Virtual Compton Scattering Oo the Nucleon at Low Energies Typeset Using Revt E X

We investigate the low-energy behavior of the four-point Green's function ? describing virtual Compton scattering oo the nucleon. Using Lorentz invariance, gauge invariance, and crossing symmetry, we derive the leading terms of an expansion of the operator in the four-momenta q and q 0 of the initial and nal photon, respectively. The model-independent result is expressed in terms of the electromagnetic form 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 q 0), whereas the orders O(q q) and O(q 0 q 0) are contained in the low-energy theorem for ? , i.e., no new parameters appear. We discuss 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.