Charmonium Decays: J/psi to e+e- and etac to Gamma Gamma

The authors compute the Ο(αs2) correction to the decay rate ηc → γγ and discuss its implications for precision quarkonium physics. They study the suitability of the ratio Γ(J/ψ → e+e-)/Γ(ηc → γγ), in which the non-perturbative or soft effects cancel at Ο(αs0,1), for extracting fundamental parameters of QCD at low energies. They show that the QCD-based theory of charmonia is not capable of predicting this ratio with any degree of confidence