From cross-section degeneracies to phase-sensitive observables in near-threshold J/psi production

Abstract

Near-threshold $J/ψ$ production is sensitive to scalar and traceless spin-two chromoelectric structure in the nucleon. We study whether present integrated and differential measurements can separate these contributions and constrain their relative phase. The GlueX integrated cross section does not determine a unique scalar contribution: production and elastic scalar terms are strongly correlated, while flatter non-forward profiles drive the fitted scalar strength to zero. In a 38-point large-$ξ$ differential analysis, the scalar term is collinear at fixed kinematics with the retained C-GFF direction, leading to nearly degenerate solutions and strong profile dependence. A common single-channel final-state factor preserves the scalar-spin-two relative phase, whereas coupled-channel rescattering can modify it. Its determination therefore requires polarization-sensitive observables together with process-specific complex helicity amplitudes.