Friday, March 13, 2009

A pion is the lightest of the meson family of elementary particles. A pion is a multiplet of three particles. The neutral pion has a mass about 264 times that of the electron. The charged pions, p+ and p-, have a mass about 273 times that of the electron. The neutral pion is its own antiparticle, and the negative pion is the antiparticle of the positive pion. Each pion is made up of a quark bound to an antiquark. Free pions are unstable. Charged pions decay with an average lifetime of 2.55 × 10-8 sec into a muon of like charge and a neutrino or antineutrino, while the neutral pion decays in about 10-15 sec, usually into a pair of photons but occasionally into a positron-electron pair and a photon.

Pions have zero spin and consist of first-generation quarks. In the quark model, an up and an anti-down quark compose a p+, while a down and an anti-up quark compose the p-, its antiparticle. The neutral combinations of up with anti-up and down with anti-down have identical quantum numbers, so they are only found in superpositions. The lowest-energy superposition is the p0, which is its own antiparticle. Together, the pions form a triplet of isospin; each pion has isospin-1 (I = 1) and third-component isospin equal to its charge (Iz = +1, 0 or -1).

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. It was first detected in cosmic rays by C. F. Powell in 1947.