Color Force

A property of quarks labeled color is an essential part of the quark model. The force between quarks is called the color force. Since quarks make up the baryons, and the strong interaction takes place between baryons which are colorless, you could say that the color force is the source of the strong interaction, or that the strong interaction is like a residual color force which extends beyond the proton or neutron to bind them together in a nucleus.

Inside a baryon, however, the color force has some extraordinary properties not seen in the strong interaction between nucleons. The color force does not drop off with distance and is responsible for the confinement of quarks. The color force involves the exhange of gluons and is so strong that the quark-antiquark pair production energy is reached before quarks can be separated. Another property of the color force is that it appears to exert little force at short distances so that the quarks are like free particles within the confining boundary of the color force and only experience the strong confining force when they begin to get too far apart. The term "asymptotic freedom" is sometimes invoked to describe this behavior of the gluon interaction between quarks.

Color

Color is the strong interaction analog to charge in the electromagnetic force. The term "color" was introduced to label a property of the quarks which allowed apparently identical quarks to reside in the same particle, for example, two "up" quarks in the proton. To allow three particles to coexist and satisfy the Pauli exclusion principle, a property with three values was needed. The idea of three primary colors like red, green, and blue making white light was attractive, and language about "colorless" particles sprang up. It has nothing whatever to do with real color, but provides three distinct quantum states. The property can be considered something like a "color charge" with three distinct values, with only color neutral particles allowed. The terms "color force" and even "quantum chromodynamics" have been used, extending the identification with color terms. The antiquarks have anti-colors, so the mesons can be colorless by having a red and an "anti-red" quark. The idea of color is supported by the fact that all commonly observed particles have either three quarks (baryons) or two (mesons), the combinations which can be "colorless" or "color neutral" with the three values of color. This does not exclude "di-baryons" with 6 quarks and other combinations of more than three. There was some experimental suggestion of the presence of a penta-quark particle, but it has not held up under closer scrutiny.

The rationale for the concept of color can be highlighted with the case of the omega-minus, a baryon composed of three strange quarks. Since quarks are fermions with spin 1/2, they must obey the Pauli exclusion principle and cannot exist in identical states. So with three strange quarks, the property which distinguishes them must be capable of at least three distinct values.