Title: Measurement of Rb and Br(b->lnuX) at LEP Using Double-Tag Methods


We present a combined measurement of 
Rb = Gamma(Z -> bb) / Gamma(Z -> hadrons}) 
and the semileptonic branching ratio of b quarks in Z
decays, Br(b->lnuX), 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->lnuX):

       Rb = 0.2174 +- 0.0015(stat.) +- 0.0028(sys.);
    Br(b->lnuX) = (10.16 +- 0.13(stat.) +- 0.30(sys.))%.

parallel session #PA 5,7
plenary session #Pl 6,3

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* paper 185 *  European Physical Journal C 13/1, 47-61 (2000)
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