Study of $\overline{B}\to {\Xi }_c{\overline{\Lambda }}_c^{-}$ and $\overline{B}\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}\overline{K}$ decays at BABAR

We report measurements of $B$-meson decays into two- and three-body final states containing two charmed baryons using a sample of $230\times {10}^6$ ${\rm Y}(4S)\to B\overline{B}$ decays. We find significant signals in two modes, measuring branching fractions $\mathcal{B}(B^{-}\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}{K}^{-})=(1.14\pm 0.15\pm 0.17\pm 0.60)\times {10}^{-3}$ and $\mathcal{B}(B^{-}\to {\Xi }_c^0{\overline{\Lambda }}_c^{-})\times \mathcal{B}({\Xi }_c^0\to {\Xi }^{-}{\pi }^{+})=(2.08\pm 0.65\pm 0.29\pm 0.54)\times {10}^{-5}$, where the uncertainties are statistical, systematic, and from the branching fraction $\mathcal{B}({\Lambda }_c^{+}\to p{K}^{-}{\pi }^{+})$, respectively. We also set upper limits at the 90% confidence level on two other modes: $\mathcal{B}({\overline{B}}^0\to {\Xi }_c^{+}{\overline{\Lambda }}_c^{-})\times \mathcal{B}({\Xi }_c^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+})<5.6\times {10}^{-5}$ and $\mathcal{B}({\overline{B}}^0\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}{\overline{K}}^0)<1.5\times {10}^{-3}$. We observe structure centered at an invariant mass of $2.93\mathrm{GeV}/c^2$ in the ${\Lambda }_c^{+}{K}^{-}$ mass distribution of the decay $B^{-}\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}{K}^{-}$.

Figure 1

The $m_{\mathrm{ES}}$ and $\Delta E$ distributions for $B^{-}\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}{K}^{-}$ candidates, summing over five different final states. Plot (b) shows the scatterplot of $m_{\mathrm{ES}}$ vs $\Delta E$, and (a) and (c) show the $m_{\mathrm{ES}}$ and $\Delta E$ projections for $|\Delta \mathrm{E}|<0.022\mathrm{GeV}$ and for ${\mathrm{m}}_{\mathrm{ES}}>5.27\mathrm{GeV}/c^2$, respectively. The dashed horizontal and vertical lines in (b) indicate the signal regions used for the projection in (a) and (c), respectively.

Figure 2

The fitted $m_{\mathrm{ES}}$ distributions observed for the decay modes (a) $B^{-}\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}{K}^{-}$, combining 5 exclusive final states; (b) $B^{-}\to {\Xi }_c^0{\overline{\Lambda }}_c^{-}$, combining 2 exclusive final states; (c) ${\overline{B}}^0\to {\Xi }_c^{+}{\overline{\Lambda }}_c^{-}$; (d) ${\overline{B}}^0\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}{\overline{K}}^0$. Points with error bars represent the data, dashed lines the background PDF, and solid lines the sum of the signal and background PDFs.

Figure 3

Reconstructed $B^{-}\to {\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}{K}^{-}$ candidates in the signal region (${\mathrm{m}}_{\mathrm{ES}}>5.27\mathrm{GeV}/c^2$, $\Delta E<22\mathrm{MeV}$), shown as (a) the Dalitz plot, (b) the ${\Lambda }_c^{+}{K}^{-}$ invariant mass distribution, and (c) the ${\Lambda }_c^{+}{\overline{\Lambda }}_c^{-}$ invariant mass distribution. Data from the signal region are shown as black points. Signal events from a phase-space simulation are shown as small gray points in (a) and as a histogram in (b) and (c). Data from the sideband region $5.20<{\mathrm{m}}_{\mathrm{ES}}<5.26\mathrm{GeV}/c^2$ are shown as a shaded histogram in (b) and (c), normalized according to the expected background yield in the signal region. The masses of the $B$-meson candidates are not constrained.