MEASUREMENT OF GAMMA(Z(0)-]B(B)OVER-BAR)GAMMA(Z(0)-]HADRONS) USING IMPACT PARAMETERS AND LEPTONS

A measurement of GAMMA(bbBAR)/GAMMA(had) = GAMMA(Z0 --> bbBAR)/GAMMA(Z0 --> hadrons) is presented using a '' mixed tag '' method involving the identification of Z0 --> bbBAR events by two different techniques. The first uses the large impact parameter of tracks emerging from the decay of b-flavoured hadrons and the second their semi-leptonic decay. The identification efficiencies are measured from the data using all possible combinations of the two tags in opposite hemispheres. The method is therefore insensitive to Monte Carlo modelling of bottom quark production and of b-flavoured hadron production and decay properties, and depends only weakly on the simulation of the detector. The data sample collected by OPAL at LEP in 1990 and 1991 is considered. The result is: GAMMA(bbBAR)/GAMMA(had) = 0.218 +/- 0.006 (stat) +/- 0.007 (syst) +/- 0.007 (GAMMA(ccBAR)/GAMMA(had)), where the systematic uncertainty due to the charm quark partial width has been separated from the other systematic uncertainties. Combination with previous OPAL measurements gives: GAMMA(bbBAR)/GAMMA(had) = 0.220 +/- 0.004 (stat) +/- 0.006 (syst) +/- 0.006 (GAMMA(ccBAR)/GAMMA(had)).