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

TL;DR

This paper predicts a narrow peak in the cross section for producing the $X(3872)$ and a photon in electron-positron annihilation, caused by a triangle singularity near the $D^{*0} ar D^{*0}$ threshold, and discusses the limitations of previous approximations.

Contribution

It introduces a detailed prediction of the cross section peak due to a triangle singularity in $e^+ e^-$ annihilation producing $X(3872)$ and a photon, highlighting the inadequacy of prior absorptive approximations.

Findings

A narrow peak in the cross section is predicted 2.2 MeV above the $D^{*0} ar D^{*0}$ threshold.

The triangle singularity significantly influences the cross section shape.

Previous absorptive calculations do not accurately describe the peak.

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 P-wave charm-meson pair into the $X$ and a photon. A triangle singularity produces a narrow peak in the cross section for $e^+ e^- \to X \gamma$ 2.2 MeV above the $D^{*0} \bar{D}^{*0}$ threshold. We predict the normalized cross section in the region of the peak. We show that the absorptive contribution to the cross section for $e^+ e^- \to D^{*0} \bar D^{*0} \to X \gamma$, which was calculated previously by Dubynskiy and Voloshin, does not give a good approximation to the peak from the triangle singularity.