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

Abstract

The photoproduction of $J/ψ$ 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 $J/ψ$ radiation terms.To shed light on the low-energy mechanism, we scrutinize the role of light-meson [$π^0(135)$, $η(548)$, $η'(958)$, $f_1(1285)$] and charmonium-meson [$η_c(1S)$, $χ_{c0}(1P)$, $χ_{c1}(1P)$, $η_c(2S)$, $χ_{c1}(3872)$] exchanges in the $t$-channel diagram. The values of the coupling constants are mostly determined by the radiative decays of the $J/ψ$ meson or the relevant charmonium mesons. The final $J/ψ$-$N$ interaction is required by the unitarity condition and is described by the gluon-exchange and direct $J/ψN$ coupling terms. The parameters of the Hamiltonian are determined by the latest GlueX and $J/ψ$-007 experiments at the Jefferson Laboratory (JLab). We find that $η(548)$ and $η'(958)$ light mesons give the most significant contribution to $γp \to J/ψp$ 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 ($E_γ\leqslant 8.9$ GeV) are strongly desired to pin down the role of the FSI term more properly.