Pion

A pion (short for pi meson) is the collective name for three subatomic particles: π0, π+ and π−. Pions are the lightest mesons and play an important role in explaining low-energy properties of the strong nuclear force.

The pion can be thought of as the particle that mediates the interaction between a pair of nucleons. This interaction is attractive; it pulls the nucleons together.

Pions have zero spin and are composed of first-generation quarks. In the quark model, an up and an anti-down quark compose a π+, while a down and an anti-up quark compose the π−, its antiparticle. The neutral combinations of up with anti-up and down with anti-down have identical quantum numbers, so they are only found in superpositions. The lowest-energy superposition is the π0, which is its own antiparticle. Together, the pions form a triplet of isospin; each pion has isospin-1 and third-component isospin equal to its charge.

The π± mesons have a mass of 139.6 MeV/c2 and a mean life of 2.6×10^−8 seconds. Their main decay mode is into a muon and its neutrino.

The π0 meson has as slightly smaller mass of 135.0 MeV/c2 and a much shorter mean life of 8.4×10^−17 seconds. Its main decay mode is into two photons.

The rate at which pions decay features prominently in many subfields of particle physics such as chiral perturbation theory. This rate is parametrized by the pion decay constant (fπ), which is about 90 MeV.

Pions are pseudoscalars under a parity transformation. Pion currents thus couple to the axial vector current and pions participate in the chiral anomaly.