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##

Strong interaction

Quantum Chromodynamics (QCD) is the theory of strong interactions and one of
the components of the
Standard Model. For the
benefits of the reader we remind briefly some of its basic features. QCD is
the gauge field theory that describes the interactions of colored fermionic
fields (quarks) and non-Abelian colored gauge fields (gluons). The (local) gauge
transformations form the group (color group) with the quarks and gluons
belonging to the fundamental (triplet) and adjoint (octet) representations of
the color group . The gluons mediate the interaction between the quarks, and
in addition, due to the non-Abelian character of QCD, the gluons also interact
with each other. The strength of interaction between colored objects is scale
dependent. The renormalized QCD coupling constant is small at high energy (asymptotic
freedom); in this domain the fundamental fields of QCD, quarks and gluons, can
be probed experimentally in a direct way, and high-precision theoretical predictions
using perturbation theory can be confirmed. On the other hand, the attraction
between two colored objects grows with increasing distance, that leads to the
confinement of quarks and gluons, so that only colorless particles (
singlets) manifest themselves as asymptotic states. At low energy the color
degrees of freedom are not directly observable. In this case the hadron dynamics
is equivalent to an * effective* field theory of colorless hadrons.

**Subsections**

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*Pionic Hydrogen Collaboration*

*1998*