VIRTUAL COMPTON-SCATTERING OFF THE NUCLEON AT LOW ENERGIES

We investigate the low-energy behavior of the four-point Green's funct ion Gamma(mu nu) describing virtual Compton scattering off the nucleon . Using Lorentz invariance, gauge invariance, and crossing symmetry, w e derive the leading terms of an expansion of the operator in the four -momenta q and q' of the initial and final photon, respectively. The m odel-independent result is expressed in terms of the electromagnetic f orm factors of the free nucleon, i.e., on-shell information which one obtains from electron-nucleon scattering experiments. Model-dependent terms appear in the operator at O(q(alpha)q(beta)'), whereas the order s O(q(alpha)q(beta)) and O(q(alpha)'q(beta)') are contained in the low -energy theorem for Gamma(mu nu), i.e., no new parameters appear. We d iscuss the leading terms of the matrix element and comment on the use of on-shell equivalent electromagnetic vertices in the calculation of ''Born terms'' for virtual Compton scattering.