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pion
The lightest type of meson. Pions are composed of up quarks, down quarks, and their anti-quark counterparts. Pions of charge +1, -1, and 0 are denoted pi+, pi-, and pi0, respectively. The pi0 (mass 135 MeV) is composed of either an up/antiup quark pair or a down/antidown quark pair; the pi+ is an up/antidownpair and the pi- is a down/antiup pair (both have a mass of 140 MeV).
Pions were predicted theoretically by Hideki Yukawa in 1935, and were discovered in cosmic ray experiments on the Pic du Midi by researchers from Bristol University, England, headed by Cceil Powell, in 1947. They are produced copiously in high energy particle collisions.
Related category
• PARTICLE PHYSICS
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