The BESIII collaboration has reported the observation of three resonant structures in the cross section of e⁺e^-→π⁺π^- h_c based on precise measurements at center-of-mass energies between 4.01 and 4.95 GeV. The cross section exhibits a plateau-like behavior between 4.3 and 4.5 GeV, followed by a sharp drop, revealing for the first time three distinct resonances. These resonances appear at masses of approximately 4.22, 4.33, and 4.47 GeV. The third structure was detected for the first time with a statistical significance exceeding 5σ. The results provide crucial insights into the nature of exotic particles, and have been published in Physical Review Letters on 11 August 2025 [Phys. Rev. Lett. 135, 071901].
Charmonium states provide an excellent platform for exploring quantum chromodynamics (QCD)—the theory governing the strong force. The existence of these resonances adds to the growing catalog of so-called "XYZ particles",a class of exotic hadrons that defy the traditional quark model. These states, especially the Y states characterized by quantum numbers J^PC=1^--, exhibit an unexpected overpopulation in this mass region, suggesting exotic internal configurations such as hybrids, tetraquarks, hadronic molecules, hadrocharmonia, or kinematically induced peaks. Clarifying the nature of these states requires investigating their decay patterns or discovering new Y partners. Transitions involving h_c meson in the final state are particularly intriguing because they are expected to be suppressed due to heavy quark spin symmetry—observing a strong production of h_c suggests the resonances have unusual internal dynamics.
Using a scan data sample corresponding to an integrated luminosity of 22.2 fb^-1 collected with the BESIII detector, the cross sections of e⁺e^-→π⁺π^- h_c is best described by a coherent sum of three Breit-Wigner (BW) functions:

M1 = 4223.6_-3.7-2.9^+3.6+2.6 MeV/c², Γ1= 58.5_-11.4-6.5^+10.8+6.7 MeV,
M2 = 4327.4_-18.8-9.3^+20.1+10.7 MeV/c², Γ2= 244.1_-27.1-18.3^+34.0+24.2 MeV,
M3 = 4467.4_-5.4-2.7^+7.2+3.2 MeV/c², Γ3= 62.8-_14.4-7.0^+19.2+9.9 MeV.

The quantum numbers of these states are 1^--, as they are produced directly via e⁺e^- annihilation. The newly observed resonances align with earlier discoveries, such as the ψ(4230) and ψ(4360), but with notable differences—including a significantly broader width for the second resonance. The third structure, however, is entirely new in this decay mode and could correspond to a predicted hybrid state and ψ(4500), a particle previously found in e⁺e^-→K⁺ K^- J/ψ.
With the world’s largest electron-positron annihilation datasets in the charmonium region, BESIII continues to offer unique opportunities to probe the nature of charmonium-like states. Nevertheless, determining their internal structures remains challenging. Further precision studies and improved modeling of the cross section line shapes across diverse decay channels will be crucial for unraveling the internal structure of these enigmatic states.