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 0 into two photons (), leading to its unusually short lifetime. We precisely measured the decay width of the process. The differential cross sections for 0 photoproduction at forward angles were measured on two targets, carbon-12 and silicon-28, yielding , 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 . Our final result has a total uncertainty of 1.50% and confirms the prediction based on the chiral anomaly in quantum chromodynamics.