Measurement of thebb¯fraction in hadronicZ0decays with precision vertex detectors

We have measured the fraction of bb¯ events in hadronic ${\mathit{Z}}^0$ decays, ${\mathit{R}}_{\mathit{b}\mathit{b}\mathrm{¯}}$, using the vertex detector system of the Mark II detector at the SLAC Linear Collider. 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 ${\mathit{R}}_{\mathit{b}\mathit{b}\mathrm{¯}}$=0.251±0.049±0.030, in good agreement with other measurements and the standard model prediction.