The decay pi0 to gamma gamma to next to leading order in Chiral   Perturbation Theory
  The two photon decay width of the neutral pion is analyzed within the combined framework of Chiral Perturbation Theory and the 1/Nc expansion up to order p^6 and p^4 times 1/Nc in the decay amplitude. The eta' is explicitly included in the analysis. It is found that the decay width is enhanced by about 4.5% due to the isospin-breaking induced mixing of the pure U(3) states. This effect, which is of leading order in the low energy expansion, is shown to persist nearly unchanged at next to leading order. The chief prediction for the width with its estimated uncertainty is 8.10+-0.08 eV. This prediction at the 1% level makes the upcomming precision measurement of the decay width even more urgent.   Observations on the eta and eta' can also be made, especially about their mixing, which is shown to be significantly affected by next to leading order corrections. 
J.L. Goity, A.M. Bernstein, and B.R. Holstein