"Quantum chromodynamics (QCD), the relativistic quantum theory that describes the interactions of quarks and gluons to make up hadrons, is a pillar of the standard model of particle physics. Yet it remains a work in progress. This is because, while QCD has obvious similarities to the much simpler quantum theory of electrodynamics that involves electrons interacting through exchanges of photons, gluons carry a color quantum number leading to a marked increase in complexity in the number of fundamental interactions."

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