Radiative charmonium decays in a contact-interaction model with dynamical quark anomalous magnetic moment

Abstract: The BESIII Collaboration has recently reported two measurements of the two-photon decay width of the $η_c$ meson. The 2024 result is significantly larger than most theoretical and empirical expectations, while a subsequent measurement published in early 2026 shows better agreement with the world average and conventional theoretical estimates. In this work, we study the $η_c\toγγ$ and $J/ψ\toγη_c$ processes within a contact interaction model that incorporates valence-quark anomalous magnetic moment effects, which are absent in standard treatments. Besides achieving agreement with modern lattice QCD estimates for these observables, we find that the 2024 central value for $η_c\toγγ$ lies above the range that could be accommodated by the present framework, whereas the 2026 result is naturally consistent with it.