The Quark-Meson Coupling Model
The Quark-Meson Coupling (QMC) model, a theory which takes the radical step of incorporating self-consistent changes in the quark structure of a nucleon when it is bound in matter, has been transformed into a theory of quasi-nucleons interacting through many-body forces. This adjustment allows the QMC model to be related to the time-honored descriptions of the nucleus where nucleon structure was supposed to play no role. Of course, in experiments conducted at very high energies, it is customary to see the nucleus as a collection of quarks interacting via the exchange of gluons. At lower energies, where the spatial resolution is lower, one is apt to view the nucleus in terms of nucleons interacting via the exchange of mesons.
Actually, even in the lower energy range, one should keep the quarks in mind, because their motion inside a nucleon may change when it resides in a nucleus. That is, a nucleon is one thing when on its own and another thing when inside a nucleus, in which case it becomes a "quasi-nucleon". The QMC model takes this dichotomy into account by describing the interactions between a quark in one nucleon with a quark in another nucleon by meson exchange (see illustration at www.aip.org/png/2004/220.htm). The quarks in that nucleon are in turn interacting with the quarks in another and so on. The nucleus is now seen as quasi-nucleons interacting through forces which involve 2, 3, or even 4 bodies. The necessity of such many-body forces was empirically known from traditional nuclear physics, and the merit of the QMC model is that it explains their origin and predicts their intensity. The newer version of the QMC model will enable one to pursue more dramatic impacts of the change of hadron properties in medium, including the modification of weak and electromagnetic form factors.