Precision measurement of the neutral pion lifetime

The explicit breaking of the axial symmetry by quantum fluctuations gives rise to the so-called axial anomaly. This phenomenon is solely responsible for the decay of the neutral pion p⁰ into two photons (gg), leading to its unusually short lifetime. We precisely measured the decay width G of the p⁰ → gg process. The differential cross sections for p⁰ photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding G(p⁰ → gg) = 7.798 ± 0.056(stat.) ± 0.109(syst.) eV, where stat. denotes the statistical uncertainty and syst. the systematic uncertainty. We combined the results of this and an earlier experiment to generate a weighted average of G(p⁰ → gg) = 7.802 ± 0.052(stat.) ± 0.105(syst.) eV. Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.