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

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}.