In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ?) is any of three subatomic particles: ?0, ?+, and ??. Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more generally, the lightest hadrons. They are unstable, with the charged pions ?+ and ?? decaying with a mean lifetime of 26.033 nanoseconds (2.6033×10?8 seconds), and the neutral pion ?0 decaying with a much shorter lifetime of 84 attoseconds (8.4×10?17 seconds). Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays. Wikipedia, Pion
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