Nuclear compton scattering

The salient feature of Compton scattering with respect to photoabsorption and (e,e') is that the scattering cross section is obviously determined by the imaginary (to which on shell particles contribute) as well as by the real part of the amplitude (which is influenced by far lying states). This explains the difficulty of building a quantitative theory of Compton scattering, all the more so at high energies and backward angles where diffraction minima depress the cross section by orders of magnitude and where even minor effects may show up. It has recently been shown that a remarkable cancellation happens in the photoabsorption cross section (i.e. the imaginary part of the forward amplitude) between the low energy dipole sum rule enhancement and a hindrance in pion photoproduction above threshold. Although this has been proved in a schematic model, this shows how closely connected real and virtual pions are and suggests therefore that great care should be used in the proper handling of the real parts of the scattering amplitude and in their extrapolation. The purpose of this paper is to show, in an explicit way, the interplay between virtual and real pions