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.

Figure 1

(a) The recoil mass distribution for the $J/\psi$ signal region (points) and scaled sidebands (hatched histogram). The inset shows the MC reconstruction efficiency for $e^{+}{e}^{-}\to J/\psi q\overline{q}$ [$q=u,d,s$ (open circles)] and $e^{+}{e}^{-}\to J/\psi c\overline{c}$ events (closed circles). (b) The recoil mass distribution after refitting the $J/\psi$ candidate with a mass constraint for the $J/\psi$ signal region (points) and scaled sidebands (hatched histogram). The curve represents the fit described in the text.

Figure 2

Results of a search for associated production of $J/\psi$ and charm mesons: (a) the scatter plot $M(l^{+}{l}^{-})$ vs $M(D^0{\pi }^{+})$; (b) projection onto the $M(D^0{\pi }^{+})$ axis; (c) the scatter plot $M(l^{+}{l}^{-})$ vs $M[K^{-}{\pi }^{+}(K^{+}{K}^{-})]$; (d) projection onto the $M[K^{-}{\pi }^{+}(K^{+}{K}^{-})]$ axis. Points with error bars show the $J/\psi$ signal region and the hatched histograms show the scaled sidebands. The boxes in the scatter plots represent the signal regions defined as $|M_{{\ell }^{+}{\ell }^{-}}-M_{J/\psi }|<30\mathrm{M}\mathrm{e}\mathrm{V}/c^2$, $|M_{D^0{\pi }^{+}}-M_{D^{*+}}|<2\mathrm{M}\mathrm{e}\mathrm{V}/c^2$, $|M_{K^{-}{\pi }^{+}(K^{+}{K}^{-})}-M_{D^0}|<10\mathrm{M}\mathrm{e}\mathrm{V}/c^2$. The curves represent the fit described in the text.