Chiral-symmetry-breaking corrections in two-photon decays of pseudoscalar mesons

Chiral-symmetry-breaking corrections of order $m_P^2$/$m_V^2$ to the main effect which arises from the axial-vector anomaly in ${\pi }^0$, ${\eta }^0$, and η${\mathcal{’}}^0$→2γ decays are estimated by using gauge invariance and vector-meson dominance, which holds at about the 25% level. The inclusion of these corrections requires a large η-η’ mixing angle≊-(25±4)° and substantial violation of nonet symmetry for $F_8$ and $F_0$, the axial-vector decay constants for ${\eta }_8$ and ${\eta }_0$.