Quantum Chromodynamics

The theory of strong interactions, quantum chromodynamics (QCD), was for- mulated 30 years ago and has since been a very active field of research. The underlying equations of motion for the gauge degrees of freedom are nonlin- ear and minimally coupled to fermions with global and local SU(3) charges. This leads to spectacular problems compared with those of QED since the gauge bosons themselves interact with each other. On the other hand, it is exactly the self-interaction of the gluons which leads to asymptotic freedom and the pos- sibility to calcuate quark-gluon interaction at small distances in the framework of perturbation theory. We discover one of the most complicated but most beau- tiful gauge theories which poses extremely challenging problems on modem theoretical and experimental physics today. Quantum chromodynamics is the quantum field theory that allows us to cal- culate the propagation and interaction of colored quarks and gluons at small distances. Today's experiments do not allow these colored objects to be detected directly; instead one deals with colorless hadrons: mesons and baryons seen far away from the actual interaction point. The hadronization itself is a complicated process and not yet understood from first principles. Therefore one may won- der how the signature of quark and gluon interactions can be traced through the process of hadronization.