**Within the standard model of theoretical elementary particle physics all fundamental interactions - except gravity - are described by (relativistic) quantum field theories. Quantum chromodynamics, abbr. QCD, is the quantum field theory of strong interactions; its basic dynamical degrees of freedom are quarks and gluons, the particles mediating the strong interactions. The strong interaction binds these quarks and gluons to, for instance, protons and neutrons, and eventually protons and neutrons to nuclei.**

The theory of strong interactions exhibits two rather peculiar features:

- "
**Confinement**": Quarks and gluons do not exist as free particles; only their bound states can be observed in experiment. - "
**Dynamical breakdown of chiral symmetry**": Even if the mass parameter of a quark is strictly zero, strong interactions induce a nonvanishing mass of this quark.

Both of these phenomena are entirely non-perturbative effects: Any perturbative approach to QCD (which, by construction, represents merely an approximation and which, in the case of QCD, is particularly far from reality) cannot reveal these phenomena. Accordingly, any meaningful description of QCD bound states has to rely on different, more appropriate methods.

**Present research interests**

- formulation, investigation and application of various equations of motion aiming at the relativistic description of bound states
- description of hadrons by means of so-called sum rules (either in the form proposed by M. A. Shifman, A. I. Vainshtein and V. I. Zakharov or near the light cone)
- construction of exact (semi-) analytical bounds to the energy levels of semirelativistic bound states

The group is supported by the Austrian Science Fund (FWF).

### Staff Members

#### Wolfgang Lucha

**Group Leader Theory QCD**Theory QCD