Observation of Double $c\overline{c}$ Production in $e^{+}{e}^{-}$ Annihilation at $\sqrt{s}\approx 10.6\mathrm{G}\mathrm{e}\mathrm{V}$

We report the observation of prompt $J/\psi$ via double $c\overline{c}$ production from the $e^{+}{e}^{-}$ continuum. In this process one $c\overline{c}$ pair fragments into a $J/\psi$ meson while the remaining pair either produces a charmonium state or fragments into open charm. Both cases have been experimentally observed. We find cross sections of $\sigma [e^{+}{e}^{-}\to J/\psi {\eta }_c(\gamma )]\times \mathcal{B}({\eta }_c\to \ge 4\mathrm{\text{charged}})=({0.033}_{-0.006}^{+0.007}\pm 0.009)\mathrm{p}\mathrm{b}$ and $\sigma (e^{+}{e}^{-}\to J/\psi D^{*+}X)=({0.53}_{-0.15}^{+0.19}\pm 0.14)\mathrm{p}\mathrm{b}$ and infer $\sigma (e^{+}{e}^{-}\to J/\psi c\overline{c})/\sigma (e^{+}{e}^{-}\to J/\psi X)={0.59}_{-0.13}^{+0.15}\pm 0.12$. These results are obtained from a $46.2{\mathrm{f}\mathrm{b}}^{-1}$ data sample collected near the ${\rm Y}(4S)$ resonance, with the Belle detector at the KEKB collider.