Meson
Mesons are particles composed of an even number of quarks and antiquarks, usually one quark and one antiquark, bound together by the exchange of gluons. Since they interact using the strong nuclear force, mesons are classified as hadrons, along with the proton and neutron. About 100 different mesons are known, varying from one-fifth the mass of a proton to about 10 times the proton's mass.
Mesons were first discovered in cosmic rays, as the products of the collisions of high-energy particles, mostly fast moving protons, with the nucleons of atoms in the atmosphere. Japanese physicist Hideki Yukawa predicted their existence in 1935 as the carriers of the strong force that binds the nucleons of an atom together. Since very few mesons reach the surface of Earth before interacting with particles in the atmosphere, they were not observed until 1946. These first mesons to be identified were called pi mesons, or pions. Although pions and other mesons are involved in building the atomic nucleus, their role is more complex than the original theory and is not yet completely understood.
Most current meson research is performed in high-energy accelerators by studying the sequence of particles produced by high-energy hadron collisions. One group of mesons, the K mesons, or kaons, is especially important to the study of basic particles because kaon decay modes have provided 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 the nonconservation of parity.
All mesons are unstable particles, most of them decaying in about 10-24 seconds. This is not long enough to observe the particle directly, so its existence is known only by studying the types and motions of particles formed when these mesons decay. There are, however, some types of mesons that exist longer, up to a few billionths of a second--a very long lifetime in subatomic terms. This is long enough to observe these mesons directly with particle detectors. The existence of quarks as the basic constituents of strongly interacting particles was discovered by observing the motions of mesons and the particles that are formed as they break apart. Most of the properties of quarks are determined by the interactions of mesons. For example, the decay rate of the pi meson into two photons was used to support the hypothesis that quarks have a characteristic property, called color (not related to color as we sense it) that exists in three different states. This property is the foundation of quantum chromodynamics.
The study of mesons also leads to identification of new quarks. The J/psi particle, discovered in 1974, is made of a previously unknown kind of quark, different from the three types that had been previously named--up, down, and strange. This meson was determined to be composed of a pair of quarks, labeled the charmed quark and the charmed antiquark. Charm was a new quantum number and its existence implies that quarks are related in pairs. The discovery of another heavy meson, called upsilon, revealed the existence of the bottom quark and its corresponding antiquark. After the discovery of the bottom quark, theoretical physicists predicted the existence a sixth quark, the top quark. Another meson, composed of a top quark and a top antiquark, was discovered in 1995, confirming some of the basic assumptions of the current quark model.
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