Structure of Baryon Resonances

The structure of baryon resonances is one of the most challenging scientific problems of modern day physics. Within several theoretical approaches, much progress has been made in the understanding of the ground state properties of baryons. Nevertheless, a consistent explanation of the baryon structure, including also the excited resonances, is still a major task. In many common approaches, excited baryons are treated as three-quark bound states and therefore, strictly speaking, cannot decay. Experimentally, it is very well established that excited baryon states decay strongly and consequently models implementing only a qqq state for the description of the baryon resonance are incomplete. In constituent-quark-models (CQMs) one can define decay operators which generate the transitions between different baryon states, thereby modeling the decay process. However, in recent Poincar\`e-invariant calculations it has been seen that the theoretical results systematically underestimate the experimental data, confirming the incompleteness of the underlying CQMs and/or decay mechanism. The relativistic decay calculations also allow for the derivation of a generalised CQM which leads to resonances with complex mass eigenvalues, a manifestation of true resonance behaviour. The suggested relativistic method is based on meson-loop renormalisations of the (bare) baryon states, starting from a Bethe-Salpeter description of the meson-baryon system. An important ingredient in such an approach is the so-called (bare) vertex function, which currently is designed phenomenologically. The use of new vertex functions, which are derived directly from an underlying relativistic CQM, eliminates one of the largest uncertainties within such methods. The underlying quark-models to be investigated are based on Goldstone-boson exchange, one-gluon exchange and confinement-only interaction. Successful completion of this project will contribute to one of the most realistic descriptions of baryon resonances up to date.

The structure of baryon resonances is one of the most challenging scientific problems of modern day physics. Within several theoretical approaches, much progress has been made in the understanding of the ground state properties of baryons. Nevertheless, a consistent explanation of the baryon structure, including also the excited resonances, is still a major task. In many common approaches, excited baryons are treated as three-quark bound states and therefore, strictly speaking, cannot decay. Experimentally, it is very well established that excited baryon states decay strongly and consequently models implementing only a qqq state for the description of the baryon resonance are incomplete. In constituent-quark-models (CQMs) one can define decay operators which generate the transitions between different baryon states, thereby modeling the decay process. However, in recent Poincar\`e-invariant calculations it has been seen that the theoretical results systematically underestimate the experimental data, confirming the incompleteness of the underlying CQMs and/or decay mechanism. The relativistic decay calculations also allow for the derivation of a generalised CQM which leads to resonances with complex mass eigenvalues, a manifestation of true resonance behaviour. The suggested relativistic method is based on meson-loop renormalisations of the (bare) baryon states, starting from a Bethe-Salpeter description of the meson-baryon system. An important ingredient in such an approach is the so-called (bare) vertex function, which currently is designed phenomenologically. The use of new vertex functions, which are derived directly from an underlying relativistic CQM, eliminates one of the largest uncertainties within such methods. The underlying quark-models to be investigated are based on Goldstone-boson exchange, one-gluon exchange and confinement- only interaction. Successful completion of this project will contribute to one of the most realistic descriptions of baryon resonances up to date.