The pion is the lightest type of meson. Pions, also called
pi-mesons, 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).
All have zero spin.
|In this image from a streamer chamber, one of the
pions from a collision makes the looping track to the right, before
it decays into a muon, which then curls anticlockwise four times,
and eventually changes into an electron which moves off towards the
upper right. Image credit: Tom Kemp
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 Cecil Powell, in 1947. They are produced copiously in high-energy