A Precise measurement of the partial decay width ratio R(b)**0 = Gamma(b anti-b) / Gamma(had)

The partial decay width of the Z to bb¯ 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 Γ(Z→bb¯)Γ(Z→had)=0.21634±0.00067(stat)±0.00060(syst) was obtained, where the cc¯ production fraction was fixed to its Standard Model value.