Seminar in Hadronic Physics

Mach cone shock waves at RHIC

J. Ruppert

McGill University

Recent 2- and 3-particle correlation measurements at the relativistic heavy ion collider (RHIC) with one (semi-)hard trigger hadron and soft associate hadrons exhibit rich structures that have let to intensive discussion of how their emergence can be understood. We argue that these structures can be understood if the energy that is lost by hard partons propagating though hot and dense matter has to reappear in the medium not only in the form of heating the matter, but also as excitations of a propagating collective mode. We employ a formalism in a dynamical model of the medium that can be used to track the propagation of such a mode through the evolving medium if its dispersion relation is known and calculate the resulting distortion of hadronic spectra at freeze-out. Using this formalism, we demonstrate within a detailed picture of the evolution of the expanding system and the experimental trigger conditions that the assumption of a sound mode being excited is in line with 2-particle correlation measurements by PHENIX and STAR for a semi-hard trigger. The measurement is sensitive to the averaged speed of sound in the hot matter. We show that 3-particle correlations show higher sensitivity to the underlying medium evolution and that both diagonal and off-diagonal structures can be understood in the framework of excitation of Mach cone shock waves at RHIC.