The BESIII collaboration recently reported “Observation of D⁺ -> K_S⁰a₀(980)⁺ in the amplitude analysis of D⁺ -> K_S⁰π⁺η”, which has been published in Physical Review Letters on  March 2024 [Phys. Rev. Lett., 132, 131903 (2024)].

Perturbative quantum chromodynamics~(QCD) approaches have well explained physics of nonleptonic b-hadron decays. However, the charm quark mass is located between the perturbative and nonperturbative QCD regions, making those approaches inapplicable.
To address this, a model-independent analysis was proposed in the so-called diagrammatic approach to phenomenologically describe charmed meson decays. The diagrammatic approach represents the W-emission and weak-annihilation (W-exchange or W-annihilation) amplitudes as topological quark-graph diagrams based on SU(3) flavor symmetry. With necessary experimental inputs, it enables us to extract the contribution and study the relative importance of each amplitude.

Enriching experimental results from a series of amplitude analyses on the hadronic charmed meson decays allow the diagrammatic approach to be applied quite successfully in D -> PP and D -> VP decays. However, in the sector of D -> SP, it appears that the current experimental measurements are still insufficient (P, W, and S denote pseudoscalar, vector, and scalar mesons). The contribution of weak-annihilation amplitudes, difficult to estimate accurately, has led to a notable discrepancy between experimental results and theoretical predictions.

In this study, the decay D⁺ -> K_S⁰a₀(980)⁺ has been observed for the first time in the amplitude analysis of D⁺ -> K_S⁰π⁺η based on 2.93 fb^-1 of e⁺e^- annihilation data collected at center-of-mass energies 3.773 GeV. Notably, this decay is the only decay free of weak-annihilation contributions among D^+,0 -> a₀(980)P, providing the most sensitive constraint to the diagrammatic approach method in the sector of D -> SP, thereby contributing to our comprehension of the mechanism of nonperturbative QCD. Furthermore, the scalar particle a₀(980) is commonly considered as a candidate for exotic states, which are states other than typical quark-antiquark mesons, such as states for tetraquarks, KbarK bound states, and other possible states. The production of these exotic states essentially involves final-state interactions, such as quark exchange, resonance formation, etc. Consequently, this study offers critical information on the role of a₀(980) in charmed meson decays and the nature of a₀(980).

URL: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.131903
DOI: 10.1103/PhysRevLett.132.131903