Initial state in heavy ion collisions

TL;DR

This paper reviews recent progress in understanding the initial stages of heavy ion collisions, emphasizing QCD-based calculations, fluctuating nuclear geometries, and potential experimental signatures of initial state correlations.

Contribution

It highlights advancements in modeling the initial state from QCD and incorporating event-by-event fluctuations for more accurate heavy ion collision descriptions.

Findings

Improved modeling of initial nuclear geometry fluctuations.

Progress in connecting initial state correlations to experimental observables.

Enhanced understanding of initial conditions from QCD calculations.

Abstract

We briefly review advances in understanding the initial stages of a heavy ion collision. In particular the focus is on moving from parametrizing the initial state to calculating its properties from QCD, consistently with the description of hard probes and dilute-dense scattering experiments. Modeling the event-by-event fluctuating nuclear geometry in initial state calculations has significantly improved in recent years. We also discuss prospects of directly seeing effects of particle correlations created in the initial state in the experimental observables.