Sound Propagation on Top of the Fireball

TL;DR

This paper investigates how initial state fluctuations influence particle correlations in heavy ion collisions by modeling the propagation of perturbations on an expanding fireball using hydrodynamics, revealing viscosity effects on harmonic spectra.

Contribution

It introduces a method to analyze the evolution of initial perturbations on a conformal fireball and explores their impact on particle correlations and Fourier spectra in heavy ion collisions.

Findings

Viscosity suppresses higher harmonic modes.

Fourier spectra exhibit maxima and minima similar to cosmic microwave background.

Initial perturbation width affects final particle correlations.

Abstract

We study the effect that initial state fluctuations have on final particle correlations in heavy ion collisions. More precisely, we focus on the propagation of initial perturbations on top of the expanding fireball using the conformal solution derived by Gubser and Yarom for central collisions. For small perturbations, the hydrodynamic equations are solved by separation of variables and the solutions for different modes are added up to construct initial point-like perturbations, that are then allowed to evolve until freeze-out. The Cooper-Frye prescription is used to determine the final particle distribution. We present the two-particle correlation functions and their Fourier spectra obtained for different viscosities and different initial widths of the perturbation. We find that viscosity kills the higher harmonics, but that the Fourier spectra presents maxima and minima, similar to what is seen in the study of cosmic microwave background radiation.