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