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-1 recorded with e+e- centre-of-mass energies of Ös =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 the W-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: \textdata/\textSM = 1.002±0.011 (\textstat.) ±0.007 (\textsyst.) ±0.005 (\texttheory)dataSM=10020011(stat.)0007(syst.)0005(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.