VIRTUAL COMPTON-SCATTERING OFF SPIN-ZERO PARTICLES AT LOW ENERGIES

We discuss the low-energy behaviour of the virtual Compton scattering amplitude off a spin-zero target. We first compare various methods of obtaining a low-energy expression based either on the soft-photon appr oximation or the use of Ward-Takahashi identities. We point out that s tructure-dependent terms are defined with respect to a low-energy appr oximation of the pole terms, which commonly is separated from the full amplitude. We derive a general expression for the structure-dependent terms in an expansion in terms of the momenta k(1) and k(2) of the in itial and final virtual photon, respectively, up to and including term s of order O(k(4)). At order O(k(2)) two terms appear that are related to the usual electric and magnetic polarizabilities of real Compton s cattering. At order O(k(4)) we find nine new structures of which five can only be determined using virtual photons.