In particle physics, pion (short for 'pi meson') is the collective name for three subatomic particles discovered in 1947: π0, π+ and π−. Pions are the lightest mesons.1 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 an muon and a neutrino:
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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:
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2 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 PhysicsList of Nobel Prize laureates in Physics from 1901 to the present day. 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s External links Official site (source for all citatio.
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.
3 See also
- PioniumPionium is an exotic atom consisting of a π+ and a &pi meson. It is generally created by the interaction of a proton beam and a target nucleus in a particle accelerator. Currently, pionium is under investigation at CERN to measure its lifetime. The lif
- List of particlesElementary particles An elementary particle is a particle with no measurable internal structure, that is, it is not a composite of other particles. In a quantum field theory, these are the particles which are created and annihilated by the field operators
4 External link
Meson
