Causality violations in realistic simulations of heavy-ion collisions

TL;DR

This paper investigates causality violations in heavy-ion collision simulations, revealing that a significant portion of early-stage fluid cells exhibit superluminal behavior, which can be mitigated by including pre-equilibrium evolution.

Contribution

It identifies the extent of causality violations in current simulations and demonstrates how pre-equilibrium evolution reduces these violations, informing more accurate modeling.

Findings

Up to 75% of initial fluid cells violate causality.

Superluminal speeds reach up to 15% of light speed.

Pre-equilibrium evolution reduces acausal cells significantly.

Abstract

Causality is violated in the early stages of state-of-the-art heavy-ion hydrodynamic simulations. Such violations are present in up to 75% of the fluid cells in the initial time and only after 2-3 fm/$c$ of evolution do we find that 50% of the fluid cells are definitely causal. Superluminal propagation reaches up to 15% the speed of light in some of the fluid cells. The inclusion of pre-equilibrium evolution significantly reduces the number of acausal cells. Our findings suggests that relativistic causality may place constraints on the available parameter space of heavy-ion collision simulations when factored into more thorough statistical analyses.