Role of meson exchange and final-state interaction in $J/\psi$-meson photoproduction

Abstract

We investigate $J/\psi$ photoproduction off the nucleon using a dynamical model based on a Hamiltonian framework. First, Pomeron exchange is regarded as a background contribution that accounts for the gradual rise of the total cross section with increasing beam energy. Then, the meson-exchange and direct $J/\psi$-radiation contributions are included as part of the Born term. More specifically, we examine the contributions of the light-meson [$\pi^0(135)$, $\eta(548)$, $\eta'(958)$, $f_1(1285)$] and charmonium-meson [$\eta_c(1S)$, $\chi_{c0}(1P)$, $\chi_{c1}(1P)$, $\eta_c(2S)$, $\chi_{c1}(3872)$] exchanges in the $t$-channel diagram. Most of the coupling constants are determined from the radiative decays of the $J/\psi$ or other relevant charmonium states. Finally, we consider the $J/\psi N$ final-state interaction (FSI) within the leading-order approximation, which includes gluon-exchange and direct $J/\psi N$-coupling terms. The model parameters of the Hamiltonian are fitted to the data from the GlueX and $J/\psi$-007 experiments at Jefferson Laboratory (JLab). The light mesons $\eta(548)$ and $\eta'(958)$ are found to make the most significant contributions, whereas the charmonium mesons play only a minor role. The direct $J/\psi$-radiation term begins to come into play only in the backward scattering regions. The FSI contribution is suppressed by a factor of $10^1 - 10^2$ in comparison to the Born-term contribution when a Yukawa-type potential is employed for the charmonium-nucleon interaction. Our results agree well with the available JLab data on the total and $t$-dependent differential cross sections. High-precision, angle-dependent data in the threshold region ($E_\gamma \leqslant 8.9$ GeV) are crucial for confirming the FSI effect.