Observation of $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}{\eta }^{\mathrm{\prime }}$ decay

Using a sample of $(27.12\pm 0.14)\times {10}^8$ $\psi (3686)$ events collected with the BESIII detector, the decay $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}{\eta }^{\mathrm{\prime }}$ with ${\eta }^{\mathrm{\prime }}$ subsequently decaying into $\gamma {\pi }^{+}{\pi }^{-}$ and $\eta {\pi }^{+}{\pi }^{-}$ is observed for the first time. The branching fraction of $\psi (3686)\to \mathrm{\Lambda }\bar{\mathrm{\Lambda }}{\eta }^{\mathrm{\prime }}$ is measured to be $(7.34\pm 0.94(stat\mathrm{.})\pm 0.43(sys\mathrm{.}))\times {10}^{-6}$. No resonant structures are evident in the $\mathrm{\Lambda }{\eta }^{\mathrm{\prime }}$, $\bar{\mathrm{\Lambda }}{\eta }^{\mathrm{\prime }}$ and $\mathrm{\Lambda }\bar{\mathrm{\Lambda }}$ mass spectra.