Open charm effects in e+e- → J/ψη, J/ψπ0, and φηc

We propose to study the open charm effects in e(+)e(-) -> J/psi eta, J/psi pi(0) and phi eta(c). We show that the exclusive cross section line shapes of these processes would be strongly affected by the open charm effects. Since the final state light meson productions are through soft gluon radiations, we assume a recognition of this soft process via charmed meson loops at the hadronic level. A unique feature among these three reactions is that the D (D) over bar* + c.c. open channel is located in a relatively isolated energy, i. e. similar to 3: 876 GeV, which is sufficiently far away from the known charmonia psi(3770) and psi(4040). Therefore, the cross section line shapes of these reactions may provide an opportunity for singling out the open charm effects with relatively well-defined charmonium contributions. In particular, we find that the reaction e(+)e(-) -> J/psi pi(0) is sensitive to the open charm D (D) over bar* + c.c. Because of the dominance of the isospin-0 component at the charmonium energy region, we predict an enhanced model-independent cusp effect between the thresholds of D-0(D) over bar*(0) + c.c. and D+(D) over bar*(-) + c.c. This study can also help us to understand the X(3900) enhancement recently observed by the Belle Collaboration in e(+)e(-) -> D (D) over bar + c.c.