Soft-gluon exchange matters: isotropic screening in QCD kinetic theory

TL;DR

This paper investigates the impact of different isotropic screening prescriptions in QCD kinetic theory simulations, revealing significant early-time effects on plasma evolution and smaller shear viscosity ratios, with minimal late-time impact.

Contribution

It introduces the use of the isotropic HTL matrix element for screening in QCD kinetic theory, moving beyond the common Debye-like approximation, and analyzes its effects on plasma evolution.

Findings

Isotropic screening affects early-time plasma evolution significantly.

Maximum pressure anisotropy decreases by up to 50% with new screening.

Smaller shear viscosity to entropy density ratio (~10%) observed near equilibrium.

Abstract

QCD kinetic theory simulations are a prominent tool for studying the nonequilibrium initial stages in heavy-ion collisions. Despite their success, all implementations rely on approximations of the hard thermal loop (HTL) screened matrix elements in the collision terms. In this paper, we present our results for different isotropic screening prescriptions in the elastic collision term for gluons. In particular, we go beyond the simple Debye-like screening form that is used in all current implementations and apply the isotropic HTL matrix element instead. For isotropic systems, the evolution is nearly unchanged, but when studying the equilibration process in a Bjorken expanding plasma, we find qualitative and quantitative differences in a range of moments of the distribution function, such as a decrease in the maximum pressure anisotropy by up to 50%. In contrast, we find no significant qualitative impact on the jet quenching parameter or on the late-time hydrodynamization dynamics of anisotropic plasmas but observe systematically smaller values of $\eta/s$ by about 10% that coincide with perturbative calculations at small couplings. Our study reveals that the choice of the screening prescription can lead to large corrections at early times, but is less important close to equilibrium.