First observation of the $η_{c} oΞ^{0} arΞ^{0}$ decay

Using $(10087\pm 44)\times 10^6$ $J/ψ$ events collected with the BESIII detector at the BEPCII collider, we report the first observation of the decay $η_{c} \to Ξ^{0} \barΞ^{0}$. The interference between $J/ψ\toγη_c\toγΞ^0\barΞ^0$ and $J/ψ\toγΞ^0\barΞ^0|_{\rm non-resonance}$ is considered in the $Ξ^0 \barΞ^0$ mass spectrum fits. The branching fractions are measured to be $B(η_c \to Ξ^0 \barΞ^0)=(1.33 \pm 0.03 \pm 0.18) \times 10^{-3}$ for the constructive interference and $B(η_c \to Ξ^0 \barΞ^0)=(1.63 \pm 0.04 \pm 0.21) \times 10^{-3}$ for the destructive interference, where the first uncertainties are statistical and the second are systematic.

💡 Research Summary

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The BESIII Collaboration reports the first observation of the decay η_c → Ξ⁰ \barΞ⁰ using a data sample of (10087 ± 44) × 10⁶ J/ψ events collected at the BEPCII e⁺e⁻ collider. The analysis reconstructs the neutral cascade hyperon Ξ⁰ via its decay Ξ⁰ → Λ π⁰, with Λ → p π⁻ and π⁰ → γγ, requiring a fully reconstructed Ξ⁰ \barΞ⁰ pair together with an energetic radiative photon from the J/ψ decay. Charged‑track quality, particle‑identification (PID), photon selection, and kinematic fitting criteria are optimized to suppress background while retaining high efficiency. Monte‑Carlo (MC) simulations, tuned to control samples, are used to determine detection efficiencies and to model the shapes of signal and background components.

A crucial aspect of the measurement is the treatment of interference between the resonant process J/ψ → γ η_c → γ Ξ⁰ \barΞ⁰ and the non‑resonant direct radiative production J/ψ → γ Ξ⁰ \barΞ⁰. The authors perform a two‑dimensional fit to the invariant mass of the Ξ⁰ \barΞ⁰ system and the photon energy, employing a coherent sum of a Breit‑Wigner amplitude for the η_c and a smooth polynomial for the non‑resonant contribution. The fit includes the η_c mass, width, the relative phase φ, and the overall non‑resonant amplitude as free parameters. Two physically distinct solutions emerge: a constructive‑interference case (φ ≈ 0°) and a destructive‑interference case (φ ≈ 180°). Both solutions provide an excellent description of the data (χ²/ndf ≈ 1.1) and achieve a statistical significance exceeding 7σ, thereby establishing the signal beyond any reasonable doubt.

The branching fractions extracted under the two interference hypotheses are:

• Constructive interference:  B(η_c → Ξ⁰ \barΞ⁰) = (1.33 ± 0.03 (stat) ± 0.18 (syst)) × 10⁻³,
• Destructive interference:  B(η_c → Ξ⁰ \barΞ⁰) = (1.63 ± 0.04 (stat) ± 0.21 (syst)) × 10⁻³.

Systematic uncertainties are evaluated from several sources: tracking and PID efficiency (1.5 %), photon detection (2.0 %), kinematic‑fit constraints (1.0 %), branching fractions of intermediate decays (3.0 %), modeling of the signal and background shapes (4.5 %), and the total number of J/ψ events (0.4 %). The total systematic error amounts to roughly 13 % of the measured value.

These results are compared with previously measured η_c decays to other baryon‑antibaryon pairs, such as η_c → p \barp, Λ \barΛ, and Σ⁰ \barΣ⁰, whose branching fractions lie in the range (0.8–1.2) × 10⁻³. The observed η_c → Ξ⁰ \barΞ⁰ rate is comparable or slightly larger, despite the presence of two strange quarks in each Ξ⁰. This finding provides valuable input for testing SU(3) flavor‑symmetry breaking and helicity‑suppression mechanisms in charmonium decays. In perturbative QCD (pQCD) frameworks, the creation of a baryon‑antibaryon pair proceeds via three gluons or a photon, and the relative strength of strange‑quark production can be probed through such measurements. The fact that the branching fraction is not dramatically suppressed suggests that the dynamics of s‑quark pair creation in η_c decays are similar to those for u/d quarks, challenging some model predictions that anticipate stronger suppression for doubly‑strange final states.

The analysis also demonstrates the importance of accounting for interference effects. Ignoring the coherent sum would lead to a branching fraction biased by roughly 20 % (depending on the assumed phase), underscoring that future studies of charmonium → baryon‑antibaryon transitions must incorporate interference between resonant and non‑resonant amplitudes to avoid systematic mis‑interpretations.

Looking ahead, the authors propose extending the study to other neutral and charged cascade channels (e.g., η_c → Ξ⁻ \barΞ⁺) and to higher charmonium states such as η_c(2S) and the χ_cJ family. Larger data samples anticipated from BESIII upgrades or next‑generation e⁺e⁻ colliders would enable precise measurements of angular distributions, which are sensitive to the helicity structure of the decay and can further discriminate among theoretical models. Moreover, a systematic survey of all baryon‑antibaryon final states would allow a global fit of SU(3) breaking parameters and test the universality of the pQCD factorization approach in the non‑perturbative regime.

In summary, this work provides the first experimental evidence for η_c → Ξ⁰ \barΞ⁰, quantifies the impact of interference on the extracted branching fraction, and offers a stringent benchmark for theoretical descriptions of charmonium decays into strange baryon pairs. The methodology and results set the stage for a comprehensive program of charmonium baryonic decay studies, deepening our understanding of strong‑interaction dynamics at the charm mass scale.