Observation of a charged charmoniumlike structure Zc (4020) and search for the Zc(3900) in e+e- → π+π-hc

Abstract

We study e+e-→ π+π-hcat center-of-mass energies from 3.90 to 4.42 GeV by 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-→ π+π-J/ψ but with a different line shape. In the π±hcmass spectrum, a distinct structure, referred to as Zc(4020), is observed at 4.02 GeV/c2. The Zc(4020) carries an electric charge and couples to charmonium. A fit to the π±hcinvariant mass spectrum, neglecting possible interferences, results in a mass of (4022.9 ± 0.8 ± 2.7) MeV/c2and a width of (7.9 ± 2.7 ± 2.6) MeV for the Zc(4020), where the first errors are statistical and the second systematic. The difference between the parameters of this structure and the Zc(4025) observed in the D∗ D∗ final state is within 1.5σ, but whether they are the same state needs further investigation. No significant Zc(3900) signal is observed, and upper limits on the Zc(3900) production cross sections in π±hcat center-of-mass energies of 4.23 and 4.26 GeVare set. © 2013 American Physical Society.