Observation of $e^{+}{e}^{-}\to {\pi }^0{\pi }^0{h}_c$ and a Neutral Charmoniumlike Structure $Z_c(4020{)}^0$

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 $e^{+}{e}^{-}\to {\pi }^0{\pi }^0{h}_c$ for the first time. The Born cross sections are measured and found to be about half of those of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$ within less than $2\sigma$. In the ${\pi }^0{h}_c$ mass spectrum, a structure at $4.02\mathrm{GeV}/c^2$ is found. It is most likely to be the neutral isospin partner of the $Z_c(4020{)}^{\pm }$ observed in the process of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$ being found. A fit to the ${\pi }^0{h}_c$ invariant mass spectrum, with the width of the $Z_c(4020{)}^0$ fixed to that of its charged isospin partner and possible interferences with non-$Z_c(4020{)}^0$ amplitudes neglected, gives a mass of $(4023.9\pm 2.2\pm 3.8)\mathrm{MeV}/c^2$ for the $Z_c(4020{)}^0$, where the first error is statistical and the second systematic.

Figure 1

The $M_{{\pi }^0{\pi }^0}^{\text{recoil}}$ distribution for the events with an ${\eta }_c$ candidate. The plot shows the sum over three CME points. Dots with error bars are data; the solid curve is the best fit; the dashed black line is the background; the green shaded histogram shows the normalized ${\eta }_c$ sideband events. The inset shows the scatter plot of $M_{\gamma {\pi }^0{\pi }^0}^{\text{recoil}}$ versus $M_{{\pi }^0{\pi }^0}^{\text{recoil}}$. The two red dashed lines represent the signal region of ${\eta }_c$.

Figure 2

Sum of the simultaneous fit to the $M_{{\pi }^0}^{\text{recoil}}{|}_{\max }$ distribution at $\sqrt{s}=4.23$, 4.26, and 4.36 GeV as described in the text. Dots with errors bars are data; the green shaded histogram shows the normalized $h_c$ sideband events; the black dashed curve is the background from the fit; the red histogram shows the result from a phase space MC simulation. The solid blue line shows the total fit.