Measurement of Rb Using a Vertex Mass Tag

We report a new measurement of Rb = ΓZ0→bb̅ /ΓZ0→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 130×103 hadronic Z0 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 Rb = 0.2142±0.0034(stat)±0.0015(syst)±0.0002(Rc)