Quark-model perspectives on three-nucleon forces in nuclear physics

We stress the point that, as long as nucleons possess substructure of some kind, three-nucleon forces must exist in different forms. Using a quark model to characterize the nucleon substructure, we may classify three-nucleon forces into three distinct categories, viz.: i) Three-nucleon forces of type I, such as the Tucson-Melbourne (2π)-exchange three-body force, which can be formulated in terms of the hadron degrees of freedom, ii) three-nucleon forces of type II which describe modifications to nucleon-nucleon interactions arising from the fact that the quark substructure of the two interacting nucleons is modified by the presence of a third nucleon, and iii) three-nucleon forces of type III which are induced by the fact that, as three nucleons overlap one another, the constituents may rearrange among themselves to minimize the total energy. Possible manifestions of these three-nucleon forces, especially those of type II, are discussed in some detail. (orig.)