Measurement of branching fractions of ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0{K}_S^0$ and ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0\eta$ at Belle

We present a study of a singly Cabibbo-suppressed decay ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0{K}_S^0$ and a Cabibbo-favored decay ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0\eta$ based on 980 $\mathrm{f}{\mathrm{b}}^{-1}$ of data collected by the Belle detector, operating at the KEKB energy-asymmetric $e^{+}{e}^{-}$ collider. We measure their branching fractions relative to ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0$: $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0{K}_S^0)\mathrm{/}\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0)=(1.48\pm 0.08\pm 0.04)\times {10}^{-2}$ and $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0\eta )\mathrm{/}\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0)=(2.73\pm 0.06\pm 0.13)\times {10}^{-1}$. Combining with the world average $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0)$, we have the absolute branching fractions: $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0{K}_S^0)=(2.35\pm 0.12\pm 0.07\pm 0.12)\times {10}^{-4}$ and $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0\eta )=(4.35\pm 0.10\pm 0.20\pm 0.22)\times {10}^{-3}$. The first and second uncertainties are statistical and systematic, respectively, while the third ones arise from the uncertainty on $\mathcal{B}({\mathrm{\Lambda }}_c^{+}\to p{K}_S^0)$. The mode ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0{K}_S^0$ is observed for the first time and has a statistical significance of $>\text{\hspace{0.17em}⁣}10\sigma$. The branching fraction of ${\mathrm{\Lambda }}_c^{+}\to p{K}_S^0\eta$ has been measured with a threefold improvement in precision over previous results and is found to be consistent with the world average.