Messung der Reaktion e+ e- ZZ q anti-q l+ l mit dem L3-Detektor bei LEP
In this thesis measurements of the pair production of Z bosons and their decay into a final state with two quarks and two charged leptons are performed. The data have been recorded in the years 1997 to 2000 with the L3 experiment at CERN in Geneva. The integrated luminosity of this data set is 683 1/pb, with the center-of-mass energy sqrt(s) in a range from 183 to 208 GeV. After the selection, for which the lepton identification plays a crucial role, 53 events are selected in the data, whilst a total of 57.4 events are expected from Standard Model processes, consisting of 47.5 events from Z boson pair production and 9.9 events from the background. An efficiency of 53.7% on the signal is achieved. Firstly, the cross-section for each of the eight given energy points, but summed over the three lepton types, is computed. All measurements show good agreement with the prediction of the Standard Model. Then, the cross-section for the three different lepton types, but summed over all center-of-mass energies, is measured. The measurement is given at the averaged center-of-mass energy sqrt(s) = 196.67 GeV, and again is in good agreement with the Standard Model expectation. To understand one of the major backgrounds for the Standard Model Higgs production and to test the capability to detect a low-mass Higgs boson with the L3 experiment, the process e+ e- ---> Z Z ---> q anti-q l+ l-, a subset of the previously mentioned data, is investigated more closely. The measured cross-section of this sub-process shows good agreement with the Standard Model. A search for anomalous couplings, forbidden through the SU(2)xU(1) gauge symmetry of the Standard Model, is performed. The four anomalous couplings f4Z, f5Z, f4g, f5g are considered, which change the average polarisation and angular distribution of the Z bosons and the cross-section of the Z boson pair production. The data show no evidence for such couplings, hence limits at a 95% confidence level have been set. Recently, theories have been developed to solve the hierarchy problem by introducing additional spatial dimensions, lowering the scale MS of quantum gravity to the order of electroweak symmetry breaking O(1000 GeV). In this extra dimensions, new particles (the gravitons'') can propagate, coupling to the Standard Model fields and enhancing the cross-section of the Z boson pair production. From the measured cross-section, no evidence for such particles is found, and a limit at a 95% confidence level to the scale MS of this theory for two different scenarios can be set for two different scenarios, to absorb possible model dependence. To conclude, all measurements are in good agreement with the Standard Model, and no evidence of physics beyond the Standard Model has been found.
info:eu-repo/classification/ddc/530, Physik, Elektron-Positron-Vernichtung, Z-Boson, Paarerzeugung, Zerfall, Quark, Lepton, L3-Detektor
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