Transport coefficients of the quark-gluon plasma at the critical point and across the first-order line

TL;DR

This paper uses a holographic model to compute various transport coefficients of the quark-gluon plasma near the critical point and across the phase transition, revealing how these properties change with baryon density.

Contribution

First calculation of transport coefficients at the critical point and across the phase transition in a holographic QCD model, matching lattice results and exploring density effects.

Findings

Baryon diffusion and viscosities decrease with increasing density.

Jet quenching and heavy-quark diffusion increase with density.

Transport observables show discontinuities or infinite slopes at phase transition points.

Abstract

A bottom-up Einstein-Maxwell-Dilaton holographic model is used to compute, for the first time, the behavior of several transport coefficients of the hot and baryon-rich strongly coupled quark-gluon plasma at the critical point and also across the first-order phase transition line in the phase diagram. The observables under study are of the shear and bulk viscosities, baryon diffusion, thermal conductivity, the jet quenching parameter $\hat{q}$, as well as the heavy-quark drag force and Langevin diffusion coefficients. These calculations provide a phenomenologically promising estimate for these coefficients, given that our model quantitatively reproduces lattice QCD thermodynamics results, both at zero and finite baryon density, besides naturally incorporating the nearly-perfect fluidity of the quark-gluon plasma. We find that the diffusion of baryon charge, and also the shear and bulk viscosities, are suppressed with increasing baryon density, indicating that the medium becomes even closer to perfect fluidity at large densities. On the other hand, the jet quenching parameter and the heavy-quark momentum diffusion are enhanced with increasing density. The observables display a discontinuity gap when crossing the first-order phase transition line, while developing an infinite slope at the critical point. The transition temperatures associated with different transport coefficients differ in the crossover region but are found to converge at the critical point.