The Fate of the Initial State Perturbations in Heavy Ion Collisions

TL;DR

This paper investigates how initial perturbations from moving charges in heavy ion collisions evolve and persist through the fireball expansion, explaining observed structures via hydrodynamics and magnetohydrodynamics.

Contribution

It introduces two mechanisms—wave-splitting acoustic waves and metastable electric flux tubes—to explain the preservation of perturbations in heavy ion collisions.

Findings

Secondary sound waves can be closer to the original perturbation at freezeout.

Metastable electric flux tubes predict specific wave structures.

The models explain observed 'cone' and 'ridge' structures in data.

Abstract

Heavy ion collisions at RHIC are well described by the (nearly ideal) hydrodynamics. In the present paper we study 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 dihadron correlation function with large-$p_t$ trigger, while "soft ridge" is a similar structure seen without 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 freezeout. We study two possible solutions to it: (i) a "wave-splitting" acoustic option and (ii) a "metastable electric flux tubes". In the first case we show that rapidly variable speed of sound under certain conditions leads to secondary sound waves, which are at freezeout 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.