End-Point Behavior of Exclusive Processes: The Twilight Regime of Perturbative QCD

Abstract

A selected set of topics along the borderline between perturbative and nonperturbative QCD in exclusive reactions are studied. Specific problems, related to different mechanisms of momentum transfer to an intact hadron, are discussed. Calculations of the space-like form factors of the pion and the nucleon are reviewed within a convolution scheme of short-distance (hard) and large-distance (soft) contributions which takes into account soft gluon emission and the intrinsic transverse hadron size. The failure of this scheme to reproduce the existing experimental data signals sizeable higher-order perturbative corrections (a K-factor of order two) and/or higher-twist contributions.