Near-Threshold OPE Constraints and High-Energy Diffractive Dynamics in J/psi Photoproduction

We study the forward $J\mathrm{/}\psi N$ amplitude $M_{\psi N}(\lambda )$ using a leading-twist Coulombic operator-product expansion, exact physical-domain target-mass resummation, and modern gluon PDFs. Forward-intercept extrapolations reported by GlueX and CLAS12 constrain the amplitude modulus within a vector-meson-dominance normalization. After fitting an overall scale and a subtraction constant, the exact-TMC and no-TMC descriptions differ by only $\mathrm{\Delta }{\chi }^2=0.021$, while a two-parameter empirical exponential gives an equally good fit. The point $C_{\mathrm{s}\mathrm{u}\mathrm{b}}=0$ changes the profiled minimum by only $0.033$. Current near-threshold data therefore determine neither the target-mass correction nor the subtraction constant separately. An independent correlated fit to HERA data gives $\delta =0.695\pm 0.023$. The absence of direct-proton measurements between $W\simeq 4.73$ and $20\mathrm{G}\mathrm{e}\mathrm{V}$ prevents a data-driven determination of the crossover to high-energy diffraction.