pions

• lightest mesons, made of up and down quarks and antiquarks: π⁺, π⁻, π⁰
• mass ≈ 140 MeV/c² (charged) and 135 MeV/c² (neutral), ~15% of a proton’s
• unusually light: pseudo-Goldstone bosons of spontaneously broken chiral symmetry
• π⁺ and π⁻: lifetime ≈ 26 ns, decay into a muon and a neutrino (source of most cosmic-ray muons at the ground)
• π⁰: lifetime ≈ 8.5 × 10⁻¹⁷ s, decays into two photons
• pion exchange describes the nuclear force between protons and neutrons (Yukawa, 1935)