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. Thanks to this, the QMC model can now challenge the time-honored descriptions of the nucleus where nucleon structure was supposed to play no role.

The conventional hierarchy of nuclear matter at the smallest scale goes like this: quarks are the most elemental. Nucleons, the next bigger things, are clumps of three quarks held together by a force carried from place to place by gluons. Then the nucleus is made from nucleons held together by mesons, which are themselves clumps of two quarks. Next up in size are atoms, which consist of electrons (members of a separate category of particle called leptons) hovering around the nucleus. At all these levels different models would apply. In other words, no one theory would apply everywhere; one would need instead several "effective theories" with limited validity outside their own realm.

For instance, in experiments conducted at very high energies (many GeV)---equivalent to using a microscope able to see individual quarks inside the nucleons---it is customary to see nuclear physics as being a bunch of quarks interacting via the exchange of gluons. At lower energies, where the spatial resolution is lower (i.e., experimental studies are less able to resolve details inside the nucleon), one is apt to see nuclear physics as being a bunch 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 the latter 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."

This is what the QMC model takes into account by describing the interactions between a quark in one nucleon with a quark in another nucleon by meson exchange (see illustration). The quarks in that nucleon are in turn interacting with the quarks in another and so on. The resulting picture of the nucleus is then that of 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. This makes for a more realistic description, particularly for the border area between higher energy (a province sometimes called particle physics) and lower energy (to which the generic term "nuclear physics" applies).

The QMC theory has stood up to experimental tests for some years now. For example, it has been helpful in explaining changes in hadron masses in dense matter and there are even hints from extremely precise measurements of the ratio of electric to magnetic form factors of a proton bound in helium (at Mainz and Jefferson Lab) supporting the subtle changes predicted there.

Now, the authors of the QMC model, Pierre Guichon (Saclay, France) and Tony Thomas (Adelaide, Australia --- now Chief Scientist at Jefferson Lab), believe the newer version of their model will really help in interpreting data coming from heavy-ion collision experiments aiming to create a quark-gluon plasma state. (Physical Review Letters, upcoming article; pguichon@cea.fr, 33-1690-87207)