Generalized polarizabilities and electroexcitation of the nucleon

Generalized nucleon polarizabilities for virtual photons can be defined in terms of electroproduction cross sections as function of the 4-momentum tra nsfer Q(2). In particular, the sum of the generalized electric and magnetic polarizabilities Sigma = alpha + beta and the spin polarizability gamma ca n be expressed by virtual photon absorption cross sections integrated over the excitation energy. These quantities have been calculated within the fra mework of the recently developed unitary isobar model for pion photo- and e lectroproduction on the proton, which describes the available experimental data up to an excitation energy of about 1 GeV. Our results have been compa red to the predictions of chiral perturbation theory. (C) 1999 Elsevier Sci ence B.V. All rights reserved.