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

TL;DR

This paper reports the observation of a new neutral $Z_c(3900)^0$ state in $e^+e^- o\,pi^0\,pi^0 J/\,psi$ with high significance, measuring its properties and suggesting it as the neutral partner of a four-quark candidate.

Contribution

First observation of the neutral $Z_c(3900)^0$ state in $e^+e^-$ collisions, providing measurements of its mass, width, and production cross section.

Findings

$Z_c(3900)^0$ observed with 10.4 sigma significance.

Mass of $3894.8\,MeV/c^2$ and width of $29.6\,MeV$ measured.

Cross section and fraction attributable to $Z_c(3900)^0$ determined.

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 $3894.8\pm2.3\pm3.2$ 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 $e^+e^-\to\pi^0\pi^0 J/\psi$ 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 $Z_c(3900)^\pm$.