RENORMALIZATION OF THE NUCLEON CURRENTS IN THE NUCLEAR MEDIUM AND THE NUCLEON-NUCLEON INTERACTION

The renormalization of the effective single nucleon current operators in the nuclear medium by two-nucleon exchange currents is reviewed. The exchange current operators fall into two classes: those which are implied by the nucleon-nucleon interaction, and those which are associated with transition couplings between different nonnucleonic configurations and thus model dependent. It is shown that the quenching of the axial current coupling constant and the electromagnetic isovector spin current density are mainly caused by the model dependent exchange current contributions and second order configuration mixing of the nucleon wave functions. The large enhancement of the axial charge coupling in the nuclear medium can on the other hand be ascribed to the combined effects of the well known pion exchange charge density operator and the model independent exchange current that is determined by the nucleon-nucleon interaction.