Observation of e+e- → π+π- hc and a Neutral Charmoniumlike Structure Zc(4020)0

Abstract

Using data collected with the BESIII detector operating at the Beijing Electron Positron Collider at center-of-mass energies of √s = 4.23, 4.26, and 4.36 GeV, we observe e+e- → π+π-hc for the first time. The Born cross sections are measured and found to be about half of those of e+e-→ π+π-hcwithin less than 2σ. In the π0hc mass spectrum, a structure at 4.02 GeV/c2 is found. It is most likely to be the neutral isospin partner of the Zc(4020)± observed in the process of e+e- → π+π-hc being found. A fit to the π0hc invariant mass spectrum, with the width of the Zc(4020)0 fixed to that of its charged isospin partner and possible interferences with non- Zc(4020)0 amplitudes neglected, gives a mass of (4023.9 ± 2.2 ± 3.8) MeV/c2 for the Zc(4020)0, where the first error is statistical and the second systematic.