MESON

any member of a family of subatomic particles composed of an even number of quarks and antiquarks. Mesons are sensitive to the strong force (the force that binds the components of the nucleus) because their constituent quarks are strongly interacting. Mesons consist of an even number of quarks with half-integral spin, and so they have integral spin. They vary widely in mass, ranging from 140 MeV to nearly 10 GeV. Various types of mesons have been discovered since their existence was first predicted in 1935 by the Japanese physicist Yukawa Hideki. Of those so far identified, the pi meson and the K meson are the most important. Pi mesons, also known as pions, are chiefly responsible for the strong interactions between the protons and neutrons in atomic nuclei. K mesons, or kaons, have several competing decay modes. Investigations of these processes have led to a better understanding of parity (the property of an elementary particle or physical system that indicates whether or not its mirror image occurs in nature) and its nonconservation. Mesons serve as a useful tool for studying the properties and interactions of quarks, the fundamental units of matter that constitute all hadrons (any of the subatomic particles that react by the force of strong interaction). Although mesons are unstable, many last long enough (a few billionths of a second) to be observed with particle detectors, making it possible for researchers to reconstruct the motions of quarks. Any model attempting to explain quarks must correctly elucidate the behaviour of mesons. One of the early successes of the Eightfold Way, a forerunner of modern quark models devised by the physicists Murray Gell-Mann and Yuval Ne'eman, was the prediction and subsequent discovery of the eta meson (1962). Some years later the decay rate of the pi meson into two photons was used to support the hypothesis that quarks can take on one of three colours (i.e., values of a quantum number). Surprises in meson behaviour are also important, as attested by the study of CP violation (the violation of the combined conservation laws associated with charge and parity ) in the K-meson system. Mesons also provide a means of identifying new quarks. The J/psi particle, discovered in 1974, proved to be a meson made up of a charmed quark and its antiquark. (Up to this time, three quark types had been postulatednamely up, down, and strange.) It was the first manifestation of charm, a new quantum number the existence of which implies that quarks are related in pairs. The subsequent discovery of another heavy meson, called upsilon, revealed the existence of the bottom quark and its accompanying antiquark and gave rise to speculation about the existence of a companion particle, the top quark. This sixth quark type was discovered in 1995. Conclusive proof of its existence provided investigators with direct evidence in support of the underlying postulates of the prevailing quark model. See also quark.