On the applicability of perturbative QCD to exclusive processes at currently available momentum transfer

We re-examine the problems connected with the end-point dominance in the calculation of exclusive processes in perturbative QCD. In a re-analysis we construct nucleon quark distribution amplitudes from the respective moments obtained from a QCD sum rule approach. These functions lead to acceptable values for the e.m. Dirac form factorsF ₁ ^p,n of the nucleon if effective gluon masses of ca. 300–600 MeV are included into the hard-scattering amplitude. In addition we also find a reasonable Q₂ — dependence of the proton form factor. The results point at the importance of the end-point k-dependence of distribution amplitude and hard-scattering amplitude in the calculation of exclusive processes.Work is supported by Deutsche Forschungsgemeinschaft (Ga 153-13-1) and partially by NATO (0581/87)