pīˈŏn or pi meson, lightest of the meson family of elementary particles. The existence of the pion was predicted in 1935 by Hideki Yukawa, who theorized that it was responsible for the force of the strong interactions holding the atomic nucleus together. It was first detected in cosmic rays by C. F. Powell in 1947. The pion is actually a multiplet of three particles. The neutral pion, π⁰, has a mass about 264 times that of the electron. The charged pions, π⁺ and π⁻, each have a mass about 273 times that of the electron. The neutral pion is its own antiparticle, while the negative pion is the antiparticle of the positive pion. It is now known that each pion (and, more generally, each meson) consists of a quark bound to an antiquark. Free pions are unstable. The charged pions decay with an average lifetime of 2.55 × 10⁻⁸ sec into a muon of like charge and a neutrino or antineutrino; the neutral pion decays in about 10⁻¹⁵ sec, usually into a pair of photons but occasionally into a positron-electron pair and a photon.

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