within neutrons, protons, or other strongly interacting particles built from quarks and gluons. All these particles, as a class, are called hadrons.
In the theory of quantum chromodynamics (QCD), where quarks interact by exchanging gluons, confinement arises because the gluons interact with other gluons, as well as with quarks. As a result, two quarks interact more and more strongly as they are pulled further apart, as if they were connected by a rubber band. This behavior is in dramatic contrast to the rapidly weakening electromagnetic force that binds electrons to nuclei in atoms, and to the even more rapidly weakening force that binds protons and neutrons to each other in the nucleus. While the mechanism of confinement within QCD is understood qualitatively, its very strength makes a quantitative treatment extremely difficult. Obtaining a quantitative understanding of the confinement of quarks and gluons inside hadrons remains one of the greatest intellectual challenges facing physicists.
While studies of the issues surrounding confinement have traditionally been done at the interface with particle physics, nuclear physicists are playing an increasingly vital role in addressing this challenge, through experiments and theory. They seek answers to three basic questions:
Major efforts in nuclear theory are devoted to developing techniques for performing at least approximate QCD calculations, and for demonstrating how the conventional treatments of nuclei—as assemblies of nucleons exchanging mesons—can be viewed as an effective low-energy limit of QCD. Successful theoretical approaches must explain the structure not only of nucleons, but of other hadrons as well.