A new method is presented for determining the electroweak couplings of individual light quark flavours in hadronic Z° decays. The method, which employs double tagging of events with highly energetic hadrons, is essentially free of detailed assumptions about hadronisation and assumes SU(2) isospin symmetry and the flavour independence of the strong interactions. Under the assumption that the electroweak couplings of d and s quarks are the same, we estimate that for five million hadronic Z° decays accuracies of 5(Rd = R,)/(Rd = R,) ~ 0.05 and SApB(d = s) ~ SAfb{u) ~ 0.015 are achievable. Preliminary measurements by the OPAL Collaboration are found to be in agreement with the Standard Model predictions.
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