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

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 $3894.8\pm 2.3\pm 3.2\mathrm{MeV}/c^2$ and $29.6\pm 8.2\pm 8.2\mathrm{MeV}$, 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 $E_{\mathrm{c}.\mathrm{m}.}=4.19–4.42\mathrm{GeV}$ are also determined. We interpret this state as the neutral partner of the four-quark candidate $Z_c(3900{)}^{\pm }$.

Figure 1

Invariant mass distribution for ${\pi }^{0}J/\psi$ candidates in $E_{\mathrm{c}.\mathrm{m}.}=4.260\mathrm{GeV}$ data (points). The dashed histogram shows the MC background and the solid histogram is the sum of this background and ${\pi }^0{\pi }^{0}J/\psi$ production not from $Z_c^0$.

Figure 2

The simultaneously fitted ${\pi }^{0}J/\psi$ mass spectra (55 bins in $M_{{\pi }^{0}J/\psi }$) for (a) $E_{\mathrm{c}.\mathrm{m}.}=4.230$, (b) $E_{\mathrm{c}.\mathrm{m}.}=4.260$, and (c) $E_{\mathrm{c}.\mathrm{m}.}=4.360\mathrm{GeV}$. Dots represent the data, solid lines represent the fitted results, and dashed lines represent fitted backgrounds.

Figure 3

(a) $R$ (see text) and (b) ${\sigma }_{\text{Born}}(e^{+}{e}^{-}\to {\pi }^0{\pi }^{0}J/\psi )$ as functions of $E_{\mathrm{c}.\mathrm{m}.}$. Error bars are statistical only.