A measurement of the electroweak couplings of up and down type quarks using final state photons in hadronic Z0 decays

The production rate of final state photons in hadronic Z0 decays is measured as a function of ycut = Mij2/Ecm2 the jet resolution parameter and minimum mass of the photon-jet system. Good agreement with the theoretical expectation from an O(ααs) matrix element calculation is observed. Comparing the measurement and the prediction for ycut = 0.06, where the experimental systematic and statistical errors and the theoretical uncertainties are small, and combining this measurement with our result for the hadronic width of the Z0, we derived partial widths of up and down type quarks to be Γu = 333 ± 55 ± 72 MeV and Γd = 358 ± 37 ± 48 MeV in agreement with the standard model expectations. We compare our yield with the QCD shower models including photon radiation. At low γcut JETSET underestimates the photon yield, and ARIADNE describes the production rate well.