Observation of a Charged Charmoniumlike Structure Z_c (4020) and Search for the Z_c(3900) in e⁺e^- → π⁺π^-h_c

We study e⁺e^-→ π⁺π^-h_cat 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^±h_cmass spectrum, a distinct structure, referred to as Z_c(4020), is observed at 4.02 GeV/c₂. The Z_c(4020) carries an electric charge and couples to charmonium. A fit to the π^±h_cinvariant 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 Z_c(4020), where the first errors are statistical and the second systematic. The difference between the parameters of this structure and the Z_c(4025) observed in the D∗ D∗ final state is within 1.5σ, but whether they are the same state needs further investigation. No significant Z_c(3900) signal is observed, and upper limits on the Z_c(3900) production cross sections in π^±h_cat center-of-mass energies of 4.23 and 4.26 GeVare set.