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

The renormalization of the effective single nucleon current operators in the nuclear medium by two-nucleon exchange currents is reviewed. Th e exchange current operators fall into two classes: those which are im plied by the nucleon-nucleon interaction, and those which are associat ed with transition couplings between different nonnucleonic configurat ions and thus model dependent. It is shown that the quenching of the a xial current coupling constant and the electromagnetic isovector spin current density are mainly caused by the model dependent exchange curr ent 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 th e model independent exchange current that is determined by the nucleon -nucleon interaction.