Study of prompt photon production in hadronic Z(0) decays

From a sample of 1.5 million Z(0) --> <q(q)over bar> decays collected by the DELPHI detector during 1991, 1992 and 1993, the production cross section for isolated final state photons is measured and is compared with the O(alpha, alpha(s)) matrix element calculations implemented in the EEPRAD and GNJETS Monte Carlo generators. The observed photon yield is used to derive the electroweak couplings of charge 2/3 (u-type) and charge 1/3 (d-type) quarks to the Z(0) boson. The measured values C-2/3 = 0.91(-0.36)(+0.25) and c(1/3) = 1.62(-0.17)(+0.24) are compatible with the Standard Model prediction, C-2/3 = 1.145 and c(1/3) = 1.477.