Virtual Compton scattering off the nucleon at low energies

We investigate the low-energy behavior of the four-point Green close-quote s function Γμν describing virtual Compton scattering off 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' of the initial and final 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(qαqβ'), whereas the orders O(qαqβ) and O(qα'qβ') are contained in the low-energy theorem for Γμν, 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 open-quote open-quote Born terms close-quote close-quote for virtual Compton scattering. copyright 1996 The American Physical Society