A Precision Measurement of Neutral Pion Lifetime

The neutral pion decays via chiral anomaly and this process historically led to the discovery of the chiral anomaly. The $\pi^0$ decay amplitude is among the most precise predictions of quantum chromodynamics (QCD) at low energy. However, the current experimental results are not commensurate with theoretical predictions. The Partical Data Group (PDG) average of the experimental results is $7.74\pm0.46$ eV, which is consistent with the chiral anomaly prediction (leading order). Recent theoretical calculations (NLO and NNLO) show an increase of about 4.5\% to the LO prediction with 1\% precision. As a result, a precise measurement of the neutral pion decay amplitude would be one of the most stringent tests of low energy QCD. PrimEx-II experiment measured the neutral pion decay amplitude via the Primakoff effect using two targets, silicon and $^{12}$C. The $\pi^0\rightarrow\gamma\gamma$ decay amplitude was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value is $7.82 \pm 0.05(stat) \pm 0.10(syst)$ eV. With a total uncertainty of 1.8\%, this result is the most precise experimental estimation and is consistent with current theoretical predictions.