Equilibration of the Quark-Gluon Plasma at Finite Net-Baryon Density in QCD Kinetic Theory

We explore the out-of-equilibrium dynamics of the quark-gluon plasma at zero and finite net-baryon density based on an effective kinetic theory of quantum chromodynamics (QCD). By investigating the isotropization of the longitudinal pressure, we determine the relevant time and temperature scales for the onset of viscous hydrodynamics and quantify the dependence on the chemical composition of the quark-gluon plasma. By extrapolating our results to realistic coupling strength, we discuss phenomenological consequences regarding the role of the preequilibrium phase at different collision energies.

Figure 1

Nonequilibrium evolution of the longitudinal pressure over energy ratio $p_L/e$ for different net-baryon density (top) and different initial quark to gluon ratios at zero density (bottom). Dotted and dashed curves show results for different initial anisotropies ${\xi }_0$ (top) and different initialization times $Q_s{\tau }_0$ (bottom).

Figure 2

Nonequilibrium evolution of the energy density for different net-baryon densities, characterized by the ratio of the chemical potentials to temperature $({\mu }_B/T{)}_{\mathrm{eq}}$ at late times. Dashed and solid curves show a comparison to early and late time asymptotics in Eq. (10).

Figure 3

Nonequilibrium trajectories of temperature $T$ and baryon chemical potential ${\mu }_B$ in the QCD phase diagram for 0%–5% most central heavy-ion collisions. Dashed (dotted) lines show the preequilibrium evolution $0.2<\tilde{w}<1$ for $\eta T_{\mathrm{eff}}/(e+p)=0.08(0.16)$; solid lines correspond to the hydrodynamic evolution for $\tilde{w}>1$ which approaches the (perturbative) isentropes indicated by gray dashed lines. The black solid line indicates the QCD crossover temperature $T_c=156.5\mathrm{MeV}$ [65].