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

Speaker(s) / Presenter(s):
Raju Venugopalan (Brookhaven)
Abstract:  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 deeply inelastic scattering (DIS) of electrons off polarized protons where one observes a subtle interplay between hard parton dynamics and the topology of the QCD vacuum. Using a powerful worldline formalism, we demonstrate that the "triangle" graph provides the dominant contribution in the DIS "box" diagram both in the Bjorken limit of large Q^2 and in the Regge limit of small x_Bj. We then discuss how the infrared pole of the anomaly is canceled in this formalism leading to novel emergent dynamics governing how the spin of the proton diffuses from fast (large x) modes to slow (small x) modes. In particular, we present an effective action, consistent with anomalous chiral Ward identities, that is governed by dynamical scales corresponding to the topological charge density and the gluon saturation scale. We conjecture how the latter influences the former and emphasize that these ideas can be tested at the Electron-Ion Collider.
* title of the seminal paper by R. L. Jaffe and A. Manohar (Nucl. Phys. B 337, 509 (1990))


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