Pion




In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: π) is any of three subatomic particles: π0, π+, and π−. 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), because they are composed of the lightest quarks (the u and d quarks). They are unstable, with the charged pions π+ and π− decaying with a mean lifetime of 26 nanoseconds (×10 seconds), and the neutral pion 2.6π0 decaying with a much shorter lifetime of ×10 seconds. Charged pions most often decay into muons and muon neutrinos, while neutral pions generally decay into gamma rays. 8.4