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Pre-hydrodynamic jet momentum broadening beyond the jet quenching parameter

Alois Altenburger, Kirill Boguslavski, Florian Lindenbauer

TL;DR

The paper addresses jet-medium interactions during the nonequilibrium early stages of heavy-ion collisions by computing the collision kernel $C(\\mathbf{q}_\\perp)$, the dipole cross section $C(\\mathbf{b})$, and gluon splitting rates $\\gamma$ within nonequilibrium QCD kinetic theory using isoHTL screening. It develops a methodology to connect Glasma dynamics to kinetic theory and hydrodynamics, providing input for jet quenching calculations. Key findings reveal angular structure and enhanced small-angle momentum exchange in $C(\\mathbf{q}_\\perp)$ at early times, a time-evolving dipole cross section, and gluon-splitting rates that can differ substantially from thermal or common approximations, especially for parton energies near the hard temperature. These results offer more realistic modeling of jets in the early nonequilibrium plasma and inform improved QCD kinetic theory simulations of jet-medium interactions.

Abstract

We obtain the collision kernel and related dipole cross section during the initial nonequilibrium stages in heavy-ion collisions. These quantities are a crucial input for jet quenching calculations. We further compute the gluon splitting rates in the AMY formalism resulting from this nonequilibrium kernel. Comparing with thermal and commonly used forms, we find that particularly the gluon splitting rate for parton energies of the order of the hard effective temperature significantly differs from these approximations.

Pre-hydrodynamic jet momentum broadening beyond the jet quenching parameter

TL;DR

The paper addresses jet-medium interactions during the nonequilibrium early stages of heavy-ion collisions by computing the collision kernel , the dipole cross section , and gluon splitting rates within nonequilibrium QCD kinetic theory using isoHTL screening. It develops a methodology to connect Glasma dynamics to kinetic theory and hydrodynamics, providing input for jet quenching calculations. Key findings reveal angular structure and enhanced small-angle momentum exchange in at early times, a time-evolving dipole cross section, and gluon-splitting rates that can differ substantially from thermal or common approximations, especially for parton energies near the hard temperature. These results offer more realistic modeling of jets in the early nonequilibrium plasma and inform improved QCD kinetic theory simulations of jet-medium interactions.

Abstract

We obtain the collision kernel and related dipole cross section during the initial nonequilibrium stages in heavy-ion collisions. These quantities are a crucial input for jet quenching calculations. We further compute the gluon splitting rates in the AMY formalism resulting from this nonequilibrium kernel. Comparing with thermal and commonly used forms, we find that particularly the gluon splitting rate for parton energies of the order of the hard effective temperature significantly differs from these approximations.

Paper Structure

This paper contains 4 sections, 4 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Obtaining the collision kernel
  3. Dipole cross section and gluon splitting rates
  4. Conclusion

Figures (2)

  • Figure 1: Results for the collision kernel in momentum space. (Left): For a single time as a function of transverse momentum and angle. (Right): Angular averaged collision kernel as a function of transferred momentum for different times (color-coded). Figures from Altenburger:2025iqa.
  • Figure 2: (Left): Dipole cross section for fixed $b$ as a function of time. (Right): Gluon splitting rate for a fixed parton energy $p$ as a function of time, normalized to equilibrium. The curve obtained from the nonequilibrium collision kernel is depicted as a blue dotted line. The orange dash-dotted curve depicts the rate using a common approximation. Figures from Altenburger:2025iqa.