The Chern-Simons Diffusion Rate in Improved Holographic QCD

TL;DR

This paper calculates the Chern-Simons diffusion rate in a holographic QCD model, revealing its temperature dependence and implications for CP-odd phenomena in quark-gluon plasma.

Contribution

It provides the first computation of Gamma_{CS} in Improved Holographic QCD, showing its behavior near the deconfinement transition.

Findings

Gamma_{CS}/(sT) increases monotonically as T approaches the transition from above

The diffusion rate varies slowly at high temperatures

CP-odd phenomena may be influenced by axial glueball excitations

Abstract

In (3+1)-dimensional SU(Nc) Yang-Mills (YM) theory, the Chern-Simons diffusion rate, Gamma_{CS}, is determined by the zero-momentum, zero-frequency limit of the retarded two-point function of the CP-odd operator tr[F ^ F], with F the YM field strength. The Chern-Simons diffusion rate is a crucial ingredient for many CP-odd phenomena, including the chiral magnetic effect in the quark-gluon plasma. We compute Gamma_{CS} in the high-temperature, deconfined phase of Improved Holographic QCD, a refined holographic model for large-Nc YM theory. Our result for Gamma_{CS}/(sT), where s is entropy density and T is temperature, varies slowly at high T and increases monotonically as T approaches the transition temperature from above. We also study the retarded two-point function of tr[F ^ F] with non-zero frequency and momentum. Our results suggest that the CP-odd phenomena that may potentially occur in heavy ion collisions could be controlled by an excitation with energy on the order of the lightest axial glueball mass.