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

The partial decay width of the Z to $\bb$ quark pairs has beenmeasured by the DELPHI detector at LEP using data taken in the years1992 to 1995.Decays of b-hadrons were tagged by several methods usingtracks with large impact parameters and/or reconstructed secondary vertices, complemented by event shape variables.Combining these methods in a multivariate analysis the value\[\frac{\Gamma({\mathrm Z \rightarrow \bb}) }{ \Gamma({\mathrm {Z \rightarrow had}}) } = 0.21634 \pm 0.00067({\mathrm {stat}}) \pm 0.00060 ({\mathrm {syst}})\]was obtained, where the ${\mathrm c} \bar {\mathrm c}$ production fractionwas fixed to its Standard Model value.