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

## TL;DR

This paper predicts a narrow peak in the cross section of $e^+ e^- o X(3872) ext{ and } ext{photon}$ caused by a triangle singularity, which could be observed experimentally, providing insight into the nature of $X(3872)$ as a charm-meson molecule.

## Contribution

The paper introduces a novel prediction of a triangle singularity effect in $e^+ e^-$ annihilation producing $X(3872)$ and a photon, with specific experimental signatures.

## Key findings

- A narrow peak in the cross section near 2.2 MeV above the $D^{*0} ar D^{*0}$ threshold.
- The peak is caused by a triangle singularity in the production mechanism.
- The predicted peak is potentially observable by the BESIII detector.

## Abstract

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.