The $Z_c(3900)$ peak does not come from the "triangle singularity"

Understand

We compare contributions from the triangle diagram and the $D\bar D^*$ bubble chain to the processes of $e^{+}e^{-}\rightarrow J/\psi\pi^{+}\pi^{-}$ and $e^{+}e^{-}\rightarrow (D\bar D^*)^\mp\pi^{\pm}$.

• By fitting the $J/\psi\pi$ maximum spectrum and the $D\bar D^*$ spectrum, we find that the triangle diagram cannot explain the new data from BESIII Collaboration at center of mass at 4.23GeV and 4.26GeV, simultaneously.
• On the contrary, the molecular assignment of $Z_c(3900)$ gives a much better description.