Measurement of the e+e− →W+W− cross section and W decay branching fractions at LEP

From a total data sample of 701.1 pb recorded with ee centre-of-mass energies of √ s= 161–209 GeV with the OPAL detector at LEP, 11693 W -pair candidate events are selected. These data are used to obtain measurements of theW -pair production cross sections at 10 different centre-of-mass energies. The ratio of the measured cross sections to the standard model expectation is found to be: data/SM= 1.002±0.011(stat.)±0.007(syst.)±0.005(theory) , where the uncertainties are statistical, experimental systematics and theory systematics respectively. The data are used to determine the W boson branching fractions, which are found to be consistent with lepton universality of the charged current interaction. Assuming lepton universality, the branching ratio to hadrons is determined to be 67.41±0.37(stat.)±0.23(syst.)%, from which the CKM matrix element |Vcs| is determined to be 0.969±0.017(stat.)±0.012(syst.). The differential cross section as a function of the W production angle is measured for the qqeν and qqμν final states. The results described in this paper are consistent with the expectations from the standard model. a Deceased b e-mail: [email protected] c supported by Interuniversity Attraction Poles Programme – Belgian Science Policy