Measurement of W-pair production in e+e- collisions at 189 GeV

The production of W+W- pairs is analysed in a data sample collected by ALEPH at a mean centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 174.2 pb(-1). Cross sections are given for different topologies of W decays into leptons or hadrons. Combining all final states and assuming Standard Model branching fractions, the total W-pail cross section is measured to be 15.71 +/- 0.34(stat.) +/- 0.18(syst.) pb. Using also the W-pair data samples collected by ALEPH at lower centre-of-mass energies, the decay branching fraction of the W boson into hadrons is measured to be B(W --> hadrons) = 66.97 +/- 0.65(stat.) +/- 0.32(syst.)%, allowing a determination of the CKM matrix element /V-cs/ = 0.951 +/- 0.030(stat,) +/- 0.015(syst.). (C) 2000 Elsevier Science B.V. All rights reserved.