MEASUREMENT OF THE b6 FRACTION IN HADRONIC Z” DECAYS WITH PRECISION VERTEX DETECTORS*

We have measured the fraction of bb events in hadronic 2’ decays, R,g, using the vertex detector system of the Mark II detector at the SLC. We tag bb events by requiring the coincidence of three or more tracks with significant impact parameters. This tag is 50% efficient and results in a sample of 85% purity. We find R,b = 0.251 f 0.049 f 0.030, in good agreement with other measurements and the Standard Model prediction.