APH N.S., Heavy Ion Physics 15/3–4 (2002) 269–277 HEAVY ION PHYSICS c Akad´ emiaiKiad´o Baryon Fluctuations from the QCD Phase Transition David Bower and Sean Gavin Physics Department, Wayne State University, Detroit, MI 48201, USA Received 5 June 2001 Abstract. Extraordinary baryon fluctuations can signal a nearly first order phase transition at RHIC. We discuss how these fluctuations can be measured. Next, we apply a dissipative-hydrodynamic formulation used in condensed mat- ter physics to simulate the formation — though spinodal decomposition — and subsequent evolution of these fluctuations. Keywords: relativistic nuclear collisions, event-by-event analysis PACS: 25.75+r, 24.85.+p, 25.70.Mn, 24.60.Ky, 24.10.-k 1. Introduction A first order phase transition from quark matter to hadron gas can produce a mixed phase consisting of plasma droplets in equilibrium with a surrounding hadronic fluid. If formed in a nuclear collision, this mixed phase can suppress flow or en- hance event-by-event fluctuations, depending on the size of droplets relative to the collision volume. Small droplets turn the mixed phase into a uniform foam with a vanishing sound speed, with well-known phenomenological consequences related to flow. Large droplets can lead to large event-by-event fluctuations as the system hadronizes [1]. Extraordinary baryon fluctuations [2] can accompany a first order transition at high baryon density [3] and, possibly, a near transition at zero baryon density [4], [5]. In Ref. [2] we argued that baryon number conservation and rapid longitudinal expansion limits the extent to which post-hadronization interactions can erode fluctuations in a rapidity interval. Here, we further explore the rise and fall of these fluctuations using real-time lattice simulations [6]. At high baryon density, QCD with two massless flavors can exhibit a line of first order transitions culminating in a tricritical point at temperature T c and baryon chemical potential μ c [3]. For T>T c and μ<μ c , a second order phase transition breaks/restores chiral symmetry. If the quark masses are sufficiently large, the second order transition is replaced by a smooth transformation (chiral symmetry is explicitly broken). The first order line remains, however, with the tricritical point replaced by a critical endpoint in the same universality class as a liquid–gas transition. 1219-7580/02/ $ 5.00 c  2002 Akad´ emiai Kiad´o, Budapest