The BESIII international collaboration experiment achieved a major breakthrough in charmonium decay research. The research team led by the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences, for the first time, observed a distinct "resonance-like" structure at the dipion mass threshold. The study was published in Physical Review Letters on April 8.

Charmonium, a bound state consisting of a charm quark and an anti-charm quark, is regarded as a "natural laboratory" for investigating the strong interaction—the strongest fundamental force in nature. Based on approximately 2.7 billion ψ(3686) events collected with the BESIII detector, the research team selected about 37 million ψ(3686) → π⁺π⁻ J/ψ events for analysis.

In the dipion mass spectrum, a clear peak structure appeared near the π⁺π⁻ threshold. The measured mass of this structure is approximately 285.6±2.6 MeV/c², with a width of about 16.3±0.9 MeV. The width of this structure is much smaller than that of previously observed dipion resonant states, suggesting the presence of a different underlying physical mechanism.

To probe the nature of this structure, the research team compared two theoretical models, and found that when ψ(3686) is assumed to be a mixture of S-wave and D-wave components, and final state interactions (FSI) are introduced. The calculated results agree well with the experimental data, especially with this peculiar enhancement structure.

This finding indicates that this structure is likely not a new particle in the traditional sense, but is closely related to the complex internal structure of the ψ(3686) particle and its decay mechanism—in effect, a unique quantum dynamical effect shaped by the strong interaction during the decay process.

BESIII is currently conducting research using the decay channel with neutral pion pair in the final sates. Regardless of whether this structure is identified as a new quantum state, or another decay mechanism, due to final state interactions, this study opens a new window for understanding the complex behavior of strong interaction (quantum chromodynamics) in the low-energy non-perturbative region, and the understanding of the fundamental components of matter and their interactions.

Schematic diagram of the decay ψ(3686) → π⁺π⁻ J/ψ (Image by IHEP)