Dispersion relations for the electromagnetic form factors as a synthesis of hadron and quark dynamics

The electromagnetic form factors of nucleons are investigated via unsubtracted dispersion relations. Superconvergence conditions are imposed on the absorptive parts so as to satisfy the asymptotic constraints required by the perturbative QCD. The hadronic and QCD quantities are interrelated through the superconvergence conditions. We obtain the results which imply a new duality; the vector dominance model in the low energy region and the asymptotic behaviour of the perturbative QCD at high energy. (author)