Virtual compton scattering and generalized polarizabilities of the proton

Abstract Threshold photon electroproduction off the proton allows one to measure new electromagnetic observables which generalize the usual polarizabilities. There are—a priori—ten “generalized polarizabilities”, functions of the virtual photon mass. The purpose of this paper is to lay down the appropriate formalism to extract these quantities from the photon electroproduction cross sections. We also give a first estimate of the generalized polarizabilities in the non-relativistic quark model.