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The Collaborative International Dictionary

Meson \Mes"on\, n. [NL., fr. Gr. me`son middle, neut. of me`sos, a., middle.]

  1. (Anat.) The mesial plane dividing the body of an animal into similar right and left halves. The line in which it meets the dorsal surface has been called the dorsimeson, and the corresponding ventral edge the ventrimeson.
    --B. G. Wilder.

  2. (Physics) An elementary particle made up of two quarks; a hadron having a baryon number of zero; any hadron other than a baryon. Mesons are bosons with integral values of spin, having a mass intermediate between those of the electron and a nucleon; they may have positive or negative charges, or may be neutral. Mesons are of three types: the pion ([pi]-meson), kaon (K-mesons), and [eta]-mesons.

    mu meson (Physics) the former name for the muon, a particle which is not a true meson[2]. The term is no longer used in technical literature, except historically.

Douglas Harper's Etymology Dictionary

1951, from Greek letter pi + -on.


n. (context particle English) Any of three semistable mesons, having positive, negative or neutral charge, composed of up and down quark/antiquark.


n. a meson involved in holding the nucleus together; produced as the result of high-energy particle collision [syn: pi-meson]


In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons (and, more generally, the lightest hadrons), because they are composed of the lightest quarks (the u and d quarks). They are unstable, with the charged pions and decaying with a mean lifetime of 26  nanoseconds ( seconds), and the neutral pion decaying with a much shorter lifetime of seconds. Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays.

The exchange of virtual pions, along with the vector, rho and omega mesons, provides an explanation for the residual strong force between nucleons. Pions are not produced in radioactive decay, but are commonly produced in high energy accelerators in collisions between hadrons. All types of pion are also produced in natural processes when high energy cosmic ray protons and other hadronic cosmic ray components interact with matter in the Earth's atmosphere. Recently, the detection of characteristic gamma rays originating from the decay of neutral pions in two supernova remnant stars has shown that pions are produced copiously in supernovas, most probably in conjunction with production of high energy protons that are detected on Earth as cosmic rays.

The concept of mesons as the carrier particles of the nuclear force was first proposed in 1935 by Hideki Yukawa. While the muon was first proposed to be this particle after its discovery in 1936, later work found that it did not participate in the strong nuclear interaction. The pions, which turned out to be examples of Yukawa's proposed mesons, were discovered later: the charged pions in 1947, and the neutral pion in 1950.

Pion (disambiguation)

A pion is a type of subatomic particle.

Pion may also refer to:

  • PION, gene; see Protein pigeon homolog
  • 2S7 Pion, self-propelled gun
  • Pión District, in Peru
  • Posterior ischemic optic neuropathy

Usage examples of "pion".

When a pion, this subatomic particle, decays it becomes an electron and a positron, and they must be in antiparallel spin states so as not to violate conservation of spin angular momentum.

Actually, that makes sense: neutrons decaying into protons and pions would transmute some of the calcium to scandium, the oxygen to fluorine, and the carbon to nitrogen.

And when a proton annihilates an antiproton, it produces a pi-zero meson one-third of the time, and a charged pion, a muon, and a neutrino two-thirds of the time, after which the pions and neutrons promptly break down to electrons and positrons, photons and neutrinos.

Actually, that makes sense: neutrons decaying into protons and pions would transmute some of the calcium to scandium, the oxygen to fluorine, and the carbon to nitrogen.

The electrons make gamma rays, and the nuclei make pions, all high energy stuff.

To read even an elementary guide to particle physics nowadays you must now find your way through lexical thickets such as this: “The charged pion and antipion decay respectively into a muon plus antineutrino and an antimuon plus neutrino with an average lifetime of 2.

The whole kit-and-caboodle of pions, mesons, gluinos, antineutrinos, that whole strange charm of quarkiness, may come to seem a very twentieth-century enthusiasm.

Thy banks with pioned and twilled brims, Which spongy April at thy hest betrims, To make cold nymphs chaste crowns.

The compact fusion reactor buzzed gently, its beat-wave accelerators ramming a mixture of electrons and pions into a stream of lithium ions at just under the speed of light.