The eta (eta') 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 (NLO) 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 quark-antiquark 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. 
Peter Kroll and Kornelija Passek-Kumericki
hep-ph