A Precise measurement of Gamma ($Z \to b \bar{b}$) / Gamma ($Z \to$ hadrons)

A measurement of the partial width ratio Γ b b Γ had using a method which tags the Z → b b decays through the lif etime of the produced heavy hadrons is presented. This method relies on the tracking precision afforded by a double-sided silicon vertex detector. The tag algorithm makes a probabilistic interpretation of three-dimensional track impact parameters, using the data to measure the resolution. By tagging the two b hadrons separately, both Γ b b Γ had and the tag efficiency can be determined from the data. For a 26% efficiency of tagging a single b hadron within the vertex detector solid angle coverage, a purity of 96% is achieved. A value of Γ b b Γ had = 0.2192±0.0026( stat. )±0.0016(Γ c c / Γ had ) is found. Combining this result with other recent ALEPH Γ b b Γ had measurements gives a 95% confidence upper limit on the Standard Model top mass of M t < 228 GeV.