Radiative decays $J/ψ,\,ψ(2S)\rightarrowγη^{(\prime)}$ in perturbative QCD with relativistic corrections

We present the first calculation of the radiative decays $J/\psi,\psi(2S)\rightarrow\gamma\eta^{(\prime)}$ in perturbative QCD that includes the order-$q^{2}$ relativistic corrections in all three short-distance contributions, namely the quark-antiquark, two-gluon, and QED contributions. The amplitudes are found to be remarkably insensitive to the light-cone distribution amplitude and to the light-quark mass, a robustness that persists through order $q^{2}$ and makes the predictions correspondingly reliable. The relativistic correction enhances the $J/\psi$ branching ratios by roughly a factor of two, narrowing their shortfall from experiment, whereas for the $\psi(2S)$ it is about twice as large as for the $J/\psi$ and the low-order expansion converges poorly. In two representative $\eta$--$\eta'$ mixing schemes, the ratio $\mathcal{R}_{1S}=\mathcal{B}(\gamma\eta')/\mathcal{B}(\gamma\eta)$ proves sharply sensitive to the mixing angle and favours the smaller of the two. The predicted $\psi(2S)$ rates lie well above the data in both channels, already at leading order, and most severely for the anomalously small $\gamma\eta$ channel. Such a discrepancy suggests that a mechanism beyond the hard perturbative process is at work. As a physically motivated attempt, we explore the $\eta_{c}$-mixing contribution, which adds coherently to the perturbative one and is comparable to it in the $\gamma\eta$ channel, and find that the interference can bring the $\psi(2S)$ rates into agreement with the data, although its extraction is limited by a strong sensitivity to the mixing parameters.