Measurement of using leptons

The fraction of$$b\bar b$$ events in hadronicZ0 decays has been measured from the yield of leptons in the data samples collected by OPAL in 1990 and 1991. A sample enriched in events containing$${\rm Z}^0 \to b\bar b$$ decays was obtained by requiring the presence of an electron or muon with high momentum and high momentum component transverse to the associated hadronic jet. After accounting for backgrounds and acceptances, a value of$$\frac{{\Gamma ({\rm Z}^0 \to b\bar b)}}{{\Gamma ({\rm Z}^0 \to hadrons)}} = 0.220 \pm 0.002 \pm 0.0006 \pm 0.011$$ was obtained. The first two errors reflect the data statistics and the systematic uncertainties arising from detector modelling uncertainties, respectively. The third error includes systematic effects fromb andc fragmentation and decay uncertainties.