The Sounds of the QCD Phase Transition

Abstract

Hydrodynamic description of a fireball produced in high energy heavy ion collisions has been recently supplemented by a very successful study of acoustic perturbation created by the initial state perturbations. We discuss sound produced at later stages of the collision, as the temperature drops below critical, $T<T_c$, and originated from the Rayleigh-type collapse of the QGP clusters. In certain analytic approximation we study distorted "sound spheres" and calculate modifications of the particle spectra and two-particle correlators induced by them. Unlike for initial state perturbations studied previously, we propose to look for those late-time sounds using rapidity correlations, rather than the azimuthal angles of the particles. We then summarize known data on rapidity correlations from RHIC and LHC, suggesting that the widening of those can be the first signature of the late-time sounds.