Saturation Physics in Heavy Ion Collisions

We discuss expectations of saturation physics for various observables in heavy ion collisions. We show how simple saturation-inspired assumptions about particle production in heavy ion collisions lead to Kharzeev-Levin-Nardi model. Comparing this model to RHIC data on particle multiplicities we conclude that saturation effects may play an important role in particle production and dynamics at the early stages of Au−Au collisions already at RHIC energies. We then estimate the contribution of the initial state two-particle azimuthal correlations to elliptic flow observable v2 in Au−Au collisions by constructing a lower bound on these non-flow effects based on v2 obtained from the analysis of proton-proton (pp) collisions. INTRODUCTION Saturation/Color Glass Condensate physics is based on the observation that the small-x wave functions of ultrarelativistic hadrons and nuclei are characterized by a hard scale Qs, known as the saturation scale [1, 2, 3, 4]. The scale Qs arises due to saturation of partonic densities at small-x and is an increasing function of energy and atomic number of the nucleus [5, 6, 4]. This large scale makes the strong coupling constant small αs(Qs) ≪ 1 leading to dominance of the classical gluonic fields in all high energy processes [4, 7]. Gluon production in high energy collisions is given by the classical field of the scattering color charges [8]. Corresponding gluon production cross section was found for pA collisions in [9] and the effects of quantum evolution [5] were included in it in [10]. The gluon production cross section for heavy ion collisions (AA) at the classical level has been studied both numerically [11] and analytically [12]. Since it is quite not clear at present how to include the effects of nonlinear quantum evolution [5] in the results of [11, 12], one has to construct models to describe the actual rapidity-dependent data produced in heavy ion collisions. Below we are going to show how some of these models, based on rather basic properties of saturation physics, provide a reasonably good description of RHIC data. PARTICLE MULTIPLICITY FROM SATURATION MODELS Multiplicity at Mid-Rapidity Versus Centrality Classical field Aμ ∼ 1/g leads to produced gluon multiplicity dN d2k d2bdy ∼ 〈