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با تشکر - حسین جوادی

Color Charge

Color charge is the charge associated with strong interaction. "Color" is a whimsically named attribute of quarks and gluons that cannot be seen.

The mathematics of color charge is the mathematics of the special unitary group, SU(3). This mathematics is quite different from the ordinary arithmetic that is obeyed by electric charges.

All hadrons are color-neutral objects. This means they have no net color charge. A few of the features of the mathematics of color charges are:

There are three possible color-charges for quarks. (This is the meaning of the 3 in the name SU(3).)

A three-quark state, with one quark of each color, is a color-neutral (also called color-singlet) state. This is the combination that makes a baryon. (This is the reason for the name "color" -- mixing three primary colors of paint to give white (or colorless paint) is an analogy for the color neutral combination of three quarks.)

Going beyond this analogy:

There are three (different) anti-colors for antiquarks.

A state of three antiquarks, one of each anti-color, is also color neutral. This is an antibaryon.

If we could examine the color of the quark and antiquark in a meson, there would be equal probability of finding each color quark together with its matching (anti-color) antiquark. This too is a color-neutral combination.

Gluons have one color and one anti-color. There are, however, only eight types of gluons, not the nine you might expect. The number of gluon types can be explained -- there is one combination of color and anti-color that is color neutral. That is, the combination that has an equal probability of containing each color together with its own anti-color. Thus there is no gluon with this color combination. This apparently arbitrary statement is part of the mathematics of color charge.

Color Neutrality

All observed particles are color-neutral objects. They are either leptons, particles with no color charges inside them, or hadrons, particles made of quarks and gluons.

Quarks and gluons are only found inside hadrons, so we say they are "confined". The quarks inside a hadron are bathed in a sea of gluons (and additional quark-antiquark pairs) that are responsible for the binding forces in the hadron. Quarks continually emit and absorb gluons. Color charge is conserved in every such process. The color-mathematics always works out so that at any instant the entire hadron system is color neutral.

Gluon Emission and Multi-jet Events

When a quark emits a gluon, it can change color. Let us say a red quark becomes a green quark. The gluon it emitted must then carry the colors red and anti-green. The process of gluon emission and absorption occurs only inside hadrons and is not directly observable. The only time we can observe that gluon emission occurs is in a high energy collision process. Even then, we do not see the gluon directly, but only a collection of hadrons that are produced by it.

For example, when a high energy electron and positron meet with equal and opposite momentum, they can annihilate (that is, disappear) and produce hadrons.

If the underlying process is the production of a quark and antiquark, we see two back-to-back jets or clusters of particles.

If either the quark or the antiquark radiate a high energy gluon at a significant angle to their initial direction of travel, we see three jets or clusters of particles, lying in a single plane.

If both radiate a gluon, or one of them emits two gluons, we get four jets, and so forth.

(Check out actual multi-jet hadronic event displays!)

The theory predicts the relative rates of two, three, and more jets of particles as a function of the collision energy. It also predicts the angular distribution patterns expected for a collection of many such events. By comparing the observed patterns with the theoretical predictions, physicists have confirmed the predictions of the gluon theory of quark interactions. This theory is called QCD, short for Quantum Chromo Dynamics, the quantum dynamics of color charges.

Confinement

When quarks were first proposed they seemed a very strange idea because no one had seen particles with electric charges that were a fraction of a proton charge. Now we understand this is because quarks, and gluons too, are confined -- this means they are only found inside color-neutral hadrons.

After a high energy collision, a quark or gluon starts to move away from the rest of the formerly color-neutral object that contained it. A region of color force-field is produced between the two parts. The energy density in this color force fields is sufficient to produce additional quarks and antiquarks. The forces between the color-charged particles quickly cause the collection of quarks and antiquarks to be rearranged into color-neutral combinations. What emerges, far enough from the collision point to be detected, is always a collection or jet of color-neutral hadrons, never the initial high-energy quark or gluon alone.

One can think of confinement as a question of probability or entropy. There are possible physical states for many systems that we never observe because they are highly ordered and, thus, highly improbable compared to the many more disordered states. For example, we never expect to observe a room with all the air molecules in a one cubic centimeter region in the corner of the room. Such a state is possible, but highly improbable. Similarly, a state with a quark and an antiquark a meter apart and strong force field between them, but no additional quarks or antiquarks is a possible but highly improbable state. All the energy in the force field can readily produce a multitude of additional quarks and antiquarks and, thus, the ordered color field decays almost as fast as it forms, producing one of the more disordered states containing many hadrons.

Note that this analogy -- like all analogies -- does not tell the full story. In the case of air in a room, the ordered and disordered states both contain all the same atoms. In the case of hadron formation, new objects, quarks and antiquarks, must be produced and organized into hadrons before the ordered state of intense force-field can "fall apart."