The role of the chiral anomaly in polarized deeply inelastic scattering: Topological screening, emergent axion-like dynamics and sphaleron transitions in QCD at high energies.

The contrast between the simplicity of the QCD Lagrangian, and its emergent subtle non-perturbative dynamics, is illustrated for the cross-section for a simple process, the scattering of a polarized virtual photon off a polarized proton. This process is sensitive to the net helicity of quarks in the proton and, as first shown by Veneziano and collaborators, is sensitive to the topological susceptibility of the QCD vacuum. We employ a powerful worldline formalism to describe the rich interplay of low energy Wess-Zumino-Witten couplings and chiral Ward identities with the perturbative dynamics of spinning partons. A striking consequence is that polarized deeply inelastic scattering measurements at the future Electron-Ion Collider may provide first evidence of sphaleron transitions in nature.