Abstract: I’ll discuss the role of the chiral (triangle) anomaly in deep inelastic scattering (DIS) of electrons off polarized protons. I’ll demonstrate that the structure function g1 measured in polarized DIS is dominated by the triangle anomaly in both Bjorken and Regge asymptotics. I’ll show that the infrared pole of the anomaly appears in both limits. I’ll demonstrate the fundamental role played by a Wess-Zumino-Witten term, coupling the topological charge density to a “primordial” isosinglet meson, both in the cancellation of the infrared pole and topological mass generation of the eta prime meson.
I will introduce an axion-like effective action for g1 at small Bjorken x that follows from the cancellation of the infrared pole in the matrix element of the anomaly. It describes the interplay between gluon saturation and the topology of the QCD vacuum. In this context I’ll outline the role of “over-the-barrier” sphaleron-like transitions in spin diffusion at small x. Such topological transitions can be measured in polarized DIS at a future Electron-Ion Collider.