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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: π0, π+ and π. Pions are the lightest mesons.


Basic properties

Pions have zero spin and are composed of first generation quarks. An up and an anti-down quark compose a π+, while a down and an anti-up quark compose the π, its antiparticle. Combinations of up and anti-up, or down and anti-down, would both be neutral, but because they have the same quantum numbers are only found in superpositions. The lowest energy superposition is the π0, which is its own antiparticle.

The π± mesons have a mass of 139.6 MeV/c2 and a mean life of 2.6 × 10−8 seconds. The main decay mode is into a muon and its neutrino or antineutrino:

\pi^+\to\mu^++\nu_\mu \,
\pi^-\to\mu^-+\bar{\nu}_\mu \,

The π0 has as slightly smaller mass of 135.0 MeV/c2 and a much shorter mean life of 8.4 × 10−17 seconds. The main decay mode is into two photons:

\pi^0\to2\gamma \,


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 π0 is more difficult to observe than π±; being electrically neutral it doesn't leave a track in an emulsion. The π0 was finally identified in 1950 by its decay products.

See also

External link

Last updated: 10-24-2005 01:15:38
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