Meson fields, exchange currents and progenitor sum rules

The theory of the integrated photoabsorption cross section and the dipole sum rule is reviewed. A new progenitor sum rule, in which meson fields appear explicitly, is derived from a field theory of interacting pions and nucleons. The relation of this sum rule to previous results obtained in a potential theory is elucidated, and the role of neutron-proton correlations in nuclear ground states is discussed in terms of their effect on the dipole sum rule. The inclusion of rho-mesons (again in an oversimplified theory) and their contribution to progenitor sum rules is also mentioned. (Auth.)