Triangle Singularity in the Production of $X(3872)$ and a Photon in $e^+e^-$ Annihilation

Understand

If the $X(3872)$ is a weakly bound charm-meson molecule, it can be produced in $e^+ e^-$ annihilation by the creation of $D^{*0} \bar D^{*0}$ from a virtual photon followed by the rescattering of the charm-meson pair into $X$ and a photon.

• A triangle singularity produces a narrow peak in the cross section for $e^+ e^- \to X \gamma$ about 2.2 MeV above the $D^{*0} \bar{D}^{*0}$ threshold.
• We predict the normalized cross section in the region near the peak.
• The peak from the triangle singularity may be observable by the BESIII detector.