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Flavor Symmtry Breaking andd Mixing Effects in the eta-gamma and eta'-gamma Transition Form Factors

We reanalyze the eta-gamma and eta'-gamma transition form factors within the modified hard scattering approach on the basis of new experimental data from CLEO and L3. Our approach perfectly describes the experimental data over a wide range of the virtuality of the probing photon, 1 GeV^2 < Q^2 < 15 GeV^2. The analysis provides hints that the conventional flavor octet-singlet scheme for the eta-eta' mixing is too simple. A more general mixing scheme on the other hand, involving two mixing angles, leads to a very good description of the transition form factors and also accounts for the two-photon decay widths of the eta and eta' mesons as well as for the ratio of the widths for the J/Psi --> eta' gamma and J/Psi --> eta gamma decays. We also investigate the questions of possible deviations of the eta and eta' distribution amplitudes from the asymptotic form and of eventual intrinsic charm in the eta and eta' mesons. We estimate the charm decay constant of the eta' meson to lie within the range -65 MeV < fc(eta') <15 MeV.