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

TL;DR

This paper investigates radiative charmonium decays using a contact interaction model that includes quark anomalous magnetic moments, explaining recent experimental discrepancies with theoretical predictions.

Contribution

It introduces a contact interaction model with quark anomalous magnetic moments to better match experimental data on charmonium radiative decays.

Findings

The model aligns with lattice QCD estimates for decay widths.

The 2024 experimental value exceeds the model's range.

The 2026 experimental result is consistent with the model.

Abstract

The BESIII Collaboration has recently reported two measurements of the two-photon decay width of the $\eta_c$ 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 $\eta_c\to\gamma\gamma$ and $J/\psi\to\gamma\eta_c$ 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 $\eta_c\to\gamma\gamma$ lies above the range that could be accommodated by the present framework, whereas the 2026 result is naturally consistent with it.