The Fate of the Initial State Fluctuations in Heavy Ion Collisions. II The Fluctuations and Sounds

TL;DR

This paper investigates how initial state fluctuations in heavy ion collisions propagate as sound waves, revealing their phase correlations and fundamental scales, which can help determine the matter's properties like sound speed and viscosity.

Contribution

It introduces a detailed analysis of initial fluctuation propagation as sound waves and identifies key scales for measuring matter properties in heavy ion collisions.

Findings

Multiple low harmonic fluctuations have similar magnitudes.

Odd harmonics are phase-correlated, indicating local fluctuations.

Identification of sound and viscous horizons as fundamental scales.

Abstract

Heavy ion collisions at RHIC are well described by the (nearly ideal) hydrodynamics for average events. In the present paper we study initial state fluctuations appearing on event-by-event basis, and the propagation of perturbations induced by them. We found that (i) fluctuations of several lowest harmonics have comparable magnitudes, (ii) that at least all odd harmonics are correlated in phase, (iii) thus indicating the local nature of fluctuations. We argue that such local perturbation should be the source of the "Tiny Bang", a pulse of sound propagating from it. We identify its two fundamental scales as (i) the "sound horizon" (analogous to the absolute ruler in cosmic microwave background and galaxy distribution) and (ii) the "viscous horizon", separating damped and undamped harmonics. We then qualitatively describe how one can determine them from the data, and thus determine two fundamental parameters of the matter, the (average) {\em speed of sound} and {\em viscosity}.