Lifetime *

As the lightest and the simplest hadron, π⁰ plays a crucial role in our understanding of the symmetry structure of low-energy QCD. The π⁰→γγ decay width offers a fundamental test of the QCD predictions based on the chiral anomaly and spontaneous chiral symmetry breaking. The theoretical calculations over the past two decades have reached 1% precision in the decay amplitude of the π⁰ into two photons. The experimental measurement of this parameter with a comparable accuracy provides a stringent test of QCD. The PrimEx collaboration at Jefferson Lab has developed and performed two experiments (PrimEx I&II) to measure the π⁰ radiative decay width via the Primakoff effect. The published result, Γ(π⁰→γγ)=7.802±0.052±0.105 eV,  has a 1.5% total uncertainty. It agrees to the leading-order chiral anomaly and is about 2σ lower than the high-order QCD predictions. The details of measurement and the physics impact will be discussed.