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Quantum chromodynamics (QCD) is a theory describing the action of the strong nuclear force on quarks through the force-carrying particles gluons. Analogous to photons in quantum electrodynamics (quantum field theory of the electromagnetic force), gluons transmit the strong force between particles that carry "color," a property analogous to electric charge for the strong force. Therefore the strong nuclear force is limited to quarks and the composite particles they create, known as hadrons.
The strong nuclear force is one of the four fundamental forces of nature along with gravity, electromagnetism, and the weak force. The strong force is the strongest of the four forces and is responsible for binding fundamental particles to create larger particles such as atomic nuclei. The relative strengths of the strong force and electromagnetism are demonstrated by atomic nuclei that contain multiple protons even though they are electromagnetically repelled.
The concept of color in the context of the strong nuclear force was developed into the theory of QCD beginning in 1971 by European physicists Harald Fritzsch and Heinrich Leutwyler and American physicist Murray Gell-Mann. They utilized the general field theory developed in the 1950s by Chen Ning Yang and Robert Mills, where force carrier particles can radiate further carrier particles (unlike the electromagnetic force).
