More Particles, More Quarks
As elementary particle physics progressed through the 1970s and 1980s, physicists found more and more exotic particles, such as the psi meson, whose mass is about three times that of the proton (discovered in 1974). An accurate description of its observed properties required the addition of a fourth quark, the "charm" quark, with an electric charge of +(2⁄3)e. In 1977 discovery of the upsilon meson (ten times the proton mass) required the introduction of the "bottom" quark, with charge −(1⁄3)e. A sixth quark, the "top" quark, was found in 1994 at the Fermilab National Accelerator in Illinois, with a charge of +(2⁄3)e. and a mass of about 180 times the proton mass, which is equivalent to that of a gold atom. All of these three heavier quarks decay quickly into other particles (including other quarks).
Scientists now assume there are only six quarks, and that quarks themselves have no internal particles—that quarks are point particles like electrons, and truly fundamental. Together with electrons and the other leptons, they accurately describe the world as it is known at this time.
See also Quantum mechanics.
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Gell-Mann, Murray. The Quark and the Jaguar. New York: W. H. Freeman, 1994.
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Wilczek, Frank. "Liberating Quarks and Gluons." Nature (January 2, 1998): 330–331.
Wilczek, Frank. "Backyard Exotica." Nature (March 30, 2000): 452–453.