Two-photon width of eta_c and eta'_c from Heavy-Quark Spin Symmetry
  We evaluate the two-photon width of the pseudoscalar charmonia, Gamma(eta_c -> gamma gamma) and Gamma(eta'_c -> gamma gamma), within a Heavy-Quark Spin-Symmetry setting and show that whereas the former agrees with experiment, the latter is more than twice larger than the recent measurement by CLEO. When binding-energy effects are included in the eta'_c case, the discrepancy is worse, pointing out at a possible anomaly in the eta'_c decay. 
J.P. Lansberg and T.N. Pham