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 root 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: 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 |V(cs|) 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 qqev and qq mu v final states. The results described in this paper are consistent with the expectations from the standard model.