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Neutral Meson Decays into Two Photons from Lattice QCD

A precision determination of the neutral-pion width would improve determinations of the splitting between the up- and down-quark masses, and matrix elements for the decay of neutral mesons into two photons could play a role in the attempt to probe beyond-the-Standard Model physics in muon $g-2$ experiments. The theoretical error is dominated by hadronic light-by-light diagrams, and since direct measurements are extremely difficult, model calculations factorize it into two-photon diagrams connected by the lightest hadrons. We employ perturbative techniques to express the photon as a superposition of QCD eigenstates accessible in lattice-QCD calculations and found that vector-meson dominance is a poor description of the two-photon decay process when both photons are off shell.