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

We present a combined measurement of $\Rb = \Gamma(\mathrm{Z \rightarrow b\overline{b}}) / \Gamma(\mathrm{Z} \rightarrow\mbox{hadro ns})$ and the semileptonic branching ratio of b quarks in Z decays, $\Brbl$, 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 $\mathrm{e^+e^-}$ 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 $\Rb$ and $\Brbl$: Rb = 0.2174 \pm 0.0015\mbox{(stat.)} \pm 0.0028\mbox{(sys.)}; $\Brbl = (10.16 \pm 0.13\mbox{(stat.)} \pm 0.30\mbox{(sys.)})\%.