pion

Pion, or pi-meson, is the lightest meson and a hadron consisting of a quark and an antiquark. In the quark model the charged pions are π+ = u d̄ and π− = d ū, while the neutral pion is a quantum superposition π0 = (u ū − d d̄)/√2. They form an isospin triplet with total isospin I = 1.

Pions are pseudoscalar mesons with spin 0 and negative parity. They belong to the SU(2) isospin triplet, and the neutral pion has positive C-parity. Their masses are m(π±) ≈ 139.57 MeV/c² and m(π0) ≈ 134.98 MeV/c², making them the lightest hadrons after the photon and neutrinos.

Role in physics: pions participate in the strong interaction as quark-antiquark bound states and play a central role in low-energy quantum chromodynamics. They are often interpreted as pseudo-Goldstone bosons arising from spontaneous chiral symmetry breaking in QCD, which helps explain their relatively small masses compared with other hadrons.

Decay and lifetime: charged pions decay via the weak interaction to a muon and a muon neutrino (π+ → μ+ νμ, π− → μ− ν̄μ) with a mean lifetime of about 2.6 × 10^-8 seconds. The neutral pion decays predominantly to two photons (π0 → γγ) with a lifetime on the order of 10^-16 seconds.

Production and detection: pions are produced abundantly in high-energy hadronic collisions and cosmic-ray interactions. They also mediate the residual strong force between nucleons in the Yukawa picture and are extensively studied in particle and nuclear physics experiments to test theories of the weak interaction and chiral dynamics.