Observation of $Z_c(3900)^{0}$ in $e^+e^-\toπ^0π^0 J/ψ$

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 $10.4σ$. The mass and width are measured to be $3894.8\pm2.3\pm3.2$ MeV/$c^2$ and $29.6\pm8.2\pm8.2$~MeV, respectively, where the first error is statistical and the second systematic. The Born cross section for $e^+e^-\toπ^0π^0 J/ψ$ and the fraction of it attributable to $π^0 Z_c(3900)^{0}\toπ^0π^0 J/ψ$ 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 $Z_c(3900)^\pm$.

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