ON THE COMPARISON OF MATRIX ELEMENT CALCULATIONS OF O(ALPHA-ALPHA(S)) WITH THE MEASUREMENT OF PHOTON-EMISSION IN HADRONIC Z(0) DECAYS

The uncertainties in interpreting photon bremsstrahlung in the process e+e- --> Z0 --> qqBAR with matrix element calculations of O(alphaalpha(s)) are discussed. We address the stability of the calculations with respect to the emission of collinear photons and to higher-order QCD corrections and discuss the blas due to experimental photon isolation cuts. We analyze the resulting uncertainties for various procedures to define an event with a final state photon. Of particular interest are (i) a two-step procedure where first jets are reconstructed from hadrons alone and in a second step the photon is required to be isolated from these jets, and (ii) a 'democratic' procedure where the photon is included in the jet reconstruction but a certain maximum hadronic energy is allowed in the photon jet. In both cases we estimate that the uncertainties of the theoretical predictions, hadronization effects and the experimental photon isolation are of the order of 4%. To obtain this level of accuracy, however, the democratic procedure requires very hard cuts that reduce the event samples significantly.