Two-photon decay of heavy quarkonium from heavy-quark spin symmetry

With the recent measurements on η_c and ηc' at CLEO, Babar and Belle, and with the prospect of finding the η_b at the Tevatron, it seems appropriate to have another look at the two-photon decay of heavy quarkonium from the standpoint of an effective Lagrangian based on local operator expansion and heavy-quark spin symmetry. In this talk, I would like to discuss a recent work on the two-photon decay rates of ground states and excited states of η_c and η_b using the local operator expansion approach and heavy-quark spin symmetry and taking into account the binding-energy. We find that the predicted two-photon width for η_c agrees well with experiment, but the predicted value for η_c(2S) is twice larger than the CLEO estimation. We point out that the essentially model-independent ratio of η_b two-photon width to the Υ leptonic width and the η_b two-photon width could be used to extract the strong coupling constant α_s.