Measurement of $$R_\mathrm{b}$$ and $$\mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X})$$ at LEP using double-tag methods

Abstract. We present a combined measurement of $$R_\mathrm{b} = \Gamma(\mathrm{Z \rightarrow b\bar{b}}) / \Gamma(\mathrm{Z}\rightarrow \mbox{hadrons})$$ and the semileptonic branching ratio of b quarks in Z decays, $$\mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X})$$, using double-tag methods. Two analyses are performed on one million hadronic Z decays collected in 1994 and 1995. The first analysis exploits the capabilities of the silicon microvertex detector. The tagging of b-events is based on the large impact parameter of tracks from weakly decaying b-hadrons with respect to the $$\mathrm{e^+ e^-}$$ collision point. In the second analysis, a high-$$p_t$$ lepton tag is used to enhance the b-component in the sample and its momentum spectrum is used to constrain the model dependent uncertainties in the semileptonic b-decay. The analyses are combined in order to provide precise determinations of $$R_\mathrm{b}$$ and $$\mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X})$$: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} \[ R_\mathrm{b} = 0.2174 \pm 0.0015\mbox{ (stat.) } \pm 0.0028\mbox{ (sys.) }; \] \end{document}\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} \[ \mathrm{Br}(\mathrm{b} \rightarrow \ell\nu\mathrm{X}) = (10.16 \pm 0.13\mbox{ (stat.) } \pm 0.30\mbox{ (sys.) })\%. \] \end{document}