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Overview of the latest developments in understanding the initial state and thermalization

Kirill Boguslavski

TL;DR

The paper addresses how the non-equilibrium pre-QGP matter formed in high-energy heavy-ion collisions evolves before the quark-gluon plasma emerges and how to probe it experimentally. It synthesizes the weak-coupling narrative—from coherent Glasma fields to kinetic-theory dynamics and hydrodynamization—while extending the description to full 3D spatial dynamics and integrating multi-stage frameworks. A key focus is the quantification of pre-equilibrium transport coefficients, their anisotropies, and their impact on observables such as heavy-quark diffusion and jet quenching, with predictions for azimuthal dependences and polarization signals. The work highlights hard probes (electromagnetic and QCD) as valuable diagnostics of early-time QCD dynamics and outlines experimental opportunities at future collider runs and facilities for testing these developments.

Abstract

A proper description of the non-equilibrium matter preceding the quark-gluon plasma (QGP) in heavy-ion collisions and its observable consequences remain a major theoretical challenge, while at the same time offering new opportunities for experimental exploration. In these proceedings, I provide an overview of studies presented in talks and posters at Quark Matter 2025 on this topic. We will focus on the latest developments regarding the features and the numerical description of the non-equilibrium pre-QGP matter, as well as the potential to use hard probes as a means to study the hydrodynamization dynamics of the QCD plasma.

Overview of the latest developments in understanding the initial state and thermalization

TL;DR

The paper addresses how the non-equilibrium pre-QGP matter formed in high-energy heavy-ion collisions evolves before the quark-gluon plasma emerges and how to probe it experimentally. It synthesizes the weak-coupling narrative—from coherent Glasma fields to kinetic-theory dynamics and hydrodynamization—while extending the description to full 3D spatial dynamics and integrating multi-stage frameworks. A key focus is the quantification of pre-equilibrium transport coefficients, their anisotropies, and their impact on observables such as heavy-quark diffusion and jet quenching, with predictions for azimuthal dependences and polarization signals. The work highlights hard probes (electromagnetic and QCD) as valuable diagnostics of early-time QCD dynamics and outlines experimental opportunities at future collider runs and facilities for testing these developments.

Abstract

A proper description of the non-equilibrium matter preceding the quark-gluon plasma (QGP) in heavy-ion collisions and its observable consequences remain a major theoretical challenge, while at the same time offering new opportunities for experimental exploration. In these proceedings, I provide an overview of studies presented in talks and posters at Quark Matter 2025 on this topic. We will focus on the latest developments regarding the features and the numerical description of the non-equilibrium pre-QGP matter, as well as the potential to use hard probes as a means to study the hydrodynamization dynamics of the QCD plasma.

Paper Structure

This paper contains 8 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Pre-equilibrium features of the initial stages
  3. Initial stages go spatially 3D
  4. Better understanding of the pre-QGP era
  5. Hard probes: window into the initial stages
  6. Jet and heavy-quark transport coefficients
  7. Impact of QCD probes on observables
  8. Conclusion

Figures (2)

  • Figure 1: An overview of different stages of the pre-QGP medium from the weak-coupling perspective including their physical interpretation and theoretical description. (figures from Ipp:2017lhoKurkela:2015qoa)
  • Figure 2: Left: A sizable azimuthal correlation of $c \bar{c}$ pairs created in the Glasma remains even at relatively late times Avramescu:2024xtsAvramescu:2024poa. Center: One observes large deviations for the (differential) spectrum of a jet parton splitting into a $c \bar{c}$ pair in an anisotropic plasma as compared to the isotropic case Barata:2024bqp. Right: The jet collision kernel (that enters radiative energy loss and splitting rate calculations) of a pre-QGP plasma in the kinetic regime (using the isoHTL screening prescription Boguslavski:2024kbd) shows strong azimuthal ($\phi$) and transverse momentum ($q_\perp$) variations at early times Altenburger:2025iqaLindenbauer:2025ctw.