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

With the recent measurements on ηc and ηc' at CLEO, Babar and Belle, and with the prospect of finding the ηb 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 ηc and ηb 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 ηc agrees well with experiment, but the predicted value for ηc(2S) is twice larger than the CLEO estimation. We point out that the essentially model-independent ratio of ηb two-photon width to the Υ leptonic width and the ηb two-photon width could be used to extract the strong coupling constant αs