\pm$ 0.41&\\ $t\overline{t}$ total fitted& & 260.71 $\pm$ 20.74& & 35.73 $\pm$ 15.28&\\ $W$+jets& 39.65 $\pm$ 0.50& & 11.26 $\pm$ 0.25& &\\ $Z$+jets& 4.56 $\pm$ 0.10& & 2.38 $\pm$ 0.11& &\\ QCD& & 278.04 $\pm$ 20.74& & 409.62 $\pm$ 15.28\\ Signal significance& & 10.80& & 1.67 &\\ S/B ratio& & 0.80& & 0.08\\ \end{tabular} %\end{ruledtabular} \label{event yeild summary2} \end{table} % Table below shows the iteration process in this case and and the cross section measurement follows: \begin{table}[htbp] \label{est} \begin{center} \begin{tabular}{|c|r|r|r|} \hline Assumed $\sigma(\ttbar)$ (pb) & signal contamination for types 1 \& 2 (\%) & signal contamination for type 3 (\%) & measured $\sigma(\ttbar)$ (pb) \\ \hline \hline \multicolumn{1}{|c|}{7.46} & \multicolumn{1}{c|}{6.0} & \multicolumn{1}{c|}{3.0} & \multicolumn{1}{c|}{6.84} \\ \hline %$t\bar{t} \rightarrow \mbox{dilepton}$ & \multicolumn{1}{c|}{1.4} \\ \hline \multicolumn{1}{|c|}{6.84} & \multicolumn{1}{c|}{5.4} & \multicolumn{1}{c|}{2.7} & \multicolumn{1}{c|}{6.91} \\ \hline \multicolumn{1}{|c|}{6.91} & \multicolumn{1}{c|}{5.5} & \multicolumn{1}{c|}{2.8} & \multicolumn{1}{c|}{6.92} \\ \hline \multicolumn{1}{|c|}{6.92} & \multicolumn{1}{c|}{5.5} & \multicolumn{1}{c|}{2.8} & \multicolumn{1}{c|}{6.92} \\ \hline \end{tabular} \caption{Cross section iteration process.} \end{center} \label{iteration2} \end{table} %\newpage \begin{center}$\tau$+jets types 1 and 2 cross section: \[\sigma (t\overline{t}) = 7.03\;\;_{-0.56}^{+0.54}\;\;({\textrm{stat}})\;\;_{-0.61}^{+0.65}\;\;({\textrm{syst}})\;\;\pm 0.3\;\;({\textrm{lumi}})\;\; \rm{pb,}\] \par\end{center} \begin{center}$\tau$+jets type 3 cross section: \[\sigma (t\overline{t}) = 4.36\;\;_{-2.50}^{+2.62}\;\;({\textrm{stat}})\;\;_{-0.61}^{+0.62}\;\;({\textrm{syst}})\;\;\pm 0.3\;\;({\textrm{lumi}})\;\; \rm{pb,}\] \par\end{center} \begin{center}Combined cross section: \[\sigma (t\overline{t}) = 6.92\;\;_{-0.54}^{+0.54}\;\;({\textrm{stat}})\;\;_{-0.60}^{+0.62}\;\;({\textrm{syst}})\;\;\pm 0.3\;\;({\textrm{lumi}})\;\; \rm{pb,}\] \par\end{center} As we can see the statistical uncertainty decreases to 0.54 pb, which is in a good agreement with what we would expect if compared to the 1.2 pb measured in p17. Appendix \ref{app:xsec_nocont} shows cross section measurements when signal contamination is not taken into account for both NNelec $>$ 0.9 and no NNelec cut applied. Once again, we observed the difference caused by the NNelec requirement.