The effect of causality constraints on Bayesian analyses of heavy-ion collisions

TL;DR

This paper investigates how enforcing causality constraints in Bayesian analyses of heavy-ion collisions affects inferred medium properties, revealing that strict causality limits significantly alter initial condition preferences and extracted viscosities.

Contribution

It introduces a causality-based criterion into Bayesian parameter estimation for heavy-ion collisions, showing its impact on inferred physical properties.

Findings

Strict causality limits change initial condition preferences.

Large bulk viscosities are disfavored under causality constraints.

Causality considerations are crucial for accurate medium property extraction.

Abstract

There have long been questions about the limits to the validity of relativistic fluid dynamics, and whether it is being used outside its regime of validity in modern simulations of relativistic heavy-ion collisions. An important new tool for answering this question is a causality analysis in the nonlinear regime -- if the solutions of the evolution equations do not respect relativistic causality, they are not a faithful representation of the underlying relativistic theory (in this case, quantum chromodynamics). Using this non-linear criterion, it has recently been shown that hydrodynamics is indeed being used outside its regime of validity in simulations, at least sometimes. Here we explore the phenomenological implications, particularly the quantitative effects of demanding limits on acausality in modern Bayesian parameter estimation. We find that, while typically only a small fraction of the system's energy is initially in an acausal regime, placing strict limits on the allowed energy fraction significantly changes the preferred properties of the initial condition, which in turn alters the extracted medium properties such as bulk viscosity, where large values are no longer favored. These findings highlight the importance of developing better theoretical descriptions of the early-time, out-of-equilibrium dynamics of relativistic heavy-ion collisions.