Baryons as Relativistic Three-Quark Systems

We discuss some recent developments in the description of baryons as three-quark systems within relativistic constituent quark models. In particular we address the issues of excitation spectra, electroweak structure, and mesonic resonance decays. The necessities of implementing simultaneously the symmetries of low-energy quantum chromodynamics and of relativistic invariance are emphasized. 1 Introductory Remarks The consideration of few-quark systems has become a very exciting topic in few-body physics. On the one hand it represents a big challenge for modern few-body methods to solve few-quark problems, since the corresponding bound and continuum states have to be solved in a consistent relativistic framework. Thus, in order to treat hadron phenomena on a microscopic level the formalism of calculation has to be essentially relativistic. On the other hand the issues of quark dynamics are very important. In principle, the microscopic description of hadronic bound states and reactions amounts to solving quantum chromodynamics (QCD) at low energies. Evidently, this is not strictly possible nowadays, and one must resort to effective theories and/or models. For any of such approaches, however, it is required to implement the essential properties of low-energy QCD, which are still not known in a satisfactory manner. In working with few-quark problems it is an intriguing task to find out the forces binding the constituents of hadrons and finally governing their behaviours. Evidently it is of eminent interest from many aspects of nuclear and subnuclear physics to be able to describe and to understand the properties (and reactions) of hadrons in a microscopic manner on the basis of the fundamental theory of strong (and also electroweak) interaction. A very powerful tool for lowand intermediate-energy hadron physics consists in employing constituent quark models (CQMs). They have already witnessed a long and reasonably successful development. Starting out from the naive nonrelativistic quark model of hadrons in the 60’s one has constantly