Observation of $Z_c(3900{)}^0$ in $e^{+}{e}^{-}\to {\pi }^0{\pi }^{0}J/\psi$

Abstract

Using a data sample collected with the BESIII detector operating at the BEPCII storage ring, we observe a new neutral state $Z_c(3900{)}^0$ with a significance of $10.4\sigma$. The mass and width are measured to be $3894.8\pm 2.3\pm 3.2$ MeV/$c^2$ and $29.6\pm 8.2\pm 8.2$~MeV, respectively, where the first error is statistical and the second systematic. The Born cross section for $e^{+}{e}^{-}\to {\pi }^0{\pi }^{0}J/\psi$ and the fraction of it attributable to ${\pi }^0{Z}_c(3900{)}^0\to {\pi }^0{\pi }^{0}J/\psi$ in the range $E_{cm}=4.19-4.42$ GeV are also determined. We interpret this state as the neutral partner of the four-quark candidate $Z_c(3900{)}^{\pm }$.

7 pages, 4 figures, submit to Physics Review Letter