QCD

Quantumchromodynamics (QCD) is considered to be the theory which describes strong interactions. QCD is a gauge field theory where SU(3) is the gauge group. The gauge fields, the gluons, mediate the interactions between the fermionic particles, the quarks. Both, the quarks and the gluons carry a colour charge. Therefore the gluons do also interact among themselves.

The formulation of this theory was pushed by two important observations. Experimental results from electron-nucleon-scattering at large energy and momentum transfer showed that the electrons are scattered by nearly free particles. Any theory of strong interactions should therefore explain this so-called asymptotic freedom at small distances. In this regime it is possible to do perturbative calculations. These calculations allowed first important confirmations of the validity of QCD.

A second observation is not completely understood yet. Particles which carry a colour charge have never been detected as free particles. Because of confinement quarks will only occur in bound states like nucleons, which are composed of three quarks. In order to understand why it is not possible to separate quarks over a long distance, one has to apply non-perturbative methods, e.g. lattice QCD. On the lattice it was actually possible to understand confinement in the static limit.