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.
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.
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 particle collisions.