The η(η') gamma transition form factor and the gluon-gluon distribution amplitude

The eta gamma and eta'gamma transition form factors are analyzed to leading-twist accuracy and next-to-leading order of perturbative QCD. Using an eta-eta' mixing scheme and all currently available experimental data the lowest Gegenbauer coefficients of the distribution amplitudes for the valence octet and singlet q (q) over bar and the gluon-gluon Fock components are extracted. Predictions for the g*g*eta' vertex function are presented. We also comment on the new BELLE results for the pi gamma transition form factor.