Acta Phys. Hung. A 25/2–4 (2006) 515–524 DOI: 10.1556/APH.25.2006.2–4.43 HEAVY ION PHYSICS Diﬀusion and Two-Particle Correlations Mohamed Abdel-Aziz and Sean Gavin @ Department of Physics and Astronomy, Wayne State University 666 W. Hancock, Detroit, MI 48201, USA @ E-mail: sean@physics.wayne.edu Received 28 April 2005 Abstract. Fluctuation signals of the QCD phase transition in nuclear collisions can be dissipated due to diﬀusion. Diﬀusive modes in the standard formulation of relativistic hydrodynamics propagate with inﬁnite speed, violating causality. We develop a causal diﬀusion equation to study the dissipation of net-charge ﬂuctuations. We ﬁnd that causality restricts the extent to which diﬀusion can dissipate these ﬂuctuations. Keywords: relativistic heavy ion collisions, ﬂuctuations, diﬀusion PACS: 25.75.Ld, 24.60.Ky, 24.60.-k 1. Introduction In this talk we summarize work in [1] on charge diﬀusion and show how it is related to correlation and ﬂuctuation studies in relativistic heavy ion collisions [1]. Nu- clear collisions at RHIC create quark–gluon plasma, but we know very little about the process by which the plasma hadronizes. Probes of this hadronization include ﬂuctuations of mean p t , net-charge and baryon number ﬂuctuations [2]. Plasma hadronization can produce ﬂuctuations that diﬀer from hadronic expectations [3]. However, evolution in hadronic stage can dissipate these ﬂuctuations. The diﬀusion equation has been used as a tool to study the dissipation of net-charge and baryon number ﬂuctuations [4, 5]. However, one can question the applicability of the standard or “classic” diﬀusion equation in relativistic context because it allows signals to propagate with inﬁnite speed, violating causality. To get rid of the causality problems, we set up a causal diﬀusion equation that is suitable for ﬂuctuations studies at RHIC. Our causal formulation can be crucial for the description of net-charge ﬂuctuations, which involve rapid changes in the inhomogeneous collision environment which may reduce the net-charge ﬂuctuations compared to hadronic expectations [6]. 1219-7580/ $ 20.00 c  2006 Akad´ emiai Kiad´o, Budapest