# Radiative decays of $h_{c}$ to the light mesons $\eta^{(\prime)}$: A perturbative QCD calculation

## TL;DR

This paper calculates the radiative decays of $h_c$ to $ ext{eta}^{( extprime)}$ mesons using perturbative QCD, finding insensitivity to light quark masses and a gluonic contribution comparable to quark-antiquark content, and extracting a mixing angle that differs from some previous results.

## Contribution

It provides a perturbative QCD calculation of $h_c$ radiative decays including light quark masses and gluonic content, and determines the $ ext{eta}^{( extprime)}$ mixing angle from these decays.

## Key findings

- Branching ratios are insensitive to light quark masses and distribution amplitude shapes.
- Gluonic content contribution is comparable to quark-antiquark content in decays.
- Extracted mixing angle $ heta=33.8^ ext{o}\pm2.5^ ext{o}$ differs from Feldmann-Kroll-Stech result.

## Abstract

We study the radiative decays $h_{c}\rightarrow\gamma\eta^{(\prime)}$ in the framework of perturbative QCD and evaluate analytically the one-loop integrals with the light quark masses kept. Interestingly, the branching ratios $\mathcal{B}(h_{c}\rightarrow\gamma\eta^{(\prime)})$ are insensitive to both the light quark masses and the shapes of $\eta^{(\prime)}$ distribution amplitudes. And it is noticed that the contribution of the gluonic content of $\eta^{(\prime)}$ is almost equal to that of the quark-antiquark content of $\eta^{(\prime)}$ in the radiative decays $h_{c} \rightarrow \gamma\eta^{(\prime)}$. By employing the ratio $R_{h_{c}}=\mathcal{B}(h_{c}\rightarrow\gamma\eta)/\mathcal{B}(h_{c}\rightarrow\gamma\eta^{\prime})$, we extract the mixing angle $\phi=33.8^{\circ}\pm2.5^{\circ}$, which is in clear disagreement with the Feldmann-Kroll-Stech result $\phi=39.0^{\circ}\pm1.6^{\circ}$ extracted from the ratio $R_{J/\psi}$ with nonperturbative matrix elements $\langle 0\mid G^{a}_{\mu\nu}\tilde{G}^{a,\mu\nu}\mid\eta^{(\prime)}\rangle$, but in consistent with $\phi=33.5^{\circ}\pm0.9^{\circ}$ extracted from the asymptotic limit of the $\gamma^{\ast}\gamma-\eta^{\prime}$ transition form factor and $\phi=33.9^{\circ}\pm0.6^{\circ}$ extracted from $R_{J/\psi}$ in perturbative QCD. We also briefly discuss possible reasons for the difference in the determinations of the mixing angle.