A measurement of Rb using a lifetime-mass tag

Abstract ALEPH's published measurement of R b = Γ( Z → b b ) Γ( Z → hadrons ) using a lifetime tag is updated using the full LEP 1 data sample. Considerable effort has been devoted to understanding systematic effects. Charm background is better controlled by combining the lifetime tag with a tag based on the b c hadron mass difference. Furthermore, the algorithm used to reconstruct the event primary vertex is designed so as to reduce correlations between the two hemispheres of an event. The value of R b is measured to be 0.2167 ± 0.0011(stat) ± 0.0013(syst).