The gluon content of the $\eta$ and $\eta^{\prime}$ mesons
  We analyze power-suppressed contributions to singlet pseudoscalar $\eta$ and $\eta^{\prime}$ meson transition form factors. These corrections stem from endpoint singularities and help improve the agreement between QCD theory and the experimental data, in particular, at low-momentum transfers. Using the CLEO data, we extract information on the profile of the $\eta_1$ and $\eta_8$ distribution amplitudes in the $SU(3)_{F}$ octet-singlet basis employing both the one-angle and the two-angle mixing schemes. In the former scheme, we find good agreement with the CLEO data, while in the second case, our approach requires non-asymptotic profiles for these mesons. 
N. G. Stefanis, S. S. Agaev