Observation of $e^+e^-\to \pi^0\pi^0 h_c$ and a neutral charmoniumlike structure $Z_c(4020)^0$

Abstract

Using data collected with the BESIII detector operating at the Beijing Electron Positron Collider at center-of-mass energies of $\sqrt{s}=4.23$, 4.26, and 4.36~GeV, we observe \EE\to \pphc for the first time. The Born cross sections are measured and found to be about half of those of \EE\to \pi^+\pi^-h_c within less than 2$\sigma$. In the $\pi^0h_c$ mass spectrum, a structure at 4.02~GeV/$c^2$ is found. It is most likely to be the neutral isospin partner of the \zcp^{\pm} observed in the process of \EE\to \pi^+\pi^-h_c is found. A fit to the $\pi^0 h_c$ invariant mass spectrum, with the width of the \zcpn fixed to that of its charged isospin partner and possible interferences with non-\zcpn amplitudes neglected, gives a mass of ($4023.9\pm 2.2 \pm 3.8$)~MeV/$c^2$ for the \zcpn, where the first error is statistical and the second systematic.Comment: 7 pages, 2 figure