# Initial conditions for hydrodynamics from kinetic theory equilibration

## TL;DR

This paper uses kinetic theory to model the early pre-equilibrium phase in heavy-ion collisions, providing a smooth transition to hydrodynamics and a framework applicable to various initial conditions.

## Contribution

It introduces a kinetic theory-based method to describe pre-equilibrium dynamics and transition to hydrodynamics in heavy-ion collisions.

## Key findings

- Smooth transition from pre-equilibrium to hydrodynamics.
- Early time dynamics described by functions of scaled time variable.
- Framework applicable to different initial state models.

## Abstract

We use effective kinetic theory to study the pre-equilibrium dynamics in heavy-ion collisions. We describe the evolution of linearized energy perturbations on top of out-of-equilibrium background to the energy-momentum tensor at a time when hydrodynamics becomes applicable. We apply this description to IP-Glasma initial conditions and find an overall smooth transition to hydrodynamics. In a phenomenologically favorable range of $\eta/s$ values, early time dynamics can be accurately described in terms of a few functions of a scaled time variable $\tau T/(\eta/s)$. Our framework can be readily applied to other initial state models to provide the pre-equilibrium dynamics of the energy momentum tensor.