Sound waves in strongly coupled non-conformal gauge theory plasma

TL;DR

This study uses gauge/gravity duality to quantify sound propagation in a strongly coupled non-conformal ${\cal N}=2^*$ gauge theory plasma at high temperature. By analyzing both the equation of state and the hydrodynamic pole of the stress-energy tensor correlator in the Pilch-Warner gravity background, the authors extract the speed of sound and the bulk-to-shear viscosity ratio, including leading corrections from bosonic and fermionic mass deformations. They demonstrate that $v_s^2$ deviates from the conformal value by terms proportional to $(m_b/T)^2$ and $(m_f/T)^2$, while the bulk viscosity receives corresponding non-universal contributions with numerically determined coefficients, and they explicitly connect these to the quasinormal-mode spectrum. The results provide a quantitative non-conformal benchmark at strong coupling and indicate how transport coefficients encode the breaking of conformal symmetry, complementing earlier conformal-$\mathcal{N}=4$ results and informing QCD-related hydrodynamics. All findings are presented in the high-temperature regime with explicit formulas and numerical coefficients, laying groundwork for exploring the full parameter space and weak-coupling comparisons.

Abstract

Using gauge theory/gravity duality we study sound wave propagation in strongly coupled non-conformal gauge theory plasma. We compute the speed of sound and the bulk viscosity of N=2^* supersymmetric SU(N_c) Yang-Mills plasma at a temperature much larger than the mass scale of the theory in the limit of large N_c and large 't Hooft coupling. The speed of sound is computed both from the equation of state and the hydrodynamic pole in the stress-energy tensor two-point correlation function. Both computations lead to the same result. Bulk viscosity is determined by computing the attenuation constant of the sound wave mode.