The BESIII collaboration recently reported “Observation of Λ_c⁺→Λa₀(980)⁺ and Evidence for Σ(1380)⁺ in Λ_c⁺→Λπ⁺η”, which has been published in Physical Review Letters on Jan. 17th, 2025 [Phys. Rev. Lett. 134, 021901 (2025)].
At the microscopic level, quarks are bound together by the strong interaction to form hadrons. Besides conventional hadrons (such as mesons and baryons), the quark model also predicts more complex hadron structures. The study of exotic hadron states is an important frontier topic in particle physics, which helps to deepen the understanding and verification of the theory of strong interaction. The light scalar meson a0(980)⁺ is one of the most widely studied exotic hadrons in particle physics. However, its internal structure remains a topic of debate, with possible explanations including a two-quark meson, a compact tetraquark state, or threshold effects. Similarly, the ground-state negative-parity hyperon Σ(1380)⁺ is another exotic hadron. Theoretical predictions suggest it may consist of five quarks, but it had never been observed experimentally. From a theoretical perspective, the charm baryon Λ_c⁺ can decay into these exotic states via two specific channels: Λ_c⁺→Λa₀(980)⁺ and Λ_c⁺→Σ(1380)⁺η. These decay modes provide new pathways for studying exotic hadrons. In 2019, BESIII had already conducted the first absolute branching fraction measurement of the Λ_c⁺→Λπ⁺η decay, identifying the Σ(1385)⁺ particle in the Λπ⁺ invariant mass spectrum. However, due to limited data statistics, other possible intermediate resonances were not systematically studied. Between 2020 and 2021, BESIII collected a large amount of e⁺e^- annihilation data in the energy region around the threshold Λ_c⁺Λbar_c^- pair production, which yielded approximately 10 times more Λ_c⁺Λbar_c^- pairs than previously available. The present findings are based on this new dataset.

Figure 1: The illustration of Λ_c+ decays into a0(980)+ or Σ(1380)+.

Based on 6.1 fb⁻¹ of e⁺e^- annihilation data collected at center-of-mass energies from 4.600 GeV to 4.843 GeV with the BESIII detector, a partial wave analysis of Λ_c⁺→Λπ⁺η is performed. For the first time, the process Λ_c⁺→Λa₀(980)⁺ was observed with a statistical significance exceeding 5σ. The corresponding branching fraction was measured and found to be 1–2 orders of magnitude larger than theoretical predictions based on the short-range contributions of the tetraquark model or long-range effects from final-state interactions. This significant discrepancy suggests that the decay mechanism and internal structure of the a0(980)⁺ need to be revisited. Meanwhile, the evidence of the pentaquark candidate Σ(1380)⁺ decaying into Λπ⁺ is found with statistical significance larger than 3σ. This study provides valuable experimental proof for exploring the decay mechanisms of charm baryons and the properties of exotic hadrons. The results highlight significant gaps between experimental observations and theoretical predictions, providing critical input for refining existing models.

Figure 2: Left: Projection of the baseline model fit results in the π⁺η mass spectrum. The red line represents the process Λ_c⁺→Λa₀(980)⁺. Right: Projection of the alternative model fit results including Σ(1380)⁺ in the Λπ⁺ mass spectrum. The light blue line indicates the process Λ_c⁺→Σ(1380)⁺η.

Reference: Phys. Rev. Lett. 134, 021901 (2025)
Journal publication: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.134.021901
DOI: 10.1103/PhysRevLett.134.021901