Compton scattering by nuclei

The concept of Compton scattering by even-even nuclei from giant-resonance to nucleon-resonance energies and the status of experimental and theoretica l researches in this field are outlined. The description of Compton scatter ing by nuclei starts from different complementary approaches, namely from s econd-order S-matrix and from dispersion theories. Making use of these, it is possible to incorporate into the predicted nuclear scattering amplitudes all the information available from other channels, viz. photon-nucleon and photon-meson channels, and to efficiently make use of models of the nucleo n, the nucleus and the nucleon-nucleon interaction. The total photoabsorpti on cross section constrains the nuclear scattering amplitude in the forward direction. The specific information obtained from Compton scattering there fore stems from the angular dependence of the nuclear scattering amplitude, providing detailed insight into the dynamics of the nuclear and nucleon de grees of freedom and into the interplay between them. Nuclear Compton scatt ering in the giant-resonance energy-region provides information on the dyna mical properties of the in-medium mass of the nucleon. Most prominently, th e electromagnetic polarizabilities of the nucleon in the nuclear medium can be extracted from nuclear Compton scattering data obtained in the quasi-de uteron energy-region. In our description of this latter process special emp hasis is laid upon the exploration of many-body and two-body effects enteri ng into the nuclear dynamics. Recent results are presented for two-body eff ects due to the mesonic seagull amplitude and due to the excitation of nucl eon internal degrees of freedom accompanied by meson exchanges. Due to thes e studies the in-medium electromagnetic polarizabilities are by now well un derstood, whereas the understanding of nuclear Compton scattering in the De lta-resonance range is only at the beginning. Furthermore, phenomenological methods how to include retardation effects in the scattering amplitude are discussed and compared with model predictions. (C) 2000 Elsevier Science B .V. All rights reserved.