MEASUREMENT OF GAMMA(Z(0)-]B(B)OVER-BAR) GAMMA(Z(0)-]HADRONS) USING ADOUBLE TAGGING METHOD/

The fraction of Z(0) --> b (b) over bar events in hadronic Z(0) decays has been measured using the data collected by OPAL in 1992 and 1993. The presence of electrons or muons from semileptonic decays of bottom hadrons and the detection of bottom hadron decay vertices were used to gether to obtain an event sample enriched in Z(0) --> b (b) over bar d ecays. To reduce the systematic error on the measurement of the Z(0) - -> b (b) over bar fraction, the efficiency of the b (b) over bar event tagging was obtained from the data by comparing the numbers of events having a bottom signature in either one or both thrust hemispheres. A value of Gamma(Z(0) --> b (b) over bar)/Gamma(Z(0) --> hadrons) = 0.2 171 +/- 0.0021 +/- 0.0021 was obtained, where the first error is stati stical and the second systematic. The uncertainty on the decay width G amma(Z(0) --> c (c) over bar) is not included in these errors. A fract ional variation of this width by +/-8% about its Standard Model predic tion would result in a variation of the measured Z(0) --> b (b) over b ar fraction of double dagger 0.0015.