Study of $e^{\mathbf{+}}{e}^{\mathbf{-}}\to {\pi }^{\mathbf{+}}{\pi }^{\mathbf{-}}J/\psi$ and Observation of a Charged Charmoniumlike State at Belle

The cross section for $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ between 3.8 and 5.5 GeV is measured with a $967{\mathrm{fb}}^{-1}$ data sample collected by the Belle detector at or near the ${\rm Y}(nS)$ ($n=1,2,\dots ,5$) resonances. The $Y(4260)$ state is observed, and its resonance parameters are determined. In addition, an excess of ${\pi }^{+}{\pi }^{-}J/\psi$ production around 4 GeV is observed. This feature can be described by a Breit-Wigner parametrization with properties that are consistent with the $Y(4008)$ state that was previously reported by Belle. In a study of $Y(4260)\to {\pi }^{+}{\pi }^{-}J/\psi$ decays, a structure is observed in the $M({\pi }^{\pm }J/\psi )$ mass spectrum with $5.2\sigma$ significance, with mass $M=(3894.5\pm 6.6\pm 4.5)\mathrm{MeV}/c^2$ and width $\Gamma =(63\pm 24\pm 26)\mathrm{MeV}/c^2$, where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmoniumlike state.

Figure 1

(a) Invariant mass distributions of ${\pi }^{+}{\pi }^{-}{\ell }^{+}{\ell }^{-}$. Points with error bars are data, and the shaded histograms are the normalized $J/\psi$ mass sidebands. The solid curves show the total best fit with two coherent resonances and contribution from background. The dashed curves are for solution I, while the dotted-dashed curves are for solution II. The inset shows the distributions on a logarithmic vertical scale. The large peak around $3.686\mathrm{GeV}/c^2$ is the $\psi (2S)\to {\pi }^{+}{\pi }^{-}J/\psi$ signal. (b) Cross section of $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$ after background subtraction. The errors are statistical only.

Figure 2

Dalitz plot for $Y(4260)\to {\pi }^{+}{\pi }^{-}J/\psi$ decays for $4.15\mathrm{GeV}/c^2<M({\pi }^{+}{\pi }^{-}J/\psi )<4.45\mathrm{GeV}/c^2$. The inset shows background events from the $J/\psi$-mass sidebands (not normalized).

Figure 3

Invariant mass distributions of (a) ${\pi }^{+}{\pi }^{-}$, (b) ${\pi }^{+}J/\psi$, and (c) ${\pi }^{-}J/\psi$ for events in the $Y(4260)$ signal region. Points with error bars represent data, shaded histograms are normalized background estimates from the $J/\psi$-mass sidebands, solid histograms represent MC simulations of ${\pi }^{+}{\pi }^{-}$ amplitudes [22] (normalized $J/\psi$-mass sideband events added) and dashed histograms are MC simulation results for a $Z(3900{)}^{\pm }$ signal.

Figure 4

Unbinned maximum likelihood fit to the distribution of the $M_{\max }(\pi J/\psi )$. Points with error bars are data, the curves are the best fit, the dashed histogram is the phase space (PHSP) distribution and the shaded histogram is the non-${\pi }^{+}{\pi }^{-}J/\psi$ background estimated from the normalized $J/\psi$ sidebands.