Hadron Resonances Within a Constituent-Quark Model

In order to get a more realistic description of the hadron spectrum we extend a constituent-quark model by explicit mesonic degrees of freedom. The resulting system of constituent (anti)quarks, which are subject to an instantaneous confining force, and mesons, which couple directly to the quarks, is treated by means of a relativistic coupled-channel framework. It can be formally shown that the mass-eigenvalue problem for such a system is equivalent to a hadronic eigenvalue problem in which the eigenstates of the pure confinement potential (bare hadrons) are coupled via meson loops. Following this kind of approach we have calculated hadron masses and decay widths for a simple toy model. The resonance character of hadron excitations is usually not taken into account in constituent quark models. As a consequence most of the (perturbatively) calculated partial decay widths come out too small as compared to experiment [1]. This suggests that physical hadron resonances are not just simple bound states of valence (anti)quarks, but should also contain higher Fock components. We propose to model the additional quark-antiquark pairs by means of mesons which can couple directly to the valence (anti)quarks. A natural starting point for this kind of description is the chiral constituent quark model [2]