Study of the $B^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}{K}_S^0$ decay

The decay $B^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}{K}_S^0$ is studied at LHCb for the first time using proton-proton collision data recorded by the LHCb experiment at a center-of-mass energy of $\sqrt{s}=13$ TeV, corresponding to an integrated luminosity of 5.4 fb${}^{-1}$. The branching ratio relative to the decay $B^{+}\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}{K}^{+}$ is measured to be $$\frac{\mathcal{B}(B^0\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}{K}_S^0)}{\mathcal{B}(B^{+}\to {\mathrm{\Lambda }}_c^{+}{\bar{\mathrm{\Lambda }}}_c^{-}{K}^{+})}=0.53\pm 0.05\pm 0.05,$$ where the first uncertainty is statistical and the second is systematic. Evidence is found for contributions from two resonant states, ${\mathrm{\Xi }}_c(2923{)}^{+}$ and ${\mathrm{\Xi }}_c(2939{)}^{+}$, in the ${\mathrm{\Lambda }}_c^{+}{K}_S^0$ system. The two states show a significance of $3.9\sigma$ relative to the nonresonant hypothesis. These two ${\mathrm{\Xi }}_c^{+}$ states are consistent with being the isospin partners of the states observed in ${\mathrm{\Lambda }}_c^{+}{K}^{-}$ system.