Critical dynamics near QCD critical point

TL;DR

This thesis investigates the critical dynamics near the QCD critical point, revealing dominant hydrodynamic modes, divergence of transport coefficients, and unique critical exponents, enhancing understanding of phase transition behavior in QCD.

Contribution

It provides a comprehensive analysis of both linear and nonlinear hydrodynamic modes near the QCD critical point, including the derivation of RG equations and critical exponents, which was not previously established.

Findings

Thermal diffusion mode dominates near the critical point.

Bulk viscosity and thermal conductivity diverge strongly at the critical point.

Thermal and viscous diffusion modes slow down, while sound mode speeds up critically.

Abstract

In this thesis, we study the critical dynamics near the QCD critical point. Near the critical point, the relevant modes for the critical dynamics are identified as the hydrodynamic modes. Thus, we first study the linear dynamics of them by the relativistic hydrodynamics. We show that the thermal diffusion mode is the most relevant mode, whereas the sound mode is suppressed around the critical point. We also find that the Landau equation, which is believed to be an acausal hydrodynamic equation, has no problem to describe slowly varying fluctuations. Moreover, we find that the Israel-Stewart equation, which is a causal one, gives the same result as the Landau equation gives in the long-wavelength region. Next, we study the nonlinear dynamics of the hydrodynamic modes by the nonlinear Langevin equation and the dynamic renormalization group (RG). In the vicinity of the critical point, the usual hydrodynamics breaks down by large fluctuations. Thus, we must consider the nonlinear Langevin equation. We construct the nonlinear Langevin equation based on the generalized Langevin theory. After the construction, we apply the dynamic RG to the Langevin equation and derive the RG equation for the transport coefficients. We find that the resulting RG equation turns out to be the same as that for the liquid-gas critical point except for an insignificant constant. Consequently, the bulk viscosity and the thermal conductivity strongly diverge at the critical point. Then, a system near the critical point can not be described as a perfect fluid by their strong divergences. We also show that the thermal and viscous diffusion modes exhibit critical-slowing down with the dynamic critical exponents z_{thermal} \sim 3 and z_{viscous} \sim 2, respectively. In contrast, the sound mode shows critical-speeding up with the negative exponent z_{sound} \sim -0.8.