Observation of the $\psi(1^3D_2)$ state in $e^+e^-\to\pi^+\pi^-\gamma\chi_{c1}$ at BESIII

TL;DR

This paper reports the first observation of the $X(3823)$ state in electron-positron collisions, confirming its identity as the $ ext{psi}(1^3D_2)$ charmonium state through mass, width, and production measurements.

Contribution

The study provides the first experimental evidence of the $X(3823)$ in $e^+e^-$ collisions, establishing its properties and confirming its charmonium state assignment.

Findings

Mass of $X(3823)$ measured as 3821.7 MeV/$c^2$

Statistical significance of 6.2 sigma for the observation

Width of $X(3823)$ is less than 16 MeV at 90\

Abstract

We report the observation of the $X(3823)$ in the process $e^+e^-\to \pi^+\pi^-X(3823) \to \pi^+\pi^-\gamma\chi_{c1}$ with a statistical significance of $6.2\sigma$, in data samples at center-of-mass energies $\sqrt{s}=$4.230, 4.260, 4.360, 4.420 and 4.600~GeV collected with the BESIII detector at the BEPCII electron positron collider. The measured mass of the $X(3823)$ is $(3821.7\pm 1.3\pm 0.7)$~MeV/$c^2$, where the first error is statistical and the second systematic, and the width is less than $16$~MeV at the 90\% confidence level. The products of the Born cross sections for $e^+e^-\to \pi^+\pi^-X(3823)$ and the branching ratio $\mathcal{B}[X(3823)\to \gamma\chi_{c1,c2}]$ are also measured. These measurements are in good agreement with the assignment of the $X(3823)$ as the $\psi(1^3D_2)$ charmonium state.