Isotropization and Thermalization in Heavy Ion Collisions

We argue that isotropization and, consequently, thermalization of the system of gluons and quarks produced in an ultrarelativistic heavy ion collision does not follow from Feynman diagram analysis to any order in the coupling constant. We conclude that the apparent thermalization of quarks and gluons, leading to success of perfect fluid hydrodynamics in describing heavy ion collisions at RHIC, can only be attributed to the non-perturbative QCD effects not captured by Feynman diagrams. We proceed by modeling these non-pertrubative thermalization effects using viscous hydrodynamics. We point out that matching Color Glass Condensate inital conditions with viscous hydrodynamics leads to continuous evolution of all components of energy-momentum tensor and, unlike the case of ideal hydrodynamics, does not give a discontinuity in the longitudinal pressure. An important consequence of such a matching is a relationship between the thermalization time and shear viscosity: we observe that small viscosity leads to short thermalization time.Comment: Includes new results. 8 pages, 1 figure, based on proceedings contributions at "Quark Matter 2005" conference in Budapest, Hungary, August 4-9, 2005 and at the workshop on "Quark-Gluon Plasma Thermalization" in Vienna, Austria, August 10-12, 200

Gluon Distributions and Color Charge Correlations in a Saturated Light-cone Wavefunction

We describe the light-cone wavefunction in the saturation regime in terms of the density of gluons per unit of transverse phase space, the occupation number, and in terms of the color charge correlator. The simple McLerran- Venugopalan model gives what are claimed to be general results for the phase space gluon density, but it does not well describe the general case for the charge correlator. We derive the general momentum dependence and rapidity dependence of the color charge correlator which exhibits strong color shielding. A simplel physical picture which leads to these general results is described.Comment: 17 pages, Latex, 7 figure

Theory Summary: International Symposium on Multiparticle Dynamics 2008

I summarize the theory talks presented at the International Symposium on Multiparticle Dynamics 2008.Comment: 20 pages, 16 figures; v2: reference added; v3: typos correcte

Classical Initial Conditions for Nucleus-Nucleus Collisions

We present the results of an analytical calculation of the distribution of gluons in the state immediately following a heavy ion collision in the quasi-classical limit of QCD given by McLerran-Venugopalan model. We show that the typical transverse energy Et of the produced gluons is of the order of the saturation scale of the nuclei Qs, as predicted by Mueller. We demonstrate that due to multiple rescatterings the obtained gluon distribution remains finite (up to logarithms of k) in the soft transverse momentum limit of k << Q_s. We compare the gluon spectrum in the single nuclear wave function before the collision to the spectrum of the produced gluons. The total number of produced gluons turns out to be proportional to the total number of gluons inside the nuclear wave function before the collision with the proportionality coefficient c = 2 ln 2.Comment: 4 pages, 3 figures, to appear in the Proceedings of 15th International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (QM2001), Stony Brook, New York, 15-20 Jan 200

AdS/CFT applications to relativistic heavy ion collisions: a brief review

We review some of the recent progress in our understanding of the physics of ultrarelativistic heavy ion collisions due to applications of AdS/CFT correspondence.Comment: 20 pages, 5 figures, invited `Key Issue' mini-review for Reports on Progress in Physics; v2: minor corrections include

Toward Equilibration in the Early Stages After a High Energy Heavy Ion Collision

The early stages in the evolution of the gluons produced in the central region of a head-on high-energy heavy ion collision is studied. An equation is given for the rate of change of transverse momentum into longitudinal momentum where the longitudinal direction is along the collision axis. We are able to follow the system up to the time where equilibration seems to be setting in, but we are unable to actually follow the system as it reaches equilibrium.Comment: 18 pages, Late

High Energy QCD and the Large Nc Limit

We review recent progress in understanding QCD at high energies and the role played in it by the large Nc limit. We discuss unitarization of total hadronic cross sections and saturation of structure functions at high energies.Comment: 20 pages, 8 figures, to be published in the proceedings of "The Phenomenology of Large Nc QCD" workshop, 9-11 January 2002, Arizona State University, Tempe, A

Collinear Singularities and Running Coupling Corrections to Gluon Production in CGC

We analyze the structure of running coupling corrections to the gluon production cross section in the projectile-nucleus collisions calculated in the Color Glass Condensate (CGC) framework. We argue that for the gluon production cross section (and for gluon transverse momentum spectra and multiplicity) the inclusion of running coupling corrections brings in collinear singularities due to final state splittings completely unaffected by CGC resummations. Hence, despite the saturation/CGC dynamics, the gluon production cross section is not infrared-safe. As usual, regularizing the singularities requires an infrared cutoff Lambda_coll that defines a resolution scale for gluons. We specifically show that the cutoff enters the gluon production cross section in the argument of the strong coupling constant alpha_s(Lambda_coll^2). We argue that for hadron production calculations one should be able to absorb the collinear divergence into a fragmentation function. The singular collinear terms in the gluon production cross section are shown not to contribute to the energy density of the produced matter, which is indeed an infrared-finite quantity.Comment: 35 pages, 16 figures; v2: minor corrections, references and figures adde