Three-particle azimuthal correlations and Mach shocks

Abstract

Measurements of angular correlations of hadrons with a (semi)hard trigger hadron in heavy-ion collisions at RHIC show large angular structures opposite to the trigger which were a priori unexpected. These away side large angle correlations were first observed in two-particle correlations [S. S. Adler et al. (PHENIX Collaboration), Phys. Rev. Lett. 97, 052301 (2006) and J. Adams et al. (STAR Collaboration), Phys. Rev. Lett. 95, 152301 (2005)] and have recently also been investigated in three-particle correlation measurements [J. G. Ulery (STAR Collaboration), Nucl. Phys. A774, 581 (2006) and F. Wang, arXiv:nucl-ex/0610027]. We show that the correlation signal can be understood in terms of sonic shockwaves (`Mach cones') excited by hard partons supersonically traversing the medium. The propagation of such shocks through the medium evolution is treated in a Monte Carlo (MC) framework [T. Renk and J. Ruppert, Phys. Rev. C 73, 011901(R) (2006) and Phys. Lett. B646, 19 (2007)]. We demonstrate that two- and especially three-particle correlations offer nontrivial insight into the medium-averaged speed of sound and the evolution of flow. Our findings imply that the assumption of ``deflected jets'' is not necessary to account for the observed correlations.