Purpose of the course

The Erice International Workshop on Nuclear Physics was founded by Prof. H. Schopper in 1974. After him it was organized by Sir Denys Wilkinson till 1983. The topics of the workshop have been chosen from the very beginning to be young and fast expanding fields in the area of the interphase between nuclear, particle and astrophysics. The idea is to bring internationally highly recognized experts in the field together with young scientists and even PhD students. In the morning the experts give review lectures on the newest status of a special topic, while the afternoon is mainly devoted to seminars of the participants leaving enough time for discussions and special topic workshops.

Motivation of the Topic

Quantum Chromo-Dynamics (QCD) is generally accepted as the fundamental theory for the structure of hadrons and their strong interactions. The School/Workshop on Nuclear Physics 2007 focuses on experimental and theoretical studies of hadron properties and their interactions at energies below a few GeV. It this energy domain, it impossible to use perturbation theory to calculate different processes in QCD  since the running coupling constant is large ('strong QCD'). The principal research goal is to identify the relevant degrees of freedom and to map out the transition from a non-pertubative to a perturbative description of the observed strong interaction phenomena. Major questions of current interest include:

• what is the origin of mass in the sector of light quarks?
  
  
• what is the role of confinement in the dynamics of hadrons?
  
  
• what are the relevant degrees of freedom for the static properties and decays of light hadrons?
  
  
• what does the excitation spectrum reveal about the underlying quark-gluon degrees of freedom?
  
  
• how do the properties of hadrons evolve if one or more light quarks are replaced by 'heavy quarks' ?

In the theoretical interpretation of the experimental data several non-perturbative methods are applied:

1.  In an 'ab initio' approach one solves QCD on a discrete space-time lattice numerically to obtain the structure of hadrons and their mutual interaction. Although much progress has been made recently with the advent of high-performance computers in the Teraflops range, a major computational problem is posed by the small masses for the up, down and strange quarks. The amount of computer time scales with a large power of the inverse quark mass and in the foreseeable future one will not able to do calculations for small enough quark masses approaching the 'chiral limit' of vanishing quark masses. Thus one tries to interpolate between the results obtained on the lattice and those obtained via chiral perturbation theory. Another difficulty is the computation of excited mesons and baryons due the euclidian nature of the simulations.
   
   
2.  Another 'controlled' method is the solution of the truncated hierarchy of QCD Dyson-Schwinger equations in a particular gauge. Also in this area significant progress has been made recently for the understanding of quark- and gluon propagators in the infra-red regime, particularly through direct comparison with the lattice QCD results. Although these results are encouraging there is still a long way to construct 'first principles' hadronic propagators via the corresponding two- and three-body equations with realistic vertex constructions.
   
   
3.  A third approach is based on the exact and approximate symmetries of QCD. The global gauge invariance requires baryon- and flavor number conservation in strong interactions. In the chiral limit hadronic properties must be invariant under chiral transformations. However, the chiral symmetry is spontaneously broken in the strongly interacting vacuum that gives rise to the appearrence of the 'Goldstone' bosons. In terms of these Goldstone bosons in conjunction with heavy baryons as static sources, one can set up a systematic effective field theory, 'chiral perturbation theory', from which the static properties of hadrons and their low-energy interactions can be obtained in a gradient expansion of the pertinent fields. For this, a large number of 'low-energy' constants have to be known, either from experiment or from lattice simulation of QCD.
   
   
4.  Aside from these approaches, with direct links to QCD, there is a variety of QCD inspired models which incorporate essential features of such a 'constituent' quark masses, confinement mechanisms, non-trivial topological structures and effective quark-meson couplings. Such models are highly evolved in their treatment of the relativistic many-body problem and can be put the test by direct comparisons with experiment and lattice results.

On the experimental side, one studies properties of mesons and baryons via photo- and electro-production off protons or light nuclei as well as with hadronic probes.

1. Photon-nucleon and electron-nucleon scattering gives information about the electric and magnetic form factors of protons and the neutrons, their excitation spectrum and pertinent decay modes. Especially detailed information is obtained by double polarization experiments in which the photons or electrons as well as the target nucleons or nuclei are polarized.
    
   
2. For the role of finite quark masses and their effect on the symmetry breaking pattern of QCD it is of special importance to study the strangeness content of the nucleon. This can be done in parity violating experiments (SAMPLE at MIT, HAPEX at JLab and PV-A4 at Mainz). One measures a parity violating single spin asymmetry in the scattering of longitudinally polarized electrons off unpolarized protons or light nuclei. The longitudinal single spin asymmetry is parity violating and arises from the interference of the weak and electromagnetic one-boson exchange amplitude. It is sensitive to the strangeness contribution to the electromagnetic form factors of the nucleon.
    
   
3. Of particular importance for the interplay between pertubative and non-perturbative QCD degrees of freedom is the study of parton distribution functions in the nucleon. Major advances are to be expected from measurements of the so-called 'generalized parton distribution functions', which occur in the description of a variety of different hard processes, from inclusive to hard exclusive scattering. Experiments are presently conducted at DESY and JLab and ultimately contribute to our knowledge on the contribution of intrinsic and orbital angular momenta of quarks and gluons to the spin of the nucleon.

The Erice School/Workshop "Quarks in Hadrons and Nuclei" brings together experts in the field of strong interaction physics in the
 non-perturbative regime, including postdocs and young PhD students to present and discuss the current status of the theoretical and experimental developments in this research area in a stimulating scientific environment.