Exclusive QCD processes, quark-hadron duality, and the transition to perturbative QCD

Experiments at CEBAF will scan the intermediate-energy region of the QCDdynamics for the nucleon form factors and for Compton Scattering. Theseexperiments will definitely clarify the role of resummed perturbation theoryand of quark-hadron duality (QCD sum rules) in this regime. With thisperspective in mind, we review the factorization theorem of perturbative QCDfor exclusive processes at intermediate energy scales, which embodies thetransverse degrees of freedom of a parton and the Sudakov resummation of thecorresponding large logarithms. We concentrate on the pion and protonelectromagnetic form factors and on pion Compton scattering. New ingredients,such as the evolution of the pion wave function and the complete two-loopexpression of the Sudakov factor, are included. The sensitivity of ourpredictions to the infrared cutoff for the Sudakov evolution is discussed. Wealso elaborate on QCD sum rule methods for Compton Scattering, which provide analternative description of this process. We show that, by comparing the localduality analysis to resummed perturbation theory, it is possible to describethe transition of exclusive processes to perturbative QCD.Comment: 49 pages, 18 figures. to appear on Journ of High Energy Phys, JHEP. to appear in Journal of High Energy Physics, JHE