Alternative Relativistic Models

Abstract

So far, we have been considering current theories of nucleon—nucleon forces and relativistic models of nuclear structure (nuclear matter and finite nuclei) based on the philosophy, common to both cases, of using the concept of meson exchange (PVS models) as a tool for generating the NN interaction directly. In this chapter we consider alternative relativistic approaches. An important question in nuclear physics is whether quarks play any essential role in nuclear structure. The discovery of the EMC effect [166, 167] (the structure function of the nucleon is changed by the nuclear medium) is an indication of the fact that the quark degrees of freedom are necessary to understand deep inelastic scattering at momentum transfers of several GeV. For this reason, it is necessary to develop a theory of nuclear structure incorporating quark-gluon degrees of freedom, but this will be a challenging program. This program has been started in [511–513], the approach developed in these papers being referred to as the quark-meson coupling (QMC) model. It is a relativistic mean-field model, initially proposed for nuclear matter, the nucleons being described by the nonoverlapping MIT bag model, and the interaction between nucleons being produced by coupling of meson fields to the quarks. In this model a mean-field Dirac equation, together with the MIT bag boundary conditions, are used to describe quarks; the nucleons are also assumed to be described by a Dirac equation in the effective fields that arise from the coupling of meson fields to the quarks in the nucleons. Similar approaches have been considered by various other groups.