Observation of $\psi (3686)\to K^{-}\mathrm{\Lambda }(1520){\bar{\mathrm{\Xi }}}^{+}+c\mathrm{.}c\mathrm{.}$

Based on $(2712.4\pm 14.3)\times {10}^6$ $\psi (3686)$ events collected at the BESIII detector operating at the BEPCII collider, we present the first observation of the decay $\psi (3686)\to K^{-}\mathrm{\Lambda }(1520){\bar{\mathrm{\Xi }}}^{+}+c\mathrm{.}c\mathrm{.}$. The product branching fraction $\mathcal{B}[\psi (3686)\to K^{-}\mathrm{\Lambda }(1520){\bar{\mathrm{\Xi }}}^{+}+c\mathrm{.}c\mathrm{.}]\times \mathcal{B}[\mathrm{\Lambda }(1520)\to p{K}^{-}]$ is measured to be $(9.5\pm 0.8\pm 1.1)\times {10}^{-7}$, where the first uncertainty is statistical and the second systematic.