Color Charge Degree of Freedom in Particle Physics

Color has two facets in ► particle physics. One is as a three-valued charge degree of freedom, analogous to electric charge as a degree of freedom in electromagnetism. The other is as a ► gauge symmetry, analogous to the U(1) gauge theory of electromagnetism. Color as a three-valued charge degree of freedom was introduced by Oscar W. Greenberg [1] in 1964. Color as a gauge symmetry was introduced by Yoichiro Nambu [2] and by Moo Young Han and Yoichiro Nambu [3] in 1965. The union of the two contains the essential ingredients of ► Quantum Chromodynamics, QCD. The word “color” in this context is purely colloquial and has no connection with the the color that we see with our eyes in everyday life.

The theoretical and experimental background to the discovery of color centers around events in 1964. In 1964 Murray Gell-Mann [4] and George Zweig [5] independently proposed what are now called “quarks,” particles that are constituents of the observed strongly interacting particles, “hadrons,” such as protons and neutrons. Quarks gave a simple way to account for the ► quantum numbers of the hadrons. However quarks were paradoxical in that they had fractional values of their electric charges, but no such fractionally charged particles had been observed. Three “flavors” of quarks, up, down, and strange, were known at that time. The group SU(3)_flavor, acting on these three flavors, gave an approximate symmetry that led to mass formulas for the hadrons constructed with these quarks. However the spin 1/2 of the quarks was not included in the model. (Quarks, see also ► Mixing and Oscillations of Particles; Particle Physics; Parton Model; QCD; QFT.)