Pre-hydrodynamic evolution and its signatures in final-state heavy-ion observables

TL;DR

This paper examines how different pre-hydrodynamic models influence final-state observables in heavy-ion collisions, highlighting the importance of non-conformal approaches for accurate modeling of early-time dynamics.

Contribution

It demonstrates that flow observables are insensitive to pre-hydrodynamic details, but the $p_T$ spectra are affected, emphasizing the need for non-conformal models.

Findings

Flow observables are insensitive to pre-hydrodynamic details.

The $p_T$ spectra, especially mean transverse momentum, are affected by pre-hydrodynamic evolution.

Conformal invariance assumptions can artificially inflate out-of-equilibrium bulk pressure.

Abstract

We investigate the effects of pre-hydrodynamic evolution on final-state observables in heavy-ion collisions using state-of-the art event simulations coupled to different pre-hydrodynamic scenarios, which include the recently-developed effective kinetic transport theory evolution model KoMPoST. Flow observables are found to be insensitive to the details of pre-hydrodynamic evolution. The main effect we observe is in the $p_T$ spectra, particularly the mean transverse momentum. However, at least part of this effect is a consequence of the underlying conformal invariance assumption currently present in such approaches, which is known to be violated in the temperature regime probed in heavy-ion collisions. This assumption of early time conformal invariance leads to an artificially large out-of-equilibrium bulk pressure when switching from (conformal) pre-hydrodynamic evolution to hydrodynamics (using the non-conformal QCD equation of state), which in turn increases the transverse momentum. Our study indicates that a consistent treatment of pre-hydrodynamic evolution in heavy-ion collisions requires the use of non-conformal models of early time dynamics.