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quark(kwôrk): see elementary particleselementary particles,
the most basic physical constituents of the universe. Basic Constituents of Matter
Molecules are built up from the atom, which is the basic unit of any chemical element. The atom in turn is made from the proton, neutron, and electron.
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quark(kwork, kwark) Any of six pointlike elementary particles (the up, down, strange, charm, bottom, and top quarks) that are thought to be the structural units from which other particles (excluding leptons) are formed. In the usual scheme, quarks all have spin ½, electric charges that are –⅓ or ⅔ of the proton charge, and other innate qualities. Corresponding to each quark is an antiquark, with opposite electric charge, etc. (see antiparticles). The theory of quark interactions predicts that isolated quarks can never exist in the free state (under normal conditions of temperature and pressure); they occur only in particular pairs and triplets that form the class of particles called the hadrons. The proton, for example, is composed of one down and two up quarks, the neutron of one up and two down quarks. Quarks are held together through the exchange of massless particles called gluons; these are the carriers of the strong force, one of the four fundamental forces that particles experience.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006
One of the hypothetical basic particles, having charges whose magnitudes are one-third or two-thirds of the electron charge, from which many of the elementary particles may, in theory, be built up; for example, nucleons may be formed from three quarks and mesons from quark-antiquark combinations; no experimental evidence for the actual existence of free quarks has been found.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
Physics any of a set of six hypothetical elementary particles together with their antiparticles thought to be fundamental units of all baryons and mesons but unable to exist in isolation. The magnitude of their charge is either two thirds or one third of that of the electron
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005