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meson(mē`zŏn) [Gr.,=middle (i.e., middleweight)], class of 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.
..... Click the link for more information. whose masses are generally between those of the leptonlepton
[Gr.,=light (i.e., lightweight)], class of elementary particles that includes the electron and its antiparticle, the muon and its antiparticle, the tau and its antiparticle, and the neutrino and antineutrino associated with each of these particles.
..... Click the link for more information. class of lighter particles and those of the baryonbaryon
[Gr.,=heavy], class of elementary particles that includes the proton, the neutron, and a large number of unstable, heavier particles, known as hyperons. From a technical point of view, baryons are strongly interacting fermions; i.e.
..... Click the link for more information. class of heavier particles. From a technical point of view mesons are strongly interacting bosons; i.e., they participate in the strong nuclear forceforce,
commonly, a "push" or "pull," more properly defined in physics as a quantity that changes the motion, size, or shape of a body. Force is a vector quantity, having both magnitude and direction.
..... Click the link for more information. and are described by the Bose-Einstein statistics, which apply to all particles not covered by the Pauli exclusion principleexclusion principle,
physical principle enunciated by Wolfgang Pauli in 1925 stating that no two electrons in an atom can occupy the same energy state simultaneously. The energy states, or levels, in an atom are described in the quantum theory by various values of four different
..... Click the link for more information. . The lightest meson is the pionpion
or pi meson,
lightest of the meson family of elementary particles. The existence of the pion was predicted in 1935 by Hideki Yukawa, who theorized that it was responsible for the force of the strong interactions holding the atomic nucleus together.
..... Click the link for more information. , whose mass is about 270 times that of the electronelectron,
elementary particle carrying a unit charge of negative electricity. Ordinary electric current is the flow of electrons through a wire conductor (see electricity). The electron is one of the basic constituents of matter.
..... Click the link for more information. . Heavier mesons include the kaon (K meson), eta meson, and a number of higher-mass recurrences of the lighter mesons. The heaviest mesons are heavier than some baryons, such as the proton and neutron, but their classification as mesons is based on their behavior rather than on their mass. The existence of mesons was first predicted in 1935 by Hideki Yukawa, who theorized that they could be responsible for the force holding the nucleusnucleus,
in physics, the extremely dense central core of an atom. The Nature of the Nucleus
Atomic nuclei are composed of two types of particles, protons and neutrons, which are collectively known as nucleons.
..... Click the link for more information. of an atom together. In 1936 a particle was discovered by Carl D. Anderson and Seth Neddermeyer that had a mass close to that predicted for the Yukawa particle. However, the behavior of this particle, the muonmuon
, elementary particle heavier than an electron but lighter than other particles having nonzero rest mass. The name muon is derived from mu meson, the former name of the particle. The muon was first observed in cosmic rays by Carl D.
..... Click the link for more information. , did not correspond to that of the theory at all. The muon was subsequently reclassified as a lepton rather than a meson. The particle predicted by Yukawa was the pion, which was not discovered until 1947 by C. F. Powell and coworkers. Both the muon and the pion were first observed in secondary cosmic rayscosmic rays,
charged particles moving at nearly the speed of light reaching the earth from outer space. Primary cosmic rays consist mostly of protons (nuclei of hydrogen atoms), some alpha particles (helium nuclei), and lesser amounts of nuclei of carbon, nitrogen, oxygen, and
..... Click the link for more information. , being produced in the upper atmosphere by collisions between primary cosmic rays and the atoms of the atmosphere. Since then mesons have been produced and observed in large numbers in laboratories where high-energy particle collisions can be achieved with the aid of a particle acceleratorparticle accelerator,
apparatus used in nuclear physics to produce beams of energetic charged particles and to direct them against various targets. Such machines, popularly called atom smashers, are needed to observe objects as small as the atomic nucleus in studies of its
..... Click the link for more information. . It is now known that each type of meson consists of a quark bound to an antiquark.
The generic name for any hadronic particle with baryon number zero. Such particles were first envisaged in 1935 by H. Yukawa, who pointed out that the main features of nuclear forces would be explained if these forces were transmitted between nucleons through an intermediate field coupled with nucleons, provided that its quanta (nuclear force mesons) were massive [200–300 electron masses (me)] and could carry electric charge between the nucleons. See Baryon, Hadron, Nuclear structure, Quantum field theory
All mesons are unstable. Those with relatively long lifetimes are referred to as semistable. Nearly 200 highly unstable mesons are established, with lifetimes shorter than 10-22 s, and more continue to be discovered. These mesons decay to lighter mesons through the strong nuclear (hadronic) interactions, whereas the hadronic decays of the semistable mesons are forbidden or strongly suppressed. Alternative decay modes involve the weak interactions or the electromagnetic interactions, which are much weaker than the strong interactions and therefore lead to much smaller decay rates and longer lifetimes. The longest-lived mesons are those that decay only through the weak interactions; these include the charged &pgr; mesons (pions) and the K mesons (kaons), with lifetimes of about 10-8 to 10-10 s. See Fundamental interactions, Weak nuclear interactions
Hadrons are now considered to be composite, consisting of spin-1/2 quarks (q), corresponding antiquarks (q), and some number of gluons (g), the last being the quanta of the intermediate field which binds the quarks and antiquarks to form hadrons. Baryon number B = + holds for a quark q, B = - for antiquark q, while B = 0 holds for a gluon. In this view, the simplest possibility is that each meson is a quark-antiquark pair bound together by the gluon field, and this model does account quite well for most of the known mesons and their properties. However, more complicated systems (for example, consisting of two quarks with two antiquarks) can be considered and may even be required by some of the present data. The quarks must be assigned fractional charge values, relative to the proton charge. See Gluons, Quarks
an unstable elementary particle that belongs to the class of strongly interacting particles (hadrons); in contrast to baryons, mesons have zero baryon charge and have zero or integral spin, that is, mesons are bosons. The term “meson”— from the Greek mesos, meaning “intermediate”—was chosen because the masses of the first mesons that were discovered—the pion (7r-meson) and kaon (π-meson)—have values intermediate between the masses of the proton and electron. (Muons, which originally were called μ-mesons, are not mesons since they have a spin of ½ and do not take part in strong interactions.) Many other mesons with very short lifetimes (called boson resonances) were discovered subsequently, and the mass of most exceeds that of the proton. Mesons are the carriers of nuclear forces. Mesons are created especially intensively during collisions of highenergy hadrons.
Mesons may be neutral or charged (with positive or negative elementary electric charge) and may have zero (for example, pions) and nonzero (for example, kaons) strangeness. Mesons with an isotopic spin of 0, Vi, or 1 form, respectively, isotopic singlets, doublets, and triplets. According to the current classification of elementary particles, mesons with properties that are similar with respect to the processes caused by the strong interaction are combined in groups (“supermultiplets”) that consist of eight, nine, and ten particles.