Observation of ψ(3686) →Ξ−KS0Ω¯+ + c.c.

Using a sample of (2.712 ± 0.014) × 10⁹ψ(3686) events collected with the BESIII detector at the electron positron collider BEPCII, the decay ψ(3686) → Ξ−KS0Ω¯+ + 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 ± 0.47 ± 0.33) × 10⁻⁶, where the first and second uncertainties are statistical and systematic, respectively. The ratio between Bψ3686→Ξ−KS0Ω¯++c.c and Bψ3686→Ω−K+Ξ¯0+c.c is determined to be 1.05 ± 0.23 ± 0.14, which deviates from the isospin symmetry conservation predicted value of 0.5 by 2.1σ.