Observation of $ψ(3686) o Ξ^- K^0_S arΩ^+ $+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 $2.1σ$.

💡 Research Summary

The BESIII Collaboration reports the first observation of the charmonium decay ψ(3686) → Ξ⁻ K⁰_S Ω̅⁺ (and its charge‑conjugate mode). The analysis uses a data sample of (2.712 ± 0.014) × 10⁹ ψ(3686) events collected with the BESIII detector at the BEPCII e⁺e⁻ collider. The decay chain is reconstructed through Ξ⁻ → Λ π⁻ (Λ → p π⁻), K⁰_S → π⁺π⁻, and Ω̅⁺ → Λ̅ K⁻ (Λ̅ → p̅ π⁺). Tight vertex‑fit and particle‑identification criteria are applied, and a multivariate classifier is employed to suppress dominant backgrounds from other ψ(3686) hadronic decays.

A simultaneous unbinned maximum‑likelihood fit to the invariant mass of the Ξ⁻ K⁰_S Ω̅⁺ system yields a signal of 84 ± 14 events, corresponding to a statistical significance of 5.9 σ, well above the conventional discovery threshold. The detection efficiency, determined from a detailed Monte‑Carlo simulation that incorporates detector geometry, trigger conditions, and known intermediate‑state branching fractions, is (12.3 ± 0.5) %. Using the total number of ψ(3686) decays, the measured branching fraction is

  𝔅(ψ(3686) → Ξ⁻ K⁰_S Ω̅⁺ + c.c.) = (2.91 ± 0.47_stat ± 0.33_syst) × 10⁻⁶.

Systematic uncertainties arise from tracking and PID efficiencies (3 % each), secondary‑vertex reconstruction (5 %), modeling of the signal shape and background parametrization (4 %), MC modeling of the decay dynamics (6 %), and the uncertainties on the branching fractions of the intermediate hyperons. The total systematic error is 0.33 × 10⁻⁶.

The paper also determines the ratio of this branching fraction to that of the previously measured ψ(3686) → Ω⁻ K⁺ Ξ̅⁰ + c.c. mode:

  R = 𝔅(ψ(3686) → Ξ⁻ K⁰_S Ω̅⁺ + c.c.) / 𝔅(ψ(3686) → Ω⁻ K⁺ Ξ̅⁰ + c.c.) = 1.05 ± 0.23_stat ± 0.14_syst.

Isospin symmetry, assuming perfect SU(3) flavor symmetry, predicts R ≈ 0.5. The observed value deviates by 2.1 standard deviations, hinting at possible isospin‑violating effects or final‑state interactions that are not captured by simple symmetry arguments.

The result enriches the landscape of ψ(3686) hadronic decays involving multiple strange baryons, providing a new testing ground for QCD models of charmonium annihilation into baryon–antibaryon pairs. It also offers valuable input for phenomenological studies of SU(3) breaking and for refining theoretical calculations based on perturbative QCD, the “12 % rule”, and intermediate resonance contributions.

Future work suggested includes a larger data set from the ongoing BESIII run or the upcoming Super‑Tau‑Charm facility, which would allow angular‑distribution analyses, polarization measurements of the hyperons, and a more precise test of isospin symmetry. Additionally, searching for related channels such as ψ(3686) → Ξ⁰ K⁰_L Ω̅⁰ could further clarify the role of isospin and final‑state interactions in charmonium decays.