The cross section for ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}J/\ensuremath{\psi}$ between 3.8 and $5.5\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ is measured using a $548\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ data sample collected on or near the $\ensuremath{\Upsilon}(4S)$ resonance with the Belle detector at KEKB. A peak near $4.25\text{ }\text{ }\mathrm{GeV}/{c}^{2}$, corresponding to the so called $Y(4260)$, is observed. In addition, there is another cluster of events at around $4.05\text{ }\text{ }\mathrm{GeV}/{c}^{2}$. A fit using two interfering Breit-Wigner shapes describes the data better than one that uses only the $Y(4260)$, especially for the lower-mass side of the 4.25 GeV enhancement.
Measurement of thee+e−→π+π−J/ψCross Section Via Initial-State Radiation at Belle | Litlas
