Observation of $ψ(3686) \to Ξ^- K^0_S \barΩ^+ $+c.c

Using a sample of $(2.712\pm0.014) \times 10^{9}$ $ψ(3686)$ events collected with the BESIII detector at the electron positron collider BEPCII, the decay $ψ(3686) \to Ξ^- K^0_S \barΩ^+ +c.c.$ is observed for the first time, which has a significance of 5.9 standard deviations. The branching fraction of this decay is measured to be $(2.91\pm0.47\pm0.33)\times 10^{-6}$, where the first and second uncertainties are statistical and systematic, respectively. The ratio between $\mathcal{B}_{ψ(3686) \to Ξ^- K^0_S \barΩ^+ +c.c.}$ and $\mathcal{B}_{ψ(3686) \to Ω^- K^+ \barΞ^0 +c.c.}$ is determined to be $1.05\pm0.23\pm0.14 $, which deviates with the isospin symmetry conservation predicted value of 0.5 by $2.1σ$.