Messung des hadronischen Wirkungsquerschnitts des Prozesses e+e- -> W+W- mit dem DELPHI-Detektor

In this dissertation the first analysis measuring the hadronic cross–section of the W–pair production e +e − → W+W− consistently at all Lep2 energies is presented. Using a feed–forward neural network for the event selection the performance of the analysis was improved significantly compared to the previous Delphi selections. From the data sample collected with the Delphi detector corresponding to a total integrated luminosity of about 670 pb−1 the cross–section is determined for ten different centre–of–mass energies between 161 and 207 GeV. The results from all energies above 180 GeV are combined to obtain the ratio of the measured and the predicted cross–section, RWW→qqqq, using Gentle 2.0, RacoonWW and Yfsww. Especially for the most recent and most precise predictions using the double–pole approximation for a full calculation of O(α) corrections the results agree very well with the expectation, e.g. the ratio for RacoonWW is: RWW→qqqq = 1.005 ± 0.017 (stat) ± 0.014 (syst) ± 0.005 (theo). Because of the very good performance the neural network is used in different Delphi analyses on WW physics, e.g. in studies of the triple and quartic gauge couplings. It covers the hadronic channel in the analysis of the total cross–section of the W–pair production where additionally the W branching fractions are measured. All results agree with the Standard Model predictions. From the leptonic branching fraction a precise determination of the CKM matrix element VCS is derived.