Open charm scenarios
We discuss possibilities of identifying open charm effects in direct production processes, and propose that direct evidence for the open charm effects can be found in $e^+ e^-\to J/\psi\pi^0$. A unique feature with this process is that the $D\bar{D^*}+c.c.$ open channel is located in a relatively isolated energy, i.e. $\sim 3.876$ GeV, which is sufficiently far away from the known charmonia $\psi(3770)$ and $\psi(4040)$. Due to the dominance of the isospin-0 component at the charmonium energy region, an enhanced model-independent cusp effect between the thresholds of $D^0\bar{D^{*0}}+c.c.$ and $D^+ D^{*-}+c.c.$ can be highlighted. An energy scan over this energy region in the $e^+e^-$ annihilation reaction can help us to understand the nature of X(3900) recently observed by Belle Collaboration in $e^+ e^-\to D\bar{D}+c.c.$, and establish the open charm effects as an important non-perturbative mechanism in the charmonium energy region.