Evidence of $\mathrm{\Upsilon }(1S)\to J/\psi +{\chi }_{c1}$ and search for double-charmonium production in $\mathrm{\Upsilon }(1S)$ and $\mathrm{\Upsilon }(2S)$ decays

Using data samples of $102\times {10}^6$ $\mathrm{\Upsilon }(1S)$ and $158\times {10}^6$ $\mathrm{\Upsilon }(2S)$ events collected with the Belle detector, a first experimental search has been made for double-charmonium production in the exclusive decays $\mathrm{\Upsilon }(1S,2S)\to J/\psi ({\psi }^{\prime })+X$, where $X={\eta }_c$, ${\chi }_{cJ}(J=0,1,2)$, ${\eta }_c(2S)$, $X(3940)$, and $X(4160)$. No significant signal is observed in the spectra of the mass recoiling against the reconstructed $J/\psi$ or ${\psi }^{\prime }$ except for the evidence of ${\chi }_{c1}$ production with a significance of $4.6\sigma$ for $\mathrm{\Upsilon }(1S)\to J/\psi +{\chi }_{c1}$. The measured branching fraction $\mathcal{B}(\mathrm{\Upsilon }(1S)\to J/\psi +{\chi }_{c1})$ is $[3.90\pm 1.21(\text{stat})\pm 0.23(\text{syst})]\times {10}^{-6}$. The 90% C.L. upper limits on the branching fractions of the other modes having a significance of less than $3\sigma$ are determined. These results are consistent with theoretical calculations using the nonrelativistic QCD factorization approach.

Figure 1

Distributions of the invariant masses of $J/\psi$ identified with lepton pairs and ${\psi }^{\prime }$ identified with both ${\ell }^{+}{\ell }^{-}$ and $J/\psi {\pi }^{+}{\pi }^{-}$ with the $J/\psi ({\psi }^{\prime })$ recoil mass within $2.2\mathrm{GeV}/c^2$ and $4.6\mathrm{GeV}/c^2$ from left to right. The upper and lower three graphs are for $\mathrm{\Upsilon }(1S)$ and $\mathrm{\Upsilon }(2S)$ decays, respectively. The arrows show the signal regions of $J/\psi$ or ${\psi }^{\prime }$ masses.

Figure 2

Distributions of the recoil masses against the reconstructed $J/\psi (\to {\ell }^{+}{\ell }^{-})$, ${\psi }^{\prime }(\to {\ell }^{+}{\ell }^{-})$, ${\psi }^{\prime }(\to J/\psi {\pi }^{+}{\pi }^{-})$ within the $J/\psi$ or ${\psi }^{\prime }$ mass signal regions from left to right. The upper and lower three graphs are for $\mathrm{\Upsilon }(1S)$ and $\mathrm{\Upsilon }(2S)$ decays, respectively. The red-shaded histograms are from the normalized continuum sample at $\sqrt{s}=10.52\mathrm{GeV}$, and the green ones represent the scaled $J/\psi ({\psi }^{\prime })$ mass sideband backgrounds from $\mathrm{\Upsilon }$ decays.

Figure 3

Fit to the recoil mass spectra against the (a) $J/\psi (\to {\ell }^{+}{\ell }^{-})$, (b) ${\psi }^{\prime }(\to {\ell }^{+}{\ell }^{-})$ and (c) ${\psi }^{\prime }(\to J/\psi {\pi }^{+}{\pi }^{-})$ in $\mathrm{\Upsilon }(1S)$ decays from data (points with error bars). The red solid curves are the nominal fits and the blue-dashed curves show the total background. The fitted normalized continuum contributions are represented by the cyan-shaded histograms.

Figure 4

Fit to the recoil mass spectra against (a) $J/\psi (\to {\ell }^{+}{\ell }^{-})$, (b) ${\psi }^{\prime }(\to {\ell }^{+}{\ell }^{-})$ and (c) ${\psi }^{\prime }(\to J/\psi {\pi }^{+}{\pi }^{-})$ in $\mathrm{\Upsilon }(2S)$ decays from data (points with error bars). The representations of the curves and histograms in each graph match those in Fig. 3.