Non-Forward OPE Kernel versus Scalar Form Factor in Near-Threshold $J/ψ$ Photoproduction

Abstract

Near-threshold $J/ψ$ photoproduction has been used to infer scalar gluonic structure from integrated cross-section data. We test whether such an inference is stable against the assumed non-forward continuation of the OPE kernel. With an exponential diffractive slope fixed, the threshold fit favors an additional scalar $G_S(t)$-like contribution. However, the residuals show a systematic $W$-dependent trend. When either the effective slope or the kernel shape is replaced by a physically motivated form-factor continuation, the trend disappears and the fitted scalar coefficient is driven to zero. Thus integrated $σ(W)$ data cannot determine the non-forward OPE kernel or isolate a scalar $G_S(t)$ contribution. Differential $dσ/dt$ data are required.