[en] We review the interaction of real photons with nuclei up to the GeV region. The common microscopic description of exchange effects below threshold and of the corresponding real photoproduction above, is emphasized. The theoretical problems connected with π photoproduction in Δ region and vector meson photoproduction are spelled out and solved. The gross features of the reaction mechanism are shown to explain both the low energy region, the bulk properties around the Δ resonance as well as the appearance of shadowing only above ρ threshold

[en] Guided by heuristic arguments based on the reinterpretation of the Thomas-Reiche-Kuhn sum rule and on neutron multiplicity measurements, the author proposes a mechanism in which, in contrast to the quasi deuteron model and to the traditional microscopic calculations, exchange effects are mediated by the photoproduced pion. This pion is renormalized by the repulsive S and attractive P wave of the pion optical potential V/sub opt/, describing its interaction with the nucleus, but above all, it acquires a finite width (due to the imaginary part of V/sub opt/) describing its absorption. In such a way, and by making strong assumptions about the extrapolation of the optical potential parameters, exchange effects below pion threshold appear naturally as an off-shell continuation of real pion production above threshold

No abstract available

[en] The properties of quasi-elastic electron scattering with respect to the scaling variable y=ß/q-q/2M are examined. It is shown that different sets of scattering experiments can indeed be related through the scaling variable. (orig.)

[en] A microscopic calculation of pp-bar → ππ, KK-bar reactions is attempted. A considerable number of resonances both in the s and t channel are found to contribute. The theory is predictive only at a semi-quantitative level. The suppression of KK-bar P-wave threshold annihilation remains unexplained

[en] The 2γ branching ratio, energy distribution and angular correlation are derived from a gauge invariant effective Hamiltonian using energy weighted sum rules

No abstract available

[en] Recent photoabsorption experiments in the Δ resonance region in heavy nuclei are recently obtained, in addition to the older ones in light elements. This prompts a renewed theoretical investigation of the effects in the Δ photoexcitation in nuclei. The authors discuss in the present paper both the gross properties of the cross section, particularly its behavior as a function of the atomic number A, as well as more detailed dynamical features. As regards the first point the authors show that, under certain assumptions, the integrated cross section takes naturally the form of an energy weighted sum rule, whereas more sophisticated microscopical models must be used in order to explain the detailed shape of the cross section

[en] 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

No abstract available