Nucleon tomography from a calibrated diquark spectator model
Generalized parton distributions (GPDs) provide a unified description of the longitudinal-momentum and transverse spatial structure of the nucleon. We calculate the twist-2 vector GPDs $H^{q}$ and $E^{q}$ in a spectator model containing both scalar and axial-vector diquark configurations. A central new result is the explicit analytic expression for the axial-vector-diquark contribution to the helicity-flip GPD $E^{q}$. To regulate the transverse-momentum integrations while retaining a physically sensible dependence on the momentum transfer, we introduce a momentum-transfer-dependent quasi-hard cutoff. The model parameters are determined through two complementary calibrations: one using proton observables only and another simultaneously including proton and neutron observables. Electromagnetic moments and radii, together with selected constraints on gravitational form factors, determine the quark and diquark masses, the relative scalar and axial-vector contributions, and the transverse-momentum scale. The two calibrations yield compatible parameter ranges, while the inclusion of neutron observables reduces the parameter uncertainties and more strongly constrains the flavor structure. From the resulting zero-skewness GPDs $H^{u,d}(x,0,t)$ and $E^{u,d}(x,0,t)$, we calculate the valence-quark PDFs and the impact-parameter distributions of unpolarized up and down quarks in both unpolarized and transversely polarized nucleons. The transverse distributions exhibit the expected localization of large-$x$ quarks near the transverse center of momentum and a flavor-dependent distortion when the nucleon is transversely polarized. Our results provide a unified phenomenological description of the longitudinal-momentum and transverse-position structure of valence quarks in the nucleon.