A measurement of Rbo using the DELPHI detector at LEP

A measurement of Rbo using the DELPHI detector at LEP is presented. Rbo is the fraction of Zo → bb-bar events in hadronic Zo decays, or the ratio of the partial width Γbb-bar to the total hadronic width Γhad. Since the mass of the top quark has now been measured, a precision measurement of Rbo forms a test of the Standard Model and allows possible new physics to be probed. A b tagging algorithm, based on the reconstruction of the secondary decay vertices of long-lived bottom hadrons, was developed. Traditionally, such analyses use some measure of the distance of the secondary vertex from the interaction point to improve purity. In this analysis, several other vertex properties were combined in a multivariate approach to allow better control over the background. An efficiency of 24.4% was achieved for a purity of 89.6%. Rbo was then measured in the 1994 data sample (corresponding to 1,484,000 hadronic events) using the double tag technique which allows the b-tagging efficiency to be measured in the data. This removes many of the uncertainties associated with both b physics and detector modelling and decreases the systematic error. This analysis found; Rbo = 0.2101 ± 0.0021(stat) ± 0.0031(sys) - 0.18(Rc - 0.172) where the first error is statistical and the second systematic. This value is consistent with the Standard Model prediction of 0.2158 ± 0.0003 at the level of 1.5σ. (author)