Puzzles in charmonium decays

Abstract: The open charm effects via intermediate hadron loop transitions seem to play a crucial role in the understanding of several existing "puzzles" in charmonium exclusive decays, such as the $\psi(3770)$ non-$D\bar{D}$ decays, and "$\rho\pi$ puzzle" etc. In the charmonium energy region, non-perturbative mechanisms could be still sizeable, and as a consequence the intermediate hadron loop transitions also provide a mechanism for the helicity-selection-rule (HSR) violation. We report our recent progress on those existing puzzles.