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# 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.