Radiative and leptonic decays of the pseudoscalar charmonium state $\eta_c$

Understand

The radiative and leptonic decays of $\eta_c\to \gamma\gamma$ and $\eta_c\to l^+l^-$ are studied.

• For $\eta_c\to \gamma\gamma$ decay, the second-order electromagnetic tree-level diagram gives the leading contribution.
• The decay rate of $\eta_c\to \gamma\gamma$ is calculated, the prediction is in good agreement with the experimental data.
• For $\eta_c\to l^+\l^-$, both the tree and loop diagrams are calculated.