Quantum chromodynamics (abbreviated as QCD) is the theory of the strong interaction (color force), a fundamental force describing the interactions of the quarks and gluons found in hadrons (such as the proton, neutron or pion). QCD is a quantum field theory of a special kind called a non-abelian gauge theory. It is an important part of the Standard Model of particle physics. A huge body of experimental evidence for QCD has been gathered over the years.
http://en.wikipedia.org/wiki/Quantum_chromodynamicsA key factor is that quarks, the particles which build hadrons (pion, nucleon and baryonic resonances) are not free, but confined in pairs or triplets, and perhaps quintuplets.
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