pions

Pions, or pi mesons, are the lightest mesons and form the isospin triplet of light pseudoscalar mesons in the strong interaction. They come in three charge states: π+, π0, and π−. Their quark content is π+ = u d̄, π− = d ū, and π0 ≈ (u ū − d d̄)/√2. The charged pions have a mass of about 139.6 MeV/c², while the neutral pion is about 134.98 MeV/c². Lifetimes differ markedly: π± live about 2.6×10^−8 seconds, whereas the π0 is extremely short-lived, about 8×10^−17 seconds.

Pions are understood as the pseudo-Goldstone bosons of spontaneously broken chiral symmetry in quantum chromodynamics (QCD) with light up and down quarks. As such, they dominate low-energy hadron dynamics and are central to effective theories of strong interactions, such as chiral perturbation theory. They participate in the strong force and, at longer ranges within nuclei, are exchanges in the Yukawa description of the nuclear interaction.

In terms of decays, charged pions decay via the weak interaction to muons and muon neutrinos (π+ → μ+ νμ; π− → μ− ν̄μ), with the muon subsequently decaying further. The neutral pion decays primarily into two photons (π0 → γ γ) through a quantum anomaly, reflecting its role in electromagnetic processes as well.

Pions are produced abundantly in high-energy hadronic collisions and cosmic-ray interactions, and they serve as essential probes of nonperturbative QCD and of symmetries in particle physics.