DOE OSTI2012
Top quark, the heaviest and most intriguing among the six known quarks, can be created via two independent production mechanisms in {\pp} collisions. The primary mode, strong {\ttbar} pair production from a $gtt$ vertex, was used by the {\d0} and CDF collaborations to establish the existence of the top quark in March 1995. The second mode is the electroweak production of a single top quark or antiquark, which has been observed recently in March 2009. Since single top quarks are produced at hadron colliders through a $Wtb$ vertex, thereby provide a direct probe of the nature of $Wtb$ coupling and of the Cabibbo-Kobayashi-Maskawa matrix element, $V_{tb}$. So this mechanism provides a sensitive probe for several, standard model and beyond standard model, parameters such as anomalous $Wtb$ couplings. In this thesis, we measure the cross section of the electroweak produced top quark in three different production modes, $s+t$, $s$ and $t$-channels using a technique based on the Bayesian neural networks. This technique is applied for analysis of the 5.4 $fb^{-1}$ of data collected by the {\d0} detector. From a comparison of the Bayesian neural networks discriminants between data and the signal-background model using Bayesian statistics, the cross sections of the top quark produced through the electroweak mechanism have been measured as: \[\sigma(p\bar{p}→tb+X,tqb+X) = 3.11^{+0.77}_{-0.71}\;\rm pb\] \[\sigma(p\bar{p}→tb+X) = 0.72^{+0.44}_{-0.43}\;\rm pb\] \[\sigma(p\bar{p}→tqb+X) = 2.92^{+0.87}_{-0.73}\;\rm pb\] % The $s+t$-channel has a gaussian significance of $4.7\sigma$, the $s$-channel $0.9\sigma$ and the $t$-channel~$4.7\sigma$. The results are consistent with the standard model predictions within one standard deviation. By combining these results with the results for two other analyses (using different MVA techniques) improved results \[\sigma(p\bar{p}→tb+X,tqb+X) = 3.43^{+0.73}_{-0.74}\;\rm pb\] \[\sigma(p\bar{p}→tb+X) = 0.68^{+0.38}_{-0.35}\;\rm pb\] \[\sigma(p\bar{p}→tqb+X) = 2.86^{+0.69}_{-0.63}\;\rm pb\] % were obtained with a significance of $5.4\sigma$, $1.8\sigma$ and $5.0\sigma$ respectively for $s+t$, $s$ and $t$-channels. Using this measured cross section and constraining $0 \leq |V_{tb}|^2 \leq 1$, the lower limit has been calculated to be $|V_{tb}| > 0.79$ with 95\% confidence level (C.L.). Another measurement of $tqb$ production cross section is done using the same dataset and discriminant but without any assumption on the $tb$ production rate. From this measurement, we obtain a cross section of $2.90^{+0.59}_{-0.59}$~pb for $t$-channel and corresponding significance of $5.5\sigma$. In addition to the above mentioned work, a search is made for the anomalous $Wtb$ couplings in single top quark production. Within the Standard Model, the $Wtb$ vertex is purely left-handed, and its amplitude is given by the $V_{tb}$, related to weak interaction between a top and a $b$-quark. In a more general way, additional anomalous couplings such as right-handed vectorial couplings and left and right-handed tensorial couplings can also be considered. An analysis based on the Bayesian neural networks method is used to separate the signal from expected backgrounds. We find no evidence for anomalous couplings and set 95\% C.L. limits on these couplings as $|V_{tb} \cdot f_{L_T}|^2<0.06$, $|V_{tb} \cdot f_{R_V}|^2<0.93$ and $|V_{tb} \cdot f_{R_T}|^2<0.13$. This result represents the most stringent direct constraints on anomalous $Wtb$ interactions. This work has been done in collaboration with \d0 experiment but the analyses and results presented in this thesis are my contribution.
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗