The OPE-Regge Transition in $J/ψ$ Photoproduction

Abstract

We compare forward $J/ψ$ photoproduction data from threshold to HERA with a threshold-normalized OPE direct-convolution calculation using modern NNLO gluon PDFs. The formal continuation of the threshold-normalized OPE baseline is used only as a diagnostic reference; its nearly PDF-independent mismatch with HERA data, of order $6$-$10$, quantifies the separation from the high-energy diffractive regime. We then describe the forward cross section by matching the OPE contribution to an effective Regge component at the cross-section level. The fit gives $W_m\!=\!7.4\pm0.2$~GeV and an effective Regge exponent $δ_{\rm eff}\!=\!0.93\pm0.01$. Within this two-component description, $W_m$ estimates the phenomenological transition from the OPE-dominated threshold region to diffractive high-energy dynamics.