Anomaly, mixing and transition form factors of pseudoscalar mesons

We derive the exact non-perturbative QCD sum rule for the transition form factors of η and η′ using the dispersive representation of axial anomaly. This sum rule allows to express the transition form factors entirely in terms of meson decay constants. Using this sum rule several mixing schemes were analyzed and compared to recent experimental data. A good agreement with experimental data on η,η′ transition form factors in the range from real to highly virtual photons was obtained.