Electromagnetic Transition Form Factor of Pseudoscalar Meson and   $\eta-\eta'$ Mixing
  The electromagnetic transition form factors of $\eta$ and $\eta'$ are calculated in the light-cone perturbation theory. We show that it is unreliable to determine the \eta-\etap'$ mixing angle without any additional normalization conditions other than their decay widthes to two photons. The possible intrinsic $c\bar{c}$ component in the flavor singlet is investigated. The heavy quark pair has distinct properties from the light ones in electromagnetic transition processes of pseudoscalar mesons. It is possible to explore the size of $c\bar{c}$ component and our numerical results disfavor a large portion of $c\bar{c}$ component. 
Jun Cao, Fu-Guang Cao, Tao Huang, and Bo-Qiang Ma