Quantum Chromodynamics (QCD) is the quantum field theory of quarks and gluons. There are two properties of QCD which are fundamental for a microscopic description of matter: 'confinement' and 'dynamical mass generation'. Confinement implies that free quarks and gluons cannot be observed in nature. Instead, they are constituents of bound states ('hadrons') as for example protons and neutrons. Inside hadrons quarks and gluons interact strongly. The dynamics of this interaction generates the constituent masses of quarks. This mechanism accounts for 99 percent of all mass observed in our daily life. Contributions from a variety of theoretical and experimental approaches are necessary to understand the origins of confinement and quark-mass generation. Within a nonperturbative functional integral framework we are working in three research fields related to this challenge.
Firstly, the strong self-interaction of gluons generates hadrons which almost entirely consist of glue, the so-called glueballs. An intensive experimental investigation of glueballs is one of the primary goals of the GLUEX project at Jefferson Lab (USA) and the planned PANDA experiment at FAIR/GSI. We are investigating the structure and spectra of these glueballs in both quenched and unquenched QCD.
Secondly, we work on our understanding of the structure of the quark-gluon interaction which is behind the wealth of experimentally observed hadron states. These include two and three-quark configurations as well as exotic hadrons which cannot be understood within a valence-quark model. We are studying the quark-gluon interaction from its defining equation of motion and we are evaluating resulting properties of a range of meson states.
Thirdly, the properties of the quark-gluon plasma are of tremendous interest. This state of matter is generated by heating and/or compressing hadronic matter. The corresponding phase transition generates matter that may be described as a perfect liquid of quarks and gluons. This so called quark-gluon plasma is explored in experiments at RHIC (Brookhaven) and planned experiments at ALICE/LHC (Genf) and CBM/FAIR (Darmstadt). We are investigating the properties of the chiral phase transition and the propagation of quarks and possible bound states in the plasma phase.