Measurement of the branching ratio of the Z 0 into heavy quarks ∗

We measure the hadronic branching ratios of the Z boson into heavy quarks: Rb = ΓZ0→bb/ΓZ0→hadrons and Rc = ΓZ0→cc/ΓZ0→hadrons using a multi-tag technique. The measurement was performed using about 400,000 hadronic Z events recorded in the SLD experiment at SLAC between 1996 and 1998. The small and stable SLC beam spot and the CCD-based vertex detector were used to reconstruct bottom and charm hadron decay vertices with high efficiency and purity, which enables us to measure most efficiencies from data. We obtain, Rb = 0.21610± 0.00098(stat.)± 0.00073(syst.)∓ 0.00012(Rc) and, Rc = 0.1745± 0.0031(stat.)± 0.0020(syst.)∓ 0.0006(Rb) ∗ Work supported by Department of Energy contract DE-AC03-76SF00515 (SLAC).