Observation of $Z_c(3900)$ both by BESIII and Belle

Using $525$ pb$^{-1}$ data collected with the BESIII detector at $e^+e^-$ central-of-mass energy 4.26 GeV, the BESIII Collaboration observed a charged charmoniumlike state $Z_c(3900)\toπ^{\pm}J/ψ$ with mass $M[Z_c(3900)]=3899.0\pm 3.6 \pm 4.9$ MeV/c$^2$, and width $Γ[Z_c(3900)]=46\pm 10 \pm 20$ MeV. The significance of this state was estimated to be $>8σ$ in all kinds of systematic tests. Using $967$ fb$^{-1}$ data recorded by the Belle detector on or near $Υ(nS)$, n=1, 2, $\cdots$, 5 resonances, the $e^+e^- \to π^+π^-J/ψ$ process has been studied through initial-state-radiation (ISR) method. Except for the $Y(4260)$ resonance, an intermediate state $Z(3900)^{\pm}$ has been observed in the $π^{\pm}J/ψ$ mass spectrum, with significance $>5.2σ$. The measured mass $M[Z(3900)^{\pm}]=3894.5\pm6.6\pm4.5$ MeV/c$^2$ and width $Γ[Z(3900)^{\pm}]=63\pm24\pm26$ MeV, shows good agreement with the BESIII measurement within errors, which means $Z_c(3900)$ and $Z(3900)^{\pm}$ is the same state.

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