Separating fact from fantasy in the proton's spin*: the chiral anomaly and the proton spin puzzle

Theoretical Physics Colloquium by Dr. Raju Venugopalan.
This presentation was held live on August 19, 2020 as part of the theoretical physics colloquium series hosted by Prof. Igor Shovkovy at the Arizona State University. The series features high-quality interactive presentations on current advances in theoretical physics by active researchers in the field of theoretical physics.
Abstract for this presentation:
Anomalies in gauge theories are of great interest because they represent topological features of the theory that are not contained in the Lagrangian. We will discuss here the role of the chiral anomaly in polarized deeply inelastic scattering (DIS) where one observes a subtle interplay between hard parton dynamics and the topology of the QCD vacuum. Using a powerful worldline formalism, we demonstrate in particular how the cancelation of the infrared pole of the anomaly leads to novel emergent dynamics governing how the spin of the proton diffuses from fast (large x) partons to a slow (small x) regime governed by the many-body dynamics of ``saturated" gluons at maximal occupancy. We present an effective action that we conjecture captures both the physics of the anomaly and of gluon saturation and outline how the latter may influence the former. These ideas can be tested at the future Electron Ion Collider.
*In homage to 30 years of a seminal paper by R. L. Jaffe and A. Manohar [Nucl. Phys. B 337, 509 (1990)]