Fate of the initial state perturbations in heavy ion collisions

Abstract

Heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) are well described by the (nearly ideal) hydrodynamics. In the present paper, we study the propagation of perturbations induced by moving charges (jets) on top of the expanding fireball, using hydrodynamics and (dual) magnetohydrodynamics. Two experimentally observed structures, called a ``cone'' and a ``hard ridge'', have been discovered in a dihadron correlation function with a large-${p}_{t}$ trigger, while a ``soft ridge'' is a similar structure seen without a hard trigger. All three can be viewed as traces left by a moving charge in matter, on top of overall expansion. A puzzle is why those perturbations are apparently rather well preserved at the time of the fireball freeze-out. We study two possible solutions: (A) a ``wave-splitting'' acoustic option and (B) a ``metastable electric flux tube'' option. In the first case, we show that rapidly variable speed of sound under certain conditions leads to secondary sound waves, which are at freeze-out time closer to the original location and have larger intensities than the first wave. In the latter case, we rely on (dual) magnetohydrodynamics, which also predicts two cones or cylinders of the waves. We also briefly discuss metastable electric flux tubes in the near-${T}_{c}$ phase and their relation to clustering data.