# Measurement of $e^{+}e^{-}\rightarrow \pi^{+}\pi^{-}\psi(3686)$ from 4.008 to 4.600$\sim$GeV and observation of a charged structure in the $\pi^{\pm}\psi(3686)$ mass spectrum

## TL;DR

This paper measures the cross section of the process $e^{+}e^{-}
ightarrow \pi^{+}\pi^{-}\psi(3686)$ across energies from 4.008 to 4.600 GeV, discovering two resonances and a charged structure, with improved precision over previous results.

## Contribution

First observation of a lower-mass resonance in $e^{+}e^{-}
ightarrow \\pi^{+}\pi^{-}\\psi(3686)$ with high significance, and identification of a charged charmonium-like structure in the mass spectrum.

## Key findings

- Two resonances at 4209.5 MeV and 4383.8 MeV with widths around 80 MeV.
- First observation of the lower-mass resonance with 5.8 sigma significance.
- Charged structure observed in the \\pi^{\\pm}\\psi(3686) spectrum at 4.416 GeV.

## Abstract

We study the process $e^{+}e^{-} \rightarrow \pi^{+}\pi^{-}\psi(3686)$ using 5.1 $fb^{-1}$ of data collected at 16 center-of-mass energy ($\sqrt{s}$) points from 4.008 to 4.600$\sim$GeV by the BESIII detector operating at the BEPCII collider. The measured Born cross sections for $e^{+}e^{-}\rightarrow \pi^{+}\pi^{-}\psi(3686)$ are consistent with previous results, but with much improved precision. A fit to the cross section shows contributions from two structures: the first has $M=4209.5\pm7.4\pm1.4$$\sim$MeV/$c^{2}$ and $\Gamma=80.1\pm24.6\pm2.9$$\sim$MeV, and the second has $M=4383.8\pm4.2\pm0.8$$\sim$MeV/c$^{2}$ and $\Gamma=84.2\pm12.5\pm2.1$$\sim$MeV, where the first errors are statistical and the second systematic. The lower-mass resonance is observed in the process $e^{+}e^{-}\rightarrow \pi^{+}\pi^{-}\psi(3686)$ for the first time with a statistical significance of $5.8\sigma$. A charged charmonium-like structure is observed in the $\pi^{\pm}\psi(3686)$ invariant mass spectrum for data at $\sqrt{s} = 4.416~$GeV. A fit with an $S$-wave Breit-Wigner function yields a mass $M=4032.1\pm2.4$~$MeV$^{2}$, where the errors are statistical only. However, there are still unresolved discrepancies between the fit model and data. The width of the intermediate state varies in a wide range for different kinematic regions within the data set. Therefore no simple interpretation of the data has been found, and a future data sample with larger statistics and more theoretical input will be required to better understand this issue.