Chiral Anomaly and Eta-Eta' Mixing

We determine the $\eta -{\eta }^{\mathrm{\prime }}$ mixing angle via a procedure relatively independent of theoretical assumptions by simultaneously fitting $\eta -et{a}^{\mathrm{\prime }}$ reactions involving the anomaly–$\eta ,{\eta }^{\mathrm{\prime }}\to \gamma \gamma ,{\pi }^{+}{\pi }^{-}\gamma$. We extract reasonably precise renormalized values of the octet and singlet pseudoscalar decay constants $F_8,F_0$ as well as the mixing angle $\theta$.