quantum chromodynamics

Quantum chromodynamics (QCD) is a quantum field theory that describes the properties of the strong interactions between quarks and between protons and neutrons in the framework of quantum theory and the special theory of relativity.


Color charge

Quarks have a distinctive property called color charge which governs their binding together to form other elementary particles. Analogous to electric charge in charged particles, color charge (or, simply, "color") comes in three varieties, arbitrarily known as red, blue, and yellow, and – analogous to positive and negative charges – three anticolor varieties, yellow, magenta and cyan.


Just as positively and negatively charged particles form electrically neutral atoms, colored quarks form particles with no net color. Quarks interact by emitting and absorbing massless particles called gluons, each of which carries a color-anticolor pair. Eight kinds of gluons are required to transmit the strong force between quarks, e.g., a blue quark might interact with a yellow quark by exchanging a blue-antiyellow gluon.


Quarks are not, of course, actually colored – this is just a metaphor for the fact that quarks are always found in groups consisting of all three colors or anticolors (the baryons) or one color and its anticolor (the mesons) or even both (the pentaquark consists of a triplet and a pair). Thus the three colors (or color-anticolor pair) of every particle combine to make white.


The concept of color was proposed by the American physicist Oscar Greenberg and independently by the Japanese physicist Yoichiro Nambu in 1964. The theory was confirmed in 1979 when quarks were shown to emit gluons during studies of high-energy particle collisions at the German national laboratory in Hamburg. QCD is nearly identical in mathematical structure to quantum electrodynamics (QED) and to the unified theory of weak and electromagnetic interactions advanced by American physicist Steven Weinberg and Pakistani physicist Abdus Salam (see electroweak force).


Color force

Color force is the force between particles that carry color charge, namely quarks and gluons. This is the force due to the strong interaction. One effect of the color force is to ensure that particles made of quarks (like protons and neutrons) are "white."