Virtual Compton scattering off the nucleon at low energies.

We investigate the low-energy behavior of the four-point Green’s function Γ 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(qαq ′ β), whereas the orders O(qαqβ) and O(q ′ αq ′ β) are contained in the low-energy theorem for Γ μν , 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. 13.40.Gp,13.60.Fz,14.20.Dh Typeset using REVTEX ∗Supported by Deutsche Forschungsgemeinschaft