The quark is an elementary particle whose existence was independently theorized in 1963–1964 by two physicists, Murray Gell-Mann (1929– ) and George Zweig (1938– ). Although quarks may be “unobservable,” physicists accept indirect evidence of their existence. Both Gell-Mann and Zweig proposed that a class of elementary particles made up the other subatomic particles and that their qualities explained the behavior of the others. The name itself was derived from a line in a novel by James Joyce, Finnegans Wake, in which the dreaming hero hears seagulls crying, “Three quarks for Muster Mark.”
The original theory of quarks described them as falling into three types, known as up, down, and strange (u, d, and s). Later, other quark varieties, also named in the same whimsical fashion, were discovered, known as the beauty (or bottom), the truth (or top), and the charmed (or c) quarks. Each of these six types was said to represent a “flavor” of a quark, and in addition they possess another quality, known as “color.” The six flavors of quarks come in three colors, called green, red, and blue.
Whether or not quarks can be said to have been “discovered” or only postulated is a difficult philosophical question. However, since the qualities ascribed to quarks describe effects later produced in particle accelerators, most nuclear physicists now accept their existence as providing an explanation for the behavior of a wide variety of subatomic particles. The 1990 Nobel Prize in physics was awarded to three scientists whose work with linear accelerators demonstrated the existence of quarks: Jerome I. Friedman, Henry W. Kendall, and Richard E. Taylor.