Observation of a charged charmoniumlike structure Z_c(4020) and search for the Z_c(3900) in e+e- to pi+pi-h_c

TL;DR

This study reports the observation of a charged charmoniumlike structure Z_c(4020) in e+e- collisions, measures its properties, and searches for the Z_c(3900), providing new insights into exotic hadron states.

Contribution

First observation and detailed measurement of the Z_c(4020) structure in e+e- to pi+pi-h_c, and a search for the Z_c(3900) in the same process.

Findings

Z_c(4020) observed at 4.02 GeV with specific mass and width

No significant Z_c(3900) signal found in the studied energy range

Cross sections of e+e- --> pi+pi-h_c are comparable to those of pi+pi-J/psi

Abstract

We study e+e- --> pi+pi-h_c at center-of-mass energies from 3.90 GeV to 4.42 GeV using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross sections are measured at 13 energies, and are found to be of the same order of magnitude as those of e+e- --> pi+pi-J/psi but with a different line shape. In the \pi^\pm h_c mass spectrum, a distinct structure, referred to as Z_c(4020), is observed at 4.02 GeV/c^2. The Z_c(4020) carries an electric charge and couples to charmonium. A fit to the \pi^\pm h_c invariant mass spectrum, neglecting possible interferences, results in a mass of (4022.9\pm 0.8\pm 2.7) MeV/c^2 and a width of (7.9\pm 2.7\pm 2.6) MeV for the Z_c(4020), where the first errors are statistical and the second systematic. No significant Z_c(3900) signal is observed, and upper limits on the Z_c(3900) production cross sections in \pi^\pm h_c at center-of-mass energies of 4.23 and 4.26 GeV are set.