Pion
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In particle physics, pion (short for the Greek pi meson = 'P middle') is the collective name for three subatomic particles discovered in 1947: π^{0}, π^{+} and π^{−}. Pions are the lightest mesons.
Contents 
Basic properties
Pions have zero spin and are composed of first generation quarks. In the quark model, an up and an antidown quark compose a π^{+}, while a down and an antiup quark compose the π^{−}, its antiparticle. Combinations of up and antiup, or down and antidown, would both be neutral, but because they have the same quantum numbers are only found in superpositions. The lowest energy superposition is the π^{0}, which is its own antiparticle.
The π^{±} mesons have a mass of 139.6 MeV/c^{2} and a mean life of 2.6 × 10^{−8} seconds. The main decay mode is into a muon and its neutrino or antineutrino:
 <math>\pi^+\to\mu^++\nu_\mu \,<math>
 <math>\pi^\to\mu^+\bar{\nu}_\mu \,<math>
The π^{0} has as slightly smaller mass of 135.0 MeV/c^{2} and a much shorter mean life of 8.4 × 10^{−17} seconds. The main decay mode is into two photons:
 <math>\pi^0\to2\gamma \,<math>
History
After theoretical work by Hideki Yukawa in 1935 had predicted the existence of mesons as the carrier particles of the strong nuclear force, charged pions were found experimentally in 1947 by a team led by Cecil Powell. The age of particle accelerators had yet to arrive in those days. Instead, the team lifted photographic emulsions by balloon to high altitude, where they were exposed to cosmic rays. After recovery of the balloon, microscopic inspection of the emulsion revealed the tracks of charged particles, among which the pions were discovered. Their achievements earned Yukawa in 1949 and Powell in 1950 the Nobel Prize in Physics.
The π^{0} is more difficult to observe than π^{±}; being electrically neutral it doesn't leave a track in an emulsion. The π^{0} was finally identified in 1950 by its decay products.
The pions are considered to be the pseudo NambuGoldstone bosons of a spontaneously broken symmetry. This is also the reason why the pion masses are considerably lighter than the masses of other mesons like the <math>\eta<math>Meson (547.75 MeV/c^{2}).
Theoretical overview
The pion can be thought of as the particle that mediates the interaction between a pair of nucleons. This interaction is attractive, that is, it pulls the nucleons together. Written in a nonrelativistic form, it is called the Yukawa potential. The pion, being a meson, has kinematics described by the KleinGordon equation. In the general terms of quantum field theory, the Lagrangian describing the pionnucleon interaction is called the Yukawa interaction.
The nearly identical masses of <math>\pi^\pm<math> and <math>\pi^0<math> imply that there must be a symemtry at play; this symmetry is called the SU(2) flavour symmetry or isospin. The reason that there are three pions, <math>\pi^+<math>, <math>\pi^<math> and <math>\pi^0<math> is that these are understood to belong to the triplet representation or the adjoint representation of SU(2). By contrast, the up and down quarks transform accodring to the irreducible, semisimple fundamental representation of SU(2), whereas the the antiquarks transform according to the conjugate representation. Thus, one has
 <math>SU(2) \otimes \overline{SU(2)} \approx SO(3) \oplus U(1)<math>
which is one of the many relationships which lends weight to the quark model of pions and nucleons.
With the addition of the strange quark, one can say that the pions participate in an SU(3) symmetry with the strange paritcles, belonging to the octect representation (the adjoint representation of SU(3)). With the addition of the charmed, top, and bottom quarks, one can argue that the full flavour symmetry is SU(6) but the argument is strained because the large masses of these other quarks implies that the masses of the corresponding particles are nowhere near the pion mass.
Pions are pseudoscalars under a parity transformation. Pion currents thus couple to the axial vector current and pions participate in the chiral anomaly.
See also
References
 Gerald Edward Brown and A. D. Jackson, The NucleonNucleon Interaction, (1976) NorthHolland Publishing, Amsterdam ISBN 0720403359
External link
 Mesons (http://pdg.lbl.gov/2004/tables/mxxx.pdf) at the Particle Data Group
 Mesons (http://hyperphysics.phyastr.gsu.edu/hbase/particles/hadron.html) at Hyperphysics
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