Constraining Light and Strange Flavor Equilibration in Relativistic Heavy-Ion Collisions

Abstract

Relativistic heavy-ion collisions are the only known means of creating the quark-gluon plasma, a deconfined state of QCD matter. Hydrodynamic modeling has established that this medium behaves as a strongly coupled, low-viscosity fluid, but typically assumes that its quark and gluon composition reaches chemical equilibrium by the onset of the hydrodynamic phase. This assumption remains uncertain: gluon-dominated initial states suggest that quark production continues well into the hydrodynamic stage, with potentially different rates for light and strange quarks. This dissertation develops a dynamical framework for light and strange quark chemical equilibration. Incomplete equilibration of each flavor is described through time-dependent quark fugacities that modify both the equation of state and the particlization of the medium. Implemented within a multistage framework of fluctuating initial conditions, viscous hydrodynamics, particlization, and hadronic transport, the model is used to study the effects of equilibration on the hydrodynamic evolution and on hadronic and electromagnetic observables. The framework is embedded within a Bayesian analysis in which Gaussian process emulators trained on the model calculations infer the initial light and strange quark fugacities, their equilibration timescales, and selected transport coefficients from Au+Au collision data at RHIC. The strange sector is found to begin substantially undersaturated, more suppressed than the light sector; the data further favor a slower approach to equilibrium for strangeness, though the timescales themselves remain weakly constrained. These results favor a quark-gluon plasma whose flavor composition is still approaching chemical equilibrium during the hydrodynamic phase rather than fixed at its outset, and motivate extensions to collision systems of differing size and energy.