Rescattering effects in near-threshold $J/\psi$ photoproduction

Abstract

We investigate near-threshold $J/\psi$ photoproduction off the nucleon, focusing on hadronic rescattering effects induced by open-charm meson-baryon intermediate states. Beyond the conventional Pomeron-exchange mechanism, the $\bar D^0\Lambda_c^+$ and $\bar D^{*0} \Lambda_c^+$ channels are incorporated within an effective Lagrangian framework. The relevant production amplitudes are evaluated at tree level from $t$-, $s$-, and $u$-channel diagrams in a gauge-invariant manner. The resulting total and $t$-dependent differential cross sections are compared with recent near-threshold data from the GlueX, $J/\psi$-007, and CLAS experiments at Jefferson Lab. We find that the open-charm rescattering contributions significantly improve the description of the data, particularly at large momentum transfer, and naturally generate cusp-like structures near the $\bar D^0 \Lambda_c^+$ and $\bar D^{*0} \Lambda_c^+$ thresholds in the GlueX data. We further present predictions for the associated open-charm processes $\gamma p \to \bar D^{(*)0} \Lambda_c^+$, whose cross sections are estimated to be of the order of 5 nb.