A Precise Measurement of $\Gamma_{Z^{0} \to b\overline{b}} / \Gamma_{Z^{0} \to hadrons}$

The ratio Rb = T z0->bb/T z0-> hadrons has been measured using about 3.8 million hadronic Z 0 decays recorded by the ALEPH detector at LEP during the period 1992-95. A new high-purity b-tag was developed, relying on both the large impact parameters of tracks from B hadrons and on the large mass of the b quark compared to the c quark. A new primary vertex finder reduced systematic uncertainty due to hemisphere-hemisphere correlation in tagging efficiency. Improved understanding of the silicon vertex detector efficiency and its simulation in the Monte Carlo helped to reduce systematic errors due to uncertain ties in the detector simulation.