Observation of e⁺e⁻ → π⁰π⁰h_c and a Neutral Charmoniumlike Structure Z_c(4020)°

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⁻ → π⁰π⁰h_c for the first time. The Born cross sections are measured and found to be about half of those of e⁺e⁻ → π⁺π⁻h_c within less than 2σ In the π⁰h_c mass spectrum, a structure at 4.02 GeV/c² is found. It is most likely to be the neutral isospin partner of the Z_c(4020)^± observed in the process of e⁺e⁻ → π⁺π⁻h_c being found. A fit to the π°h_c invariant mass spectrum, with the width of the Z_c(4020)° fixed to that of its charged isospin partner and possible interferences with non- Z_c(4020)° amplitudes neglected, gives a mass of (4023.9 ± 2.2 ± 3.8) MeV/c² for the Z_c(4020)°, where the first error is statistical and the second systematic.