Quark
Quark, a particle that physicists believe is a basic constituent of protons, neutrons, and certain other subatomic particles collectively known as hadrons. The most widely accepted theory that has been developed to explain the properties of quarks and how they interact is called quantum chromodynamics, or QCD. According to this theory, there are several kinds, or "flavors" of quarks and each quark possesses one of three varieties of a type of charge called "color." ("Flavor," "color," and other terms used in describing quarks are whimsical names and have nothing to do with the ordinary meanings of these words.)
Physicists have discovered six kinds of quarks, which have been given the names up, down, strange, charmed, bottom, and top. In addition, there is an antiquark (an antiparticle) that corresponds to each flavor and color of quark. According to QCD, quarks are always combined in groups of three quarks (of different colors) or as pairs consisting of a quark and an antiquark (of the same color). A proton, for example, consists of a group of three quarks. The quarks in a group or pair are bound by a force transmitted by particles called gluons.
The existence of quarks was first proposed in 1964 by Murray Gell-Mann and, independently, by George Zweig. (Gell-Mann named the particles, using a word from James Joyce's Finnegans Wake.) Originally, the quark theory described three quarks: the up, down, and strange. However, the properties of two particles (the j/psi and upsilon particles) discovered in the mid-1970's provided evidence for the existence of two additional kinds of quarks: the charmed and bottom quarks. The existence of the bottom quark in turn indicated there should be a sixth quark. The discovery of this quark, the top quark, was announced in 1995.
The existence of quarks has been established largely through indirect evidence; no experiment has succeeded in isolating individual quarks. Many physicists believe that it is not possible to isolate individual quarks because the energy required to separate them goes into creating new quarks that combine with the ones being separated.