Pion (π±)

Lightest meson, a quark-antiquark pair whose exchange binds protons and neutrons inside atomic nuclei.

Core Principles & Analysis

• Predicted by Hideki Yukawa in 1935 as the carrier of the nuclear force between nucleons.
• Discovered in 1947 in cosmic-ray tracks on photographic emulsions by Cecil Powell's Bristol group.
• The charged pion decays almost exclusively to a muon and its neutrino.

Specifications & Metrics

• SPIN: 0
• SYMBOL: π±
• CHARGE: ±1 e
• MASS: 139.57039 ± 1.8 × 10⁻⁴ MeV/c²
• MEAN LIFE: 2.6033 × 10⁻⁸ ± 5 × 10⁻¹² s
• PDG ID: S008
• PDG EDITION: PDG 2026

Principal Decay Modes

• pi+ → mu+ nu_mu — 99.98770 ± 0.00004%
• pi+ → mu+ nu_mu gamma — 0.0200 ± 0.0025%
• pi+ → e+ nu_e — 0.01230 ± 0.00004%
• pi+ → e+ nu_e gamma — 0.0000739 ± 0.0000005%
• pi+ → e+ nu_e pi0 — 0.000001036 ± 0.000000006%

Branching fractions: Particle Data Group, 2026 edition (CC BY 4.0).

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