Study of e^+ e^- \rightarrow\pi^+ \pi^- J/\psi and observation of a charged charmoniumlike state at Belle
Z. Q. Liu et al.
Accepted
The cross section for $e^+ e^- \to \pi^+ \pi^- J/\psi$ between 3.8~GeV and 5.5~GeV is measured with a 967~fb$^{-1}$ data sample collected by the Belle detector at or near the $\Upsilon(nS)$ ($n = 1,2,...,5$) 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 parameterization 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 $M(\pi^\pm\jpsi)$ mass spectrum with $5.2\sigma$ significance, with mass $M=(3894.5\pm 6.6\pm 4.5)~{\rm MeV}/c^2$ and width $\Gamma=(63\pm 24\pm 26)$~MeV/$c^{2}$, where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmonium-like state.