Axial anomaly and mixing: from real to highly virtual photons
  The relation for transition form factors of eta and eta' mesons is obtained by combining the exact nonperturbative QCD sum rule, following from the dispersive representation of axial anomaly, and quark-hadron duality. It is valid at all virtual photon momenta and allows one to express the transition form factors entirely in terms of meson decay constants. This relation is in a good agreement with experimental data. 
Yaroslav N. Klopot, Armen G. Oganesian and Oleg V. Teryaev
hep-ph