New puzzle in charmonium decays

By analyzing existing data on pseudoscalar charmonium decays, we obtain the ratio of the branching fractions of $η_c(2S)$ and $η_c$ decays into ten different final states with light hadrons. For the first time, we test the two existing theoretical predictions of these decays, and find that the experimental data are significantly different from both of them. The lack of observation of any decay mode with higher rate in $η_c(2S)$ than in $η_c$ decays suggests very unusual decay dynamics in pseudoscalar charmonium decays to be identified. We also report the first model-independent evaluation of the partial width of $η_c(2S)\to γγ$ ($2.21_{-0.64}^{+0.88}$ keV) and improved determination of that of $η_c\to γγ$ ($5.43_{-0.38}^{+0.41}$ keV). The latter shows a tension with the most recent lattice QCD calculation.

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