Measurement of the e_s16e- -> W_s16W- cross section and W decay branching fractions at LEP

Abstract

From a total data sample of 701.1 pb^-1 recorded with e_s16e- centre-of-mass energies of sqrt = 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 _s16- 0.011(stat.) _s16- 0.007(syst.) _s16- 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 _s16- 0.37(stat.) _s16- 0.23(syst.)%, from which the CKM matrix element Vcs is determined to be 0.96_s16-0.017(stat.)_s16-0.012(syst.). The differential cross section as a function of the W^- production angle is measured for the qqev and qqmv final states. The results described in this paper are consistent with the expectations from the Standard Model.