We study the form factors of the $\eta_c$ meson in the light-front quark model. We explicitly show that the transition form factor of $\eta_c \to \gamma^* \gamma$ as a function of the momentum transfer is consistent with the experimental data by the BaBar collaboration, while the decay constant of $\eta_c$ is found to be $f_{ \eta_{ c}}=230.5^{ +52.2}_{ -61.0}$ and $303.6^{ +115.2}_{ -116.4}$ MeV for $\eta_c\sim c\bar{ c}$ by using two $\eta_c \to \gamma \gamma$ decay widths of $5.3\pm 0.5$ and $7.2\pm2.1$ keV, given by Particle Data Group and Lattice QCD calculation, respectively.
Categories: hep-ph hep-ex