Measurement of {ital R}{sub {ital b}} Using a Vertex Mass Tag

We report a new measurement of R{sub b}={Gamma}{sub Z{sup 0}{r_arrow}b{bar b}} /{Gamma}{sub Z{sup 0}{r_arrow}hadrons} using a double tag technique, where the b hemisphere selection is based on the reconstructed mass of the B hadron decay vertex. The measurement was performed using a sample of 130{times}10{sup 3} hadronic Z{sup 0} events, collected with the SLD detector at SLC. The method utilizes the 3D vertexing abilities of the CCD pixel vertex detector and the small stable SLC beams to obtain a high b -tagging efficiency and purity. We obtain R{sub b}=0.2142{plus_minus}0.0034(stat) {plus_minus}0.0015(syst){plus_minus}0.0002( R{sub c}) . {copyright} {ital 1998} {ital The American Physical Society}