Flavor equilibration of the quark-gluon plasma

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. Even in the early times of the hydrodynamic evolution, the chemical composition of the QCD medium is still largely unknown. In this talk, 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 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 hadron yields and the development of flow are sensitive to the equilibration timescale. These observables provide an empirical way to probe the chemical equilibration of the QCD medium, and we discuss the framework for a Bayesian model-to-data comparison that will simultaneously constrain the equilibration times and transport coefficients of the QGP.