Chiral Quark Model for Baryons

Research project P 14806 Chiral Quark Model for Baryons Willibald PLESSAS 27.11.2000 The fundamental theory of strong interactions, quantum chromodynamics (QCD), is not directly solvable in a comprehensive manner at low and intermediate energies. Nevertheless there is the demand to be able to reliably explain the variety of phenomena of hadrons (i.e. the strongly interacting elementary particles), which are essentially responsible for the constitution of matter, especially in this energy domain. In this attempt one has thus to rely on alternative approaches resp. models of QCD. For them, however, it is mandatory that they meet the essential properties of QCD. A very promising approach to low-energy processes of hadrons exists in constituent quark models. They allow a widely exat calculation of hadron phenomena and are at the same time suitable to incorporate the relevant properties of QCD. Among them is undoubtedly the spontaneous breaking of chiral symmetry as a fundamental dynamical principle. In this project a chiral quark model should be investigated that relies essentially on those degrees of freedom that are brought about by the spontaneous breaking of chiral symmetry, namely constituent quarks and Goldstone bosons. This socalled Goldstone-boson-exchange constituent quark model (GBE CQM) has already proven especially successful in the description of the spectroscopy of light and strange baryons over the recent years. In particular, it was capable to solve the long-standing problems with regard to the correct level ordering in the excitation spectra, what caused a lot of interest with regard to further applications in many places. In order to demonstrate the broader relevance of the properties of the new type of dynamics for the interaction of constituent quarks one has now to clarify the important questions if the GBE CQM is also able to explain reaction processes involving light and strange baryons. Of relevance are in the first instance the electromagnetic structure of the nucleons (proton and neutron), the fundamental building blocks of atomic nuclei, further electromagnetic transitions to their excited states as well as hadronic decays of baryon resonances. For all of these processes there are ample experimental data from modem accelerator facilities, which on the one hand provide a strict test for the adequacy of theoretical models and on the other hand need urgent theoretical explanation. One may expect that the understanding and control of hadronic reactions can be advanced towards the most original constituents of matter. Therein consists the primary aim of this research project. For the planned investigations there exist already several preparatory works, which let one expect further most interesting results. Beyond the correct description of the baryon spectra already established it seems possible to reach within the GBE CQM in particular also a consistent explanation of the electromagnetic nucleon form factors. It appears that in this respect the inclusion of relativistic effect is of essential importance; for the first time it can here be achieved in a comprehensive manner for the case of a realistic quark model for baryons. The same is true with regard to electromagnetic transitions to nucleon excited states. Furthermore there is the chance to solve in an analogous way also the long-standing problem of mesonic decays of baryons, which so far has not been treated satisfactorily.