The BESIII Collaboration recently reported “Observation of charmonium h_c radiative decays to multiple light hadrons and the tensor state f₂ (1270)”. The paper has been published online in Physical Review Letter on June. 17, 2025. [Phys. Rev. Lett. 134, 241902 (2025)].
The study of charmonium radiative decay provides a unique opportunity to deepen our understanding of quantum chromodynamics (QCD) by testing theoretical predictions with experimental observations. Unlike J/ψ and ψ(3686), the study of the radiative decay of the P-wave singlet h_c charmonium state to light hadrons is limited. Besides the radiative transition  h_c→γη_c, only two radiative decays h_c→γη and h_c →γη^' have been observed. The experimental results of h_c radiative decays were significantly different from the theoretical predicted value 5.5% of B(h_c→γ+2g).
Using a data sample of (27.12±0.14)×10⁸ ψ(3686) events collected by the BESIII detector, four modes of h_c radiative decays to multiple light hadrons are observed via ψ(3686)→π⁰ h_c for the first time. The clear signal of h_c→γX (X=π⁺ π^-, π⁺ π^- η, 2(π⁺ π^-), ppbar) are shown in Fig.1 with significance larger than 5σ.  The corresponding branching fractions are precisely measured. The sum of known branching fractions of the h_c radiative decay including the results of this analysis is approximately 0.8%, which is still significantly lower than the theoretical prediction. This discrepancy highlights the need for further exploration of h_c radiative decays. Moreover, h_c radiative decay to f₂(1270) is observed with a significance of 5.1σ. This is the first observation of h_c radiative decay to a tensor state. The branching fraction of h_c→γf₂(1270)→γπ⁺ π^- is an order of magnitude smaller than that in J/ψ decay. This measurement could be employed to investigate the quark and glueball mixing parameters of f₂(1270). These findings significantly advance our understanding of charmonium decay mechanisms and provides a promising path towards searching for exotic states.

Figure 1: Fits to the M(γX) invariant mass distributions, whereXrepresents π⁺ π^-, π⁺ π^- η, 2(π⁺ π^-),  and ppbar, respectively.

URL: https://journals.aps.org/prl/abstract/10.1103/v4hf-s8x8
DOI: https://doi.org/10.1103/v4hf-s8x8