Measurement of R-b and Br(b -> lvX) at LEP using double-tag methods

We present a combined measurement of R-b = Gamma(Z --> b (b) over bar)/Gamma(Z --> hadrons) and the semileptonic branching ratio of b quarks in Z decays, Br(b --> l nu X), using double-tag methods. Two analyses are performed on one million hadronic Z decays collected in 1994 and 1995. The first analysis exploits the capabilities of the silicon microvertex detector. The tagging of b-events is based on the large impact parameter of tracks from weakly decaying b-hadrons with respect to the e(+)e(-) collision point. In the second analysis, a high-p(t) lepton tag is used to enhance the b-component in the sample and its momentum spectrum is used to constrain the model dependent uncertainties in the semileptonic b-decay. The analyses are combined in order to provide precise determinations of R-b and Br(b --> l nu X): R-b = 0.2174 +/- 0.0015 (stat.) +/- 0.0028 (sys.); Br(b --> l nu X) = (10.16 +/- 0.13 (stat.) +/- 0.30 (sys.))%.