Observation of resonance structures in $e^+e^-\to \pi^+\pi^-\psi_2(3823)$ and mass measurement of $\psi_2(3823)$

TL;DR

This paper reports the first observation of resonance structures in the process $e^+e^- o \pi^+\pi^-\psi_2(3823)$, measures the mass of $\psi_2(3823)$ with high precision, and explores possible underlying resonances.

Contribution

It provides the first observation of resonance structures in this process and offers detailed measurements of their masses and widths, advancing understanding of vector charmoniumlike states.

Findings

Resonance structures observed with >5σ significance.

Two coherent Breit-Wigner resonances fit the data.

Most precise measurement of $\psi_2(3823)$ mass.

Abstract

Using a data sample corresponding to an integrated luminosity of 11.3 $\rm fb^{-1}$ collected at center-of-mass energies from $4.23$ to $4.70$ GeV with the BESIII detector, we measure the product of the $e^+e^-\to \pi^+\pi^-\psi_2(3823)$ cross section and the branching fraction $\mathcal{B}[\psi_2(3823)\to \gamma\chi_{c1}]$. For the first time, resonance structure is observed in the cross section line shape of $e^+e^-\to \pi^+\pi^-\psi_2(3823)$ with significances exceeding $5\sigma$. A fit to data with two coherent Breit-Wigner resonances modeling the $\sqrt{s}$-dependent cross section yields $M(R_1)=4406.9\pm 17.2\pm 4.5$ MeV/$c^2$, $\Gamma(R_1)=128.1\pm 37.2\pm 2.3$ MeV, and $M(R_2)=4647.9\pm 8.6\pm 0.8$ MeV/$c^2$, $\Gamma(R_2)=33.1\pm 18.6\pm 4.1$ MeV. Though weakly disfavored by the data, a single resonance with $M(R)=4417.5\pm26.2\pm3.5$ MeV/$c^2$, $\Gamma(R)=245\pm48\pm13$ MeV is also possible to interpret data. This observation deepens our understanding of the nature of the vector charmoniumlike states. The mass of the $\psi_2(3823)$ state is measured as $(3823.12\pm 0.43\pm 0.13)$ MeV/$c^2$, which is the most precise measurement to date.