Measurement of the W-pair cross section in e+e− collisions at 172 GeV

The e+e--->W+W- cross section is measured in a data sample collected by ALEPH at a mean centre-of-mass energy of 172.09 GeV, corresponding to an integrated luminosity of 10.65 pb-1. Cross sections are given for the three topologies, fully leptonic, semi-leptonic and hadronic of a W-pair decay. Under the assumption that no other decay modes are present, the W-pair cross section is measured to be 11.7+/-1.2(stat.)+/-0.3(syst.) pb. The existence of the triple gauge boson vertex of the Standard Model is clearly preferred by the data. The decay branching ratio of the W boson into hadrons is measured to be B(W-->hadrons)=67.7+/-3.1(stat.)+/-0.7(syst.) %, allowing a determination of the CKM matrix element Vcs=0.98+/-0.14(stat.)+/-0.03(syst.).