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In particle physics, color or color charge is the property of elementary particles that gives rise to the strong nuclear force. The concept is only metaphorically related to color in everyday life. Particles with a color charge include the quarks, while the electron is an example of a color-neutral or "white" particle. According to the theory of quantum chromodynamics, the strong nuclear force is carried by gluons. Interestingly, these carrier particles are themselves color charged.
Shortly after the existence of quarks was first proposed in 1964, Oscar W. Greenberg introduced color charge to explain how quarks could coexist inside the proton in otherwise identical states and still satisfy the Pauli exclusion principle. The concept turned out to be useful. Quantum chromodynamics has been under development since the 1970s and constitutes an important ingredient in the standard model of particle physics.
Quark and antiquark colors
A quark's color can take three values: "red", "green", or "blue". Quarks of different colors are attracted and quarks of like color are repelled by the strong nuclear force. An antiquark (the antiparticle of the quark) can take on three "anticolors", sometimes called "antired", "antigreen" and "antiblue" (and sometimes graphically represented as cyan, magenta and yellow).
It should be noted that color charge is not related to electromagnetic radiation or human color perception in any way. The names of the values were arbitrarily assigned, like the names of quarks themselves. While the three types of quark color provide a rough analogy to the primary colors of light, the analogy can be misleading. For instance, a red quark combined with an antired antiquark do not form a color-neutral state, like red mixed with cyan to produce white light; only the equal quantum superposition of red-antired, green-antigreen, and blue-antiblue is color-neutral.
Color was originally introduced as a means of reconciling simple quark models of baryons with the Pauli exclusion principle. Models that fit the observed properties of baryons seemed to require that the quarks be fermions, yet have a collective wave function completely symmetric under quark exchange, a forbidden combination. The inelegant solution was to introduce an extra property, color, for which the wave function would be completely antisymmetric. However, color eventually turned out to be much more than a means of saving appearances: it is the charge associated with the strong nuclear force.
Color charge in QCD
The role of color charge in quantum chromodynamics (QCD) is analogous to that of electric charge in quantum electrodynamics (QED), the quantum theory of electricity and magnetism. The role analogous to that of the photon, the quantum of electromagnetic fields, is played in QCD by the gluons: quarks interact with one another by exchanging gluons. However, there is a major difference from QED: the gluons themselves carry color charge. Roughly speaking, each gluon carries a combination of a color and an anticolor. This would yield 9 completely independent possibilities, except that the quantum superposition of color and anticolor combinations that is completely color-neutral (the state consisting of equal amounts of red-antired, green-antigreen, and blue-antiblue) does not correspond to a gluon. So there are actually 8 gluons, each of which is a superposition of color-anticolor pairs completely orthogonal to the neutral combination.
The color charge carried by gluons means that emitting or absorbing a gluon changes a quark's color, and it also means that the gluons can interact directly with one another by exchanging other gluons. Since at ordinary energies, the interaction is strong and all these processes happen with large amplitudes, it becomes impossible to tease out QCD interactions into Feynman diagrams using perturbation theory. However, the phenomenon of asymptotic freedom effectively weakens the interaction in high-energy collisions, and makes QCD somewhat easier to study theoretically.
Color and the possible hadrons
Since particles of different color are very strongly attracted and particles of like color repel very strongly, it follows that observable particles (those that survive for an appreciable length of time, such as protons and neutrons) are color-neutral (more precisely, quarks seem to exhibit confinement). A color-neutral combination, or hadron, can be achieved by combining a quark and antiquark to create a meson (in the color-anticolor superposition described earlier), or by combining three quarks or three antiquarks to produce a baryon or its antiparticle; or with more complicated combinations such as the pentaquark. There is also the possibility of glueballs, hadrons made by combining gluons.
Mathematics of color charge
Mathematically, color charge is described by the representation theory of the Lie group SU(3). A color-neutral state, such as any hadron, corresponds to the one-dimensional representation of SU(3); the quarks and antiquarks correspond to the two three-dimensional representations, and the gluons (typically for the force-carrying particles of a gauge theory) are in the adjoint representation, which in this case is eight-dimensional.
This is a completely different application of the group SU(3) from Murray Gell-Mann, Yuval Ne'eman and George Zweig's use of it to categorize the hadrons. They thought of an approximate SU(3) symmetry which, in its three-dimensional representation, acts on the flavors of the up, down, and strange quarks, which were the constituents of all the hadrons known at the time.
Griffiths, David J., Introduction to Elementary Particles, New York: John Wiley & Sons, 1987. ISBN 0-471-60386-4
Morris, Richard, The Last Sorcerers: The Path from Alchemy to the Periodic Table, Washington, DC: Joseph Henry Press, 2003. ISBN 0-309-50593-3
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