Evidence of Two Resonant Structures in $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$

The cross sections of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$ at center-of-mass energies from 3.896 to 4.600 GeV are measured using data samples collected with the BESIII detector operating at the Beijing Electron Positron Collider. The cross sections are found to be of the same order of magnitude as those of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ and $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}\psi (2S)$, but the line shape is inconsistent with the $Y$ states observed in the latter two modes. Two structures are observed in the $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$ cross sections around 4.22 and $4.39\mathrm{GeV}/c^2$, which we call $Y(4220)$ and $Y(4390)$, respectively. A fit with a coherent sum of two Breit-Wigner functions results in a mass of $(4218.4_{-4.5}^{+5.5}\pm 0.9)\mathrm{MeV}/c^2$ and a width of $(66.0_{-8.3}^{+12.3}\pm 0.4)\mathrm{MeV}$ for the $Y(4220)$, and a mass of $(4391.5_{-6.8}^{+6.3}\pm 1.0)\mathrm{MeV}/c^2$ and a width of $(139.5_{-20.6}^{+16.2}\pm 0.6)\mathrm{MeV}$ for the $Y(4390)$, where the first uncertainties are statistical and the second ones systematic. The statistical significance of $Y(4220)$ and $Y(4390)$ is $10\sigma$ over one structure assumption.

Figure 1

The $M_{\gamma {\eta }_c}$ distribution in the ${\eta }_c$ signal region of 4.416 GeV data. Points with error bars are the data and the curves are the best fit described in the text. The inset is the scatter plot of the mass of the ${\eta }_c$ candidate $M_{{\eta }_c}$ vs the mass of the $h_c$ candidate $M_{\gamma {\eta }_c}$ for the same data sample.

Figure 2

Fit to the dressed cross section of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}{h}_c$ with the coherent sum of two Breit-Wigner functions (solid curve). The dash (dash-dot) curve shows the contribution from the two structures $Y(4220)$ [$Y(4390)$]. The dots with error bars are the cross sections for the $R$-scan data sample, the squares with error bars are the cross sections for the $XYZ$ data sample. Here the error bars are statistical uncertainty only.

Figure 3

The likelihood contours in the mass and width planes for $Y(4220)$ (left panel) and $Y(4390)$ (right panel). The filled areas are up to $3\sigma$ likelihood contours and the dots with error bars are the locations of $Y$ or $\psi$ states. The parameters of $Y(4260{)}^{\mathrm{PDG}}$ are taken from the PDG average [3] and $Y(4260{)}^{\mathrm{BESIII}}$ from the measurement of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ at BESIII [35].