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

Running Coupling Evolution for Diffractive Dissociation and the NLO Odderon Intercept

We summarize the results of including running coupling corrections into the nonlinear evolution equation for diffractive dissociation. We also document a prediction that the NLO QCD odderon intercept is zero resulting from a discussion at the Diffraction 2012 Workshop.Comment: 4 pages, 1 figure; contains an original prediction embedded in the Diffraction 2012 proceedings; v2: minor corrections include

Shock Wave Collisions and Thermalization in AdS_5

We study heavy ion collisions at strong 't Hooft coupling using AdS/CFT correspondence. According to the AdS/CFT dictionary heavy ion collisions correspond to gravitational shock wave collisions in AdS_5. We construct the metric in the forward light cone after the collision perturbatively through expansion of Einstein equations in graviton exchanges. We obtain an analytic expression for the metric including all-order graviton exchanges with one shock wave, while keeping the exchanges with another shock wave at the lowest order. We read off the corresponding energy-momentum tensor of the produced medium. Unfortunately this energy-momentum tensor does not correspond to ideal hydrodynamics, indicating that higher order graviton exchanges are needed to construct the full solution of the problem. We also show that shock waves must completely stop almost immediately after the collision in AdS_5, which, on the field theory side, corresponds to complete nuclear stopping due to strong coupling effects, likely leading to Landau hydrodynamics. Finally, we perform trapped surface analysis of the shock wave collisions demonstrating that a bulk black hole, corresponding to ideal hydrodynamics on the boundary, has to be created in such collisions, thus constructing a proof of thermalization in heavy ion collisions at strong coupling.Comment: 10 pages, 6 figures, based on talks given at the Symposium on High Energy Strong Interactions 2010 (HESI 2010), Yukawa Institute for Theoretical Physics, Kyoto, Japan, August 9-13, 2010, and at the 4th International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions (Hard Probes 2010), Eilat, Israel, October 10-15, 201

Orbital Angular Momentum at Small xx

We determine the small Bjorken $x$ asymptotics of the quark and gluon orbital angular momentum (OAM) distributions in the proton in the double-logarithmic approximation (DLA), which resums powers of $\alpha_s \ln^2 (1/x)$ with $\alpha_s$ the strong coupling constant. Starting with the operator definitions for the quark and gluon OAM, we simplify them at small $x$, relating them, respectively, to the polarized dipole amplitudes for the quark and gluon helicities defined in our earlier works. Using the small-$x$ evolution equations derived for these polarized dipole amplitudes earlier we arrive at the following small-$x$ asymptotics of the quark and gluon OAM distributions in the large-$N_c$ limit: \begin{align} L_{q + \bar{q}} (x, Q^2) = - \Delta \Sigma (x, Q^2) \sim \left(\frac{1}{x}\right)^{\frac{4}{\sqrt{3}} \, \sqrt{\frac{\alpha_s \, N_c}{2 \pi}} }, \ \ \ \ \ L_G (x, Q^2) \sim \Delta G (x, Q^2) \sim \left(\frac{1}{x}\right)^{\frac{13}{4 \sqrt{3}} \, \sqrt{\frac{\alpha_s \, N_c}{2 \pi}}} . \end{align}Comment: 27 page

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

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