Evidence for the baryonic decay ${\overline{B}}^0\to D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$

Evidence is presented for the baryonic $B$ meson decay ${\overline{B}}^0\to D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ based on a data sample of $471\times {10}^6$ $B\overline{B}$ pairs collected with the BABAR detector at the PEP-II asymmetric $e^{+}{e}^{-}$ collider located at the SLAC National Accelerator Laboratory. The branching fraction is determined to be $\mathcal{B}({\overline{B}}^0\to D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }})=({9.8}_{-2.6}^{+2.9}\pm 1.9)\times {10}^{-6}$, corresponding to a significance of 3.4 standard deviations including additive systematic uncertainties. A search for the related baryonic $B$ meson decay ${\overline{B}}^0\to D^0{\mathrm{\Sigma }}^0\overline{\mathrm{\Lambda }}$ with ${\mathrm{\Sigma }}^0\to \mathrm{\Lambda }\gamma$ is performed and an upper limit $\mathcal{B}({\overline{B}}^0\to D^0{\mathrm{\Sigma }}^0\overline{\mathrm{\Lambda }}+{\overline{B}}^0\to D^0\mathrm{\Lambda }{\overline{\mathrm{\Sigma }}}^0)<3.1\times {10}^{-5}$ is determined at 90% confidence level.

Figure 1

Leading-order Feynman diagrams for the decays ${\overline{B}}^0\to D^0\mathbf{N}\overline{\mathbf{N}}$. Setting $q=u$ leads to the $D^{0}p\overline{p}$ final state and setting $q=s$ to the $D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$, $D^0{\mathrm{\Sigma }}^0\overline{\mathrm{\Lambda }}$, $D^0\mathrm{\Lambda }{\overline{\mathrm{\Sigma }}}^0$, and $D^0{\mathrm{\Sigma }}^0{\overline{\mathrm{\Sigma }}}^0$ final states.

Figure 2

Dalitz plot for simulated $D^0\to K^{-}{\pi }^{+}{\pi }^0$ events before (gray stars) and after (black crosses) the $w_{\text{Dalitz}}>0.02$ requirement. Resonant decays are indicated.

Figure 3

Distributions for ${\overline{B}}^0\to D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ (left) and ${\overline{B}}^0\to D^0{\mathrm{\Sigma }}^0\overline{\mathrm{\Lambda }}$ reconstructed as ${\overline{B}}^0\to D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ (right) for the $D^0\to K^{-}{\pi }^{+}$ mode in simulated events.

Figure 4

Results of the combined fit. The $m_{\mathrm{ES}}$ projection is shown for $m(D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }})\in [5.15,5.31]\mathrm{GeV}/{\mathrm{c}}^2$ and the $m(D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }})$ projection for $m_{\mathrm{ES}}\in [5.272,5.286]\mathrm{GeV}/{\mathrm{c}}^2$. The solid line shows the result of the fit, the dashed curve indicates the ${\overline{B}}^0\to D^0{\mathrm{\Sigma }}^0\overline{\mathrm{\Lambda }}$ contribution, and the shaded histogram the combinatorial background. From top to bottom: $D^0\to K^{-}{\pi }^{+}$, $D^0\to K^{-}{\pi }^{+}{\pi }^0$, and $D^0\to K^{-}{\pi }^{+}{\pi }^{+}{\pi }^{-}$ subsamples.

Figure 5

Distribution of the invariant baryon-antibaryon mass for $D^0$-branching-fraction and efficiency-corrected ${\overline{B}}^0\to D^0\mathrm{\Lambda }\overline{\mathrm{\Lambda }}$ signal candidates. The data points represent the BABAR data and the shaded histogram indicates phase-space-distributed simulated events, scaled to match the area under the data.