Measurement of the electroweak top quark production cross section and the CKM matrix element Vtb with the DOe experiment

This analysis presents a measurement of the combined s+t-channel electroweak top quark production cross section at the Tevatron collider. An integrated luminosity of nearly 1 fb-1 of Run∝II data has been analyzed. After the selection of single top quark-like events, a Multi-Process Factory has been used to analyze each event under several physics process hypotheses. The novelty in this analysis is that an event is not only studied under the assumption of the signal process decay chain, but also under the assumption to originate from one of the major background physics processes. In three jet multiplicity bins a total of 13 decay chains are reconstructed and evaluated. The implementation and optimization of this technique is a first step towards analysis methods which are able to actively identify an event as a signal or background event. In this analysis the most likely decay chain of an event for each signal and background process under investigation is used to obtain decay chain dependent observables. The close connection of the observables to the partons of a physics process decay chain leads to an intuitive interpretation of the resulting distributions. Observables gained in the Multi-Process Factory are used as input to a state-of-the-art multivariate analysis technique, Boosted Decision Trees. The configuration of these Boosted Decision Trees has been optimized with the goal to maximize the separation of the expected signal contribution from the overwhelming background. Using a Bayesian approach the discriminant distributions of the twelve independent analysis channels are fitted simultaneously. This analysis measures the combined s+t channel production cross section to be σ(p anti p→ tb + X, tqb + X) = 5.51-1.31+1.62 pb for a top mass of 175 GeV and assuming a Standard Model s-channel to t-channel cross section ratio of σs/σt = 0.44. The significance of the cross section measurement is 4.2 standard deviations. The measured cross section value exceeds the Standard Model expectation by 2 standard deviations. The result of the analysis presented here is in good agreement with the result of σ(p anti p→tb+X,tqb+X)=4.8± 1.3 pb, obtained from the combination of three other analyses performed on the same data set. From the cross section measurement a measurement of the strength vertical stroke Vtb x f1L vertical stroke of the V-A coupling at the Wtb-vertex has been extracted. The result is vertical stroke Vtb x f1L vertical stroke =1.42-0.20+0.21. This value is above the Standard Model expectation by about 2∝standard deviations. The measurement agrees within uncertainties with the measurement of vertical stroke Vtb x f1L vertical stroke =1.31-0.21+0.25 obtained by another analysis performed on the same data set. Constraining the prior of this measurement to the interval [0,1], i.e. setting the strength of the left-handed coupling f1L=1, a result for the CKM matrix element vertical stroke Vtb vertical stroke has been determined to vertical stroke Vtb vertical stroke =1.00-0.08+0.00. From the posterior probability density of this measurement a lower limit for Vtb has been set at 95% confidence level: vertical stroke Vtb vertical stroke >0.79 rate at 95% C.L. (orig.)