Observation of a New Excited $Σ$ State in $ψ(3686) oar{p}K^+Σ^0+c.c.$

Using a data sample of $(2712.4\pm14.3)\times10^6\ ψ(3686)$ events collected with the BESIII detector, a partial-wave analysis of $ψ(3686)\to\bar{p}K^+Σ^0+c.c.$ is performed. A new excited $Σ$ baryon state is observed with a statistical significance of $11.9σ$, and the mass and width are measured as $(2334.7\pm7.9\pm16.0);\mathrm{MeV}/c^2$ and $(206.3\pm9.5\pm18.4);\mathrm{MeV}$, respectively. The spin-parity of the new state is favored to be $3/2^-$, and the branching fraction of $ψ(3686)\to\barΣ(2330)^0Σ^0+c.c.$ is determined to be $(4.47\pm0.58\pm1.52)\times10^{-6}$. In addition, the branching fraction of $ψ(3686)\to\bar{p}K^+Σ^0+c.c.$ is determined to be either $(2.44\pm0.20\pm0.08)\times10^{-5}$ or $(1.73\pm0.29\pm0.06)\times10^{-5}$ by considering two solutions. The first uncertainties are statistical and the second systematic.

💡 Research Summary

The BESIII Collaboration reports a comprehensive partial‑wave analysis (PWA) of the decay ψ(3686) → \bar{p}K⁺Σ⁰ + c.c. using a data set of (2 712.4 ± 14.3) × 10⁶ ψ(3686) events collected with the BESIII detector at the BEPCII e⁺e⁻ collider. After reconstructing Σ⁰ → Λγ and Λ → pπ⁻, applying stringent track, PID, photon, and four‑constraint kinematic‑fit selections, and suppressing dominant backgrounds from ψ(3686) → γχ_{cJ} → \bar{p}K⁺Λ, the authors obtain a clean sample of about 6 100 signal events with a detection efficiency of 17 %.

A maximum‑likelihood fit to the γΛ invariant mass yields the overall branching fraction for ψ(3686) → \bar{p}K⁺Σ⁰ + c.c. of (2.06 ± 0.03) × 10⁻⁵ when interference between resonant and non‑resonant amplitudes is ignored. By incorporating interference, two phase‑angle solutions are found, leading to corrected branching fractions of (2.44 ± 0.20 ± 0.08) × 10⁻⁵ and (1.73 ± 0.29 ± 0.06) × 10⁻⁵.

The PWA is performed with the TF‑PWA framework, employing helicity amplitudes, relativistic Breit–Wigner propagators, and Blatt‑Weisskopf barrier factors. All established N*, Λ*, and Σ* resonances with PDG three‑star status and spin ≤ 5/2 are considered, together with K₂(2250), K₃(2320) and a non‑resonant S‑wave term. Additional candidates N(2300), N(2570), Σ(2010) and Σ(2110) are tested. In the \bar{p}K⁺ invariant‑mass spectrum a pronounced enhancement near 2.33 GeV/c² is observed. Introducing a new Σ* resonance, denoted Σ(2330)⁰, significantly improves the fit.

Various spin‑parity hypotheses for Σ(2330)⁰ are examined. The assignment J^P = 3/2⁻ yields the largest improvement (Δ(−2 ln L) = 85.7 for 8 additional parameters) corresponding to a statistical significance of 11.9σ. The fitted mass and width are (2334.7 ± 7.9 (stat) ± 16.0 (syst)) MeV/c² and (206.3 ± 9.5 (stat) ± 18.4 (syst)) MeV, respectively, and the fit fraction is (19.8 ± 3.4) %. The branching fraction for ψ(3686) → \bar{Σ}(2330)⁰Σ⁰ + c.c. is measured to be (4.47 ± 0.58 ± 1.52) × 10⁻⁶.

Systematic uncertainties are thoroughly evaluated. Contributions arise from Λ reconstruction (1.1 %), tracking (0.3 %), PID (2.0 %), photon detection (0.5 %), kinematic fit (0.6 %), χ_{cJ} mass‑window selection (0.3 %), signal shape (0.6 %), background modeling (1.8 %), Monte‑Carlo model dependence (0.7 %), and the branching fractions of intermediate decays. The total systematic error on the branching fractions is 3.3 %. For the resonance parameters, systematic effects from adding or removing other resonances, varying background yields, changing the orbital angular momentum of the non‑resonant term, and altering the Blatt‑Weisskopf radius are considered; the largest deviations are taken as systematic uncertainties.

The observed Σ(2330)⁰ does not correspond to any listed PDG state. Recent quark‑model calculations predict a 1F Σ family around 2.33 GeV/c² with J^P = 3/2⁻, in excellent agreement (within 5 MeV) with the measured values. Alternative spin‑parity assignments (3/2⁺, 5/2⁻) also give high, but slightly lower, significances (10.7σ and 11.0σ), indicating that the current data cannot definitively exclude them.

In summary, the analysis provides the first evidence for a new excited Σ baryon, Σ(2330)⁰, with well‑determined mass, width, and preferred quantum numbers, enriching the hyperon spectrum and offering a valuable benchmark for QCD‑based baryon models. Further studies with larger ψ(3686) samples and additional decay channels are needed to confirm the spin‑parity assignment and to explore possible partner states in the same multiplet.