Quark flavor equilibration of the quark-gluon plasma

TL;DR

This paper investigates how quarks chemically equilibrate in the quark-gluon plasma during heavy-ion collisions using a novel viscous hydrodynamic model, revealing that flow development is sensitive to equilibration timescales.

Contribution

It introduces a new viscous hydrodynamic model with partial chemical equilibrium to study quark flavor equilibration in the QGP, starting from a gluon-dominated initial state.

Findings

Flow development depends on quark chemical equilibration timescale.

The model affects hadronic and electromagnetic observables.

Empirical probes of chemical equilibration are identified.

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.