A precise measurement of the partial decay width ratio ${R_{\mathrm{b}}}^0 = \Gamma_{\mathrm{b\bar{b}}}/\Gamma_{\mathrm{had}}$

Abstract.

The partial decay width of the Z to ${\rm b \bar b}$ quark pairs has been measured by the DELPHI detector at LEP using data taken in the years 1992 to 1995. Decays of b-hadrons were tagged by several methods using tracks with large impact parameters and/or reconstructed secondary vertices, complemented by event shape variables. Combining these methods in a multivariate analysis the value \[ \frac{\Gamma({\mathrm Z \rightarrow{\rm b \bar b}}) } { \Gamma({\mathrm{Z \rightarrow had}}) } = 0.21634 \pm 0.00067({\mathrm{stat}}) \pm 0.00060 ({\mathrm{syst}}) \] was obtained, where the ${\mathrm c} \bar{\mathrm c}$ production fraction was fixed to its Standard Model value.