Author(s): The SLD Collaboration Presenting author: Tracy Usher Corresponding author's e-mail: usher@slac.stanford.edu Title of the presentation: Measurement of Rb and Rc at SLD Link to the full paper: Session of 1st preference: 5-Test of the Standard Model Session of 2nd preference:   Abstract   We present a measurement of the hadronic branching fractions of the Z0 boson into bb-bar (Rb) and cc-bar (Rc) using a multitag method. This is a final measurement based on our complete 93-98 dataset of 550 000 hadronic Z0 decays. It used the 300 Mpixel high resolution CCD vertex detector of SLD and the small and stable SLC beam spot to achieve b tagging efficiencies and purities of 62% and 98% respectively. The combined statistical and systematic error on Rb is +-0.0012. Charm tagging efficiency and purity are 18% and 84% respectively for the main tag. The combined statistical and systematic error on Rc is +-0.0040, which makes it the most precise measurement in the world.