Measurement of the branching fractions and forward-backward asymmetries of the Z0 into light quarks

Using approximately 4.3 million hadronic Z0 decays collected with the OPAL detector at LEP between 1990 and 1995, we measure the branching fractions of the zo into up-type and down-type light quarks, Rq, and the forward-backward asymmetries, AFB (q) . High-momentum stable particles are used to tag parton flavours and double tagged events are used to determine the flavour tagging efficiencies. Assuming flavour independence of strong interactions and SU (2) isospin symmetry, we measure: Rd,s/(Rd + Ru+ R,) = 0.371 ± 0.016(stat.) ± 0.016 (syst.) and AE-e(d, s) = 0.068 ± 0.035(stat.)± O.Oll(syst.), when the branching fractions and forward-backward asymmetries of down and strange quarks are set to be equal. The results are essentially free of assumptions based on hadronisation models. These results are in agreement with the Standard Model expectations and are used to infer the left and right handed couplings of strange/down quarks to the Zo , yielding gf" = -0.44�g b� and g�' = +0.13�g ;�.