Some challenges in hadron physics
Matthias Lutz (Darmstadt)
In this talk I will address several topical issues in low-energy QCD. After a brief introduction and a discussion of resonance generation in QCD the role of vector-meson degrees of freedom in effective Lagrangian approaches is discussed at hand of the two-body scattering of Goldstone bosons. The thriving theme is the proper identification of the right degrees of freedom, in terms of which the experimental data set can be interpreted efficiently and systematically. It is argued that nonperturbative applications of the chiral Lagrangian that are consistent with micro-causality and coupled-channel unitarity provide a powerful tool to study low-energy hadron physics. With the availability of more and more accurate QCD lattice simulations the need for a physical interpretation of both lattice and experimental data is ever increasing. The data on the quark-mass dependence of the baryon ground states pose a particular challenge since they refuse to be described by a strict chiral expansion. It is shown that a unitarized chiral extrapolation of the baryon masses is able to accurately describe and predict the existing lattice results. The role of sum rules that follow in QCD with a large number of colors is discussed.