J/ψ-meson photoproduction off the nucleon in a dynamical model

The photoproduction of $J/\psi$ meson off the nucleon is investigated using a dynamical model based on a Hamiltonian that describes the reaction mechanisms involving meson-exchange and direct $J/\psi$-radiation terms. Pomeron exchange is also included to account for the monotonically increasing high-energy behavior of the cross section. To explore the low-energy mechanism, we scrutinize the role of light-meson [${\pi }^0\left(135\right)$, $\eta \left(548\right)$, ${\eta }^{\prime }\left(958\right)$, $f_1\left(1285\right)$] and charmonium-meson [${\eta }_c\left(1S\right)$, ${\chi }_{c0}\left(1P\right)$, ${\chi }_{c1}\left(1P\right)$, ${\eta }_c\left(2S\right)$, ${\chi }_{c1}\left(3872\right)$] exchanges in the t-channel diagram. Most of the coupling constants are determined from the radiative decays of the $J/\psi$ or the relevant charmonium mesons. The final $J/\psi$-N interaction is described by gluon-exchange and direct $J/\psi N$ coupling. The parameters of the Hamiltonian are fitted using the latest GlueX and $J/\psi$-007 experiments at the Jefferson Laboratory (JLab). We find that among the meson-exchange terms, the light mesons $\eta \left(548\right)$ and ${\eta }^{\prime }\left(958\right)$ make the most significant contributions to $\gamma p\to J/\psi p$, while the effect of charmonium mesons is relatively minor. The final-state interaction (FSI) contribution is found to be 1 - 2 orders of magnitude smaller than that of the Born term, but becomes significant near the threshold when a Yukawa-type potential is employed for the charmonium-nucleon interaction. The resulting total and t-dependent differential cross sections show good agreement with the JLab data. High-precision angular measurements near the threshold energy ($E_{\gamma }⩽8.9$ GeV) are highly desirable to clarify the role of the FSI term.