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Measurement of Rb and BR(b-> l nu X) at LEP Using Double-Tag Methods

We present a combined measurement of Rb = Γ(Z -> bb) / Γ(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 weak b-decays with respect to the e+e- collision point. In the second analysis, a high-pt 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 Rb and BR(b-> l nu X): Rb = 0.2174 +/- 0.0015 (stat.) +/- 0.0028 (sys.); BR(b-> l nu X) = (10.16 +/- 0.13 (stat.) +/- 0.30 (sys.)) %.