With Realistic Cuts at Lep Energies and Beyond

After 10 years of steadily increasing the experimental precision at LEP/SLC, there is a strong demand on an update of existing programs for fermion pair production. We present a rederivation of the O(α) Bremsstrahlung corrections to e + e − → ¯ f f for the semi-analytic program ZFITTER. We focus on observables like total cross section and forward-backward asymmetry in the leptonic case with combined cuts on acollinearity angle, acceptance angle, and minimal energy of the fermions. The outcome of our analysis is a shift of the predictions by ZFITTER at LEP 1 energies off-resonance of a few per mil while at the Z resonance numerical changes can be neglected. Thus we obtain for cross sections and asymmetries at LEP 1 a level of agreement with other programs of better than per mil, like for the kinematically simpler s ′ cut option. A preliminary analysis of ZFITTER, TOPAZ0, and other codes at LEP 2 energies showing deviations of several per cent with acollinearity cuts enforce a future examination of higher order effects with different cuts. The predictions by LEP/SLC data, however, are not affected within the experimental errors.