Nonequilibrium QCD in heavy-ion collisions: Kinetic theory and jet modifications during the initial stages

TL;DR

This thesis advances understanding of jet modifications in the nonequilibrium quark-gluon plasma during early heavy-ion collision stages by computing key parameters, improving kinetic theory models, and identifying new attractors.

Contribution

It introduces a more realistic screening mechanism in QCD kinetic theory, computes the elastic collision kernel, and uncovers a novel weak-coupling attractor for the initial plasma dynamics.

Findings

Numerical value of the jet quenching parameter during pre-equilibrium stage.

Reduced maximum anisotropy and shear viscosity with improved screening.

Significant differences in gluon splitting rates from anisotropic kernels.

Abstract

This thesis focuses on how jets are modified by the nonequilibrium quark-gluon plasma during the initial stages in heavy-ion collisions. Its influence on their propagation is typically encoded in a single medium function, the dipole cross section. Its small distance behavior is characterized by the jet quenching parameter $\hat q$, and we obtain its numerical value throughout the pre-equilibrium stage, finding values comparable in magnitude to the earlier Glasma stage. We also compute the more general elastic collision kernel, obtained by Fourier transforming the dipole cross section. This constitutes an important step to facilitate the understanding of jet-medium interactions during the initial stages in heavy-ion collisions. Additionally, we improve QCD kinetic theory simulations by employing a more realistic (HTL) screening mechanism to incorporate medium effects, which we compare with simpler screening mechanisms. An expanding plasma realized in the initial stages of heavy-ion collisions exhibits a significantly reduced maximum anisotropy and reduced specific shear viscosity $\eta/s$ when using the improved screening prescription. Moreover, we investigate the gluon splitting rates, which are typically obtained using an isotropic model for the collision kernel. Going beyond that approximation, we find that the splitting rates obtained from the nonequilibrium anisotropic kernel differ significantly both in magnitude and in their qualitative time evolution. We further identify a novel type of weak-coupling attractor, which can be observed in the ratio of the jet quenching parameter and pressure ratio, and is obtained by extrapolating to vanishing coupling. This improved kinetic theory description and novel limiting attractors contribute towards a more realistic modeling of the nonequilibrium QCD plasma and its equilibration and hydrodynamization process during the initial stages.