QCD sum rules and soft-hard interplay for hadronic form factors

Abstract

We discuss two types of contributions to hadronic form factors in QCD: hard gluon exchange and soft wave function overlap. Within the QCD sum rule approach, the hard contribution has strong numeric suppression by factor (\alpha_s/\pi) ~ 0.1 for each exchange. For this reason, the soft contribution dominates at accessible momentum transfers. The ``humpy'' distribution amplitudes used to enhance hard terms cannot be derived from QCD sum rules in a self-consistent way. The estimates of soft terms obtained within the local quark-hadronic duality approach in all cases are close to existing data, providing an experimental evidence that hard terms are small.

Comments: 9 pages, Latex, 1 figure; Contribution to proceedings of the Joint ECT* / TJNAF Workshop on N* Physics and Nonperturbative QCD, Trento, Italy, 18-29 May 1998