Quark flavor equilibration of the quark-gluon plasma

TL;DR

The paper develops a viscous hydrodynamic framework for partial chemical equilibrium in the QGP, modeling quark production with a time-dependent fugacity $\gamma_q$ and a fugacity-dependent equation of state $P(T,\gamma_q)$ matched to a fugacity-driven hadronic sector. Quark equilibration is controlled by an adjustable timescale $\tau_{eq}$ and implemented within a full heavy-ion event chain (TRENTo, MUSIC, iS3D, SMASH), with particlization on a $T_c(\gamma_q)$ surface. Key findings show that the equilibration timescale strongly affects flow observables such as $v_2$ and enhances entropy production, while charged multiplicities are surprisingly robust due to compensating temperature and fugacity effects; thermal photons display sensitivity to $\tau_{eq}$ but with sizable rate-scaling uncertainties. The framework offers a path to constrain quark chemical equilibration from LHC/RHIC data, potentially via Bayesian analyses that jointly infer $\tau_{eq}$ and transport coefficients along with flavor-specific equilibration dynamics.

Abstract

The early stage of a heavy-ion collision is marked by rapid entropy production and the transition from a gluon saturated initial condition to a plasma of quarks and gluons that evolves hydrodynamically. However, during the early times of the hydrodynamic evolution, the chemical composition of the QCD medium is still largely unknown. We present a study of quark chemical equilibration in the (Q)GP using a novel model of viscous hydrodynamic evolution in partial chemical equilibrium. Motivated by the success of gluon saturated initial condition models, we initialize the QCD medium as a completely gluon dominated state. Local quark production during the hydrodynamic phase is then simulated through the evolution of time-dependent fugacities for each independent quark flavor, with the timescales set as free parameters to compare different rates of equilibration. We present the results of complete heavy-ion collision simulations using this partial chemical equilibrium model, and show the effects on hadronic and electromagnetic observables. In particular, we show that the development of flow is sensitive to the equilibration timescale, providing an empirical way to probe the chemical equilibration of the QCD medium.

Figures (19)

• Figure 1: $T_\mathrm{c}(T,\gamma_q)$ compared to the critical temperatures for $N_f = 0$Borsanyi:2012ve, $N_f = 2$Burger:2011zcBornyakov:2009qh, and $N_f = 2+1$HotQCD:2014kol results.
• Figure 2: Energy density (left) and pressure (right) of the partial chemical equilibrium EoS, constructed by matching a linear interpolation in $\gamma_q$ of lattice data at high $T$ to a non-equilibrium hadron resonance gas low $T$. Note the first-order phase transition in the pure glue EoS at $T_\mathrm{c} = 260$ MeV. The black points indicate the respective critical temperature $T_\mathrm{c} (\gamma_q)$ for each value of $\gamma_q$.
• Figure 3: $\gamma_q$ as a function of proper time for several equilibration timescales $\tau_\mathrm{eq}$ for $\tau_0 = 0.6$ fm/c.
• Figure 4: Energy density along the $T_\mathrm{c}(\gamma_q)$ hypersurface plotted against $\gamma_q$.
• Figure 5: Total entropy per unit space-time rapidity over time for an averaged central event evolved with varying $\tau_\mathrm{eq}$ using ideal hydrodynamics.