Sum rules in photonuclear physics

Abstract

In the course of these notes, we have emphasised that the enhancement κ of the integrated total photon-nucleus cross section Σ_o (m_π) over the classical dipole sum contains integral information about explicit mesonic degrees of freedom in nuclei. The “classical” modes of excitation, like the giant dipole resonance, exhaust roughly one dipole sum, and an equal amount is left to explain, for all but the lightest nuclei, in terms of exchange current phenomena.

Two complementary approaches have been discussed. The dispersion relation sum rule of the GGT type draws global connections of analyticity between exchange currents and physical meson photoproduction, but is incapable of a microscopic description of the underlying mechanisms. The dipole sum rule calculations, on the other hand, are missing all but the low energy (static) aspects of the exchange current phenomenon. There is obviously an important link still missing. We are not yet able to understand, on a microscopic basis, the nature of exchange currents in the transitionregion just below and around the meson production threshold.