Polarized Nucleon as a Topological Dipole

We show that a polarized nucleon generically carries a dipole distribution of topological charge density. This topological dipole follows robustly from the definition of the topological form factor and the pseudoscalar nature of the topological charge density. The strength of the topological dipole is fixed, in the chiral limit, by the flavor-singlet axial charge. We demonstrate the mechanism in a two-flavor chiral soliton model with vector mesons and the $U(1)_A$ anomaly, where the rotation of the soliton induces a singlet pseudoscalar profile and realizes the predicted dipole pattern. We also discuss possible experimental probes through exclusive $\eta$, $\eta^\prime$ production and directed-flow-like pseudoscalar-meson asymmetries correlated with magnetic fields or vorticity in relativistic heavy-ion collisions.