Messung der qql$\nu$ -Endzustände der W-Paar-Produktion bei Schwerpunktsenergien von 183 GeV bis 202 GeV mit dem L3-Detektor bei LEP

The LEP storage ring at CERN allows to determine the parameters of the standard model with very high precision. Since 1996 the center of mass energy of the collider is well above the W-pair-production threshold, giving access to the decay products of the W-Bosons. Goal of this thesis was the design of a selection for the so called qqln final states (l = e,m,t) of the W-pair-production independent of the lepton flavor. In the events of this class one W-Boson decays into a lepton and the associated neutrino, the other W-Boson decays hadronicaly into two jets of particles. The so called "flavor blind selection" was used to determine the production cross section, brunching ratios, the mass and the width of the W-Boson from the data recorded with the L3 experiment between 1997 and 1999 at centre-of-mass energies between 183 and 202 GeV. Although the selection does not explicitly distinguish between the different lepton flavors, it is possible to separate electrons and muons from a leptonic tau decay from those produced directly by a W-Boson decay. This is done by a transformation of the decay particles into the W-Boson's rest frame. Neural Networks are used to identify the most probable lepton candidate in an event, to find hadronicaly decaying taus in the events and to reject background caused by other processes. The efficiency of the selection reaches from app. 55% for a tau lepton in the final state up to 82% for an electronic final state. The low number of selection cuts leads to a very transparent analysis. Every selected event is counted once, so there is no overlap between the sub samples of different lepton flavors. This avoids double counting of events in the calculation of the brunching ratios. In addition the use of just one set of cuts for the different lepton flavors improves the comparability between the different channels. With or without assumption of lepton universality the cross sections determined in this analysis follow well their theoretical predictions for the full range of studied center of mass energies. Summing up all energy points the brunching ratios of the W-Boson are determined to: BR(W ® en) = (10,58 ± 0,37) % BR(W ® mn) = ( 9,57 ± 0,36) % BR(W ® tn) = (11,91 ± 0,48) % without assumption of lepton universality and BR(W ® ln, l=e,m,t) = (10,69 ± 0,16 ) % with assumption of lepton universality. The standard model prediction is 10.8 % for all flavors. Using a kinematic fit to improve the resolution, the mass and the width of the W-Boson are extracted out of the distributions of the reconstructed invariant masses. Taking into account all energies and decay modes studied in this analysis yields: MW = ( 80,34 ± 0,09 ) GeV GW = ( 2,14 ± 0,21 ) GeV in good agreement with the results published by the Particle Data Group : MW = ( 80,394 ± 0,042 ) GeV GW = ( 2,06 ± 0,05 ) GeV (PDG '99)