$J/\psi$-meson photoproduction off the nucleon in a dynamical model

Abstract

The photoproduction of $J/\psi$ meson off the nucleon is investigated within a dynamical model approach based on a Hamiltonian which describes the reaction mechanisms of the Pomeron exchange, meson exchange, and direct $J/\psi$ radiation terms.To shed light on the low-energy mechanism, we scrutinize the role of light-meson [${\pi }^0(135)$, $\eta (548)$, ${\eta }^{\prime }(958)$, $f_1(1285)$] and charmonium-meson [${\eta }_c(1S)$, ${\chi }_{c0}(1P)$, ${\chi }_{c1}(1P)$, ${\eta }_c(2S)$, ${\chi }_{c1}(3872)$] exchanges in the $t$-channel diagram. The values of the coupling constants are mostly determined by the radiative decays of the $J/\psi$ meson or the relevant charmonium mesons. The final $J/\psi$-$N$ interaction is required by the unitarity condition and is described by the gluon-exchange and direct $J/\psi N$ coupling terms. The parameters of the Hamiltonian are determined by the latest GlueX and $J/\psi$-007 experiments at the Jefferson Laboratory (JLab). We find that $\eta (548)$ and ${\eta }^{\prime }(958)$ light mesons give the most significant contribution to $\gamma p\to J/\psi p$ among the meson-exchange terms.Meanwhile, the effect of charmonium mesons turns out to be small compared to that of light mesons.It turns out that the contribution of the FSI term is about 1 - 2 orders of magnitude smaller than that of the Born term and is significant near the threshold when the Yukawa form is used as for the charmonium-nucleon potential. The resulting total and $t$-dependent differential cross sections provide good agreement with the JLab data. The angle-dependent data with high precision near the very threshold ($E_{\gamma }⩽8.9$ GeV) are strongly desired to pin down the role of the FSI term more properly.

Comments: 10 pages, 9 figures