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Two-photon decays probe the structure of mesons and represent an importantcontribution to hadronic light-by-light scattering. For the neutral pion, thedecay amplitude tests the effects of the chiral anomaly; for a heavy quarkoniumstate, it measures the magnitude of its wavefunction at the origin. We rederivethe expression of the decay amplitude in terms of a Euclidean correlationfunction starting from the theory defined on the torus. The derivation showsthat for timelike photons the approach to the infinite-volume decay amplitudeis exponential in the periodic box size.

Lattice QCD and the two-photon decay of the neutral pion