The Chern-Simons diffusion rate in improved holographic QCD

In (3 + 1)-dimensional SU(N_c) Yang-Mills (YM) theory, the Chern-Simons diffusion rate, Γ_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 Γ_CS in the high-temperature, deconfined phase of Improved Holographic QCD, a refined holographic model for large-N_c YM theory. Our result for Γ_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.