W-Boson Analyses of the DELPHI Group of Wuppertal
Between 1996 and 2000 the center-of-mass energy of the large electron positron collider LEP at CERN was above the threshold of the W-pair production e+e- -> W+W-. A precise measurement of the cross-section of this process represents an important test of the Standard Model of particle physics and allows inferences about the existence of new physics.
At DELPHI the cross-section was determined at 10 different center-of-mass energies between 161 and 207 GeV. The results were compared to the most recent and most precise theory predictions and a very good agreement is found:
The ratio between experimenal result and theoretical prediction for all energies above 180 GeV is found to be:
R_WW = 1.001 +- 0.012 (stat) +- 0.010 (syst) +- 0.005 (theo) .
Not only the cross-section but also the branching fraction of the W-pair production was determined. The high precision of this measurement (below 0.5%) allows in addition a very precise determination of the element V_cs of the Cabbibo-Kobayashi-Maskawa matrix.
The main focus of the analyses performed in Wuppertal was the hadronic decay channel of the W-pair production (W+W- -> q qbar q qbar). To improve the selection quality neural networks were used successfully.
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