Review of the "Bottom-Up" scenario

Thermalization of a longitudinally expanding color glass condensate with Bjorken boost invariant geometry is investigated within parton cascade BAMPS. Our main focus lies on the detailed comparison of thermalization, observed in BAMPS with that suggested in the Bottom-Up scenario. We demonstrate that the tremendous production of soft gluons via $gg \to ggg$, which is shown in the Bottom-Up picture as the dominant process during the early preequilibration, will not occur in heavy ion collisions at RHIC and LHC energies, because the back reaction $ggg\to gg$ hinders the absolute particle multiplication. Moreover, contrary to the Bottom-Up scenario, soft and hard gluons thermalize at the same time. The time scale of thermal equilibration in BAMPS calculations is of order \as^{-2} (\ln \as)^{-2} Q_s^{-1}. After this time the gluon system exhibits nearly hydrodynamic behavior. The shear viscosity to entropy density ratio has a weak dependence on $Q_s$ and lies close to the lower bound of the AdS/CFT conjecture.Comment: Quark Matter 2008 Proceeding

QCD plasma thermalization, collective flow and extraction of shear viscosity

Fast thermalization and elliptic flow of QCD matter found at the Relativistic Heavy Ion Collider (RHIC) are understood as the consequence of perturbative QCD (pQCD) interactions within a (3+1) dimensional parton cascade. The main contributions stem from pQCD-inspired bremsstrahlung. We extract the shear viscosity to entropy ratio, which is between 0.08 and 0.15.Comment: 8 pages, 5 figures, Plenary talk at 20th International Conference on Ultra-Relativistic Nucleus Nucleus Collisions (Quark Matter 2008), Jaipur, India, 4-10 Feb. 200

Anisotropic flow at RHIC: How unique is the number-of-constituent-quark scaling?

The transverse momentum dependence of the anisotropic flow $v_2$ for $\pi$, $K$, nucleon, $\Lambda$, $\Xi$ and $\Omega$ is studied for Au+Au collisions at $\sqrt{s_{\rm NN}} = 200$ GeV within two independent string-hadron transport approaches (RQMD and UrQMD). Although both models reach only 60% of the absolute magnitude of the measured $v_2$, they both predict the particle type dependence of $v_2$, as observed by the RHIC experiments: $v_2$ exhibits a hadron-mass hierarchy (HMH) in the low $p_T$ region and a number-of-constituent-quark (NCQ) dependence in the intermediate $p_T$ region. The failure of the hadronic models to reproduce the absolute magnitude of the observed $v_2$ indicates that transport calculations of heavy ion collisions at RHIC must incorporate interactions among quarks and gluons in the early, hot and dense phase. The presence of an NCQ scaling in the string-hadron model results suggests that the particle-type dependencies observed in heavy-ion collisions at intermediate $p_T$ might be related to the hadronic cross sections in vacuum rather than to the hadronization process itself.Comment: 10 pages, 5 figures; A new author (H. Petersen) is added; A new figure (fig.1) on time evolution of elliptic flow and number of collisions is added; Version accepted for publication in J. Phys.

Development of relativistic shock waves in viscous gluon matter

To investigate the formation and the propagation of relativistic shock waves in viscous gluon matter we solve the relativistic Riemann problem using a microscopic parton cascade. We demonstrate the transition from ideal to viscous shock waves by varying the shear viscosity to entropy density ratio $\eta/s$. We show that an $\eta/s$ ratio larger than 0.2 prevents the development of well-defined shock waves on time scales typical for ultrarelativistic heavy-ion collisions. These findings are confirmed by viscous hydrodynamic calculations.Comment: 4 pages, 3 figures - To appear in the conference proceedings for Quark Matter 2009, March 30 - April 4, Knoxville, Tennesse

Nonequilibrium models of relativistic heavy-ion collisions

To be published in J. Phys. G - Proceedings of SQM 2004 : We review the results from the various hydrodynamical and transport models on the collective flow observables from AGS to RHIC energies. A critical discussion of the present status of the CERN experiments on hadron collective flow is given. We emphasize the importance of the flow excitation function from 1 to 50 A.GeV: here the hydrodynamic model has predicted the collapse of the v2-flow ~ 10 A.GeV; at 40 A.GeV it has been recently observed by the NA49 collaboration. Since hadronic rescattering models predict much larger flow than observed at this energy we interpret this observation as evidence for a first order phase transition at high baryon density r b. Moreover, the connection of the elliptic flow v2 to jet suppression is examined. It is proven experimentally that the collective flow is not faked by minijet fragmentation. Additionally, detailed transport studies show that the away-side jet suppression can only partially (< 50%) be due to hadronic rescattering. Furthermore, the change in sign of v1, v2 closer to beam rapidity is related to the occurence of a high density first order phase transition in the RHIC data at 62.5, 130 and 200 A.GeV

Thermal Dileptons at LHC

We predict dilepton invariant-mass spectra for central 5.5 ATeV Pb-Pb collisions at LHC. Hadronic emission in the low-mass region is calculated using in-medium spectral functions of light vector mesons within hadronic many-body theory. In the intermediate-mass region thermal radiation from the Quark-Gluon Plasma, evaluated perturbatively with hard-thermal loop corrections, takes over. An important source over the entire mass range are decays of correlated open-charm hadrons, rendering the nuclear modification of charm and bottom spectra a critical ingredient.Comment: 2 pages, 2 figures, contributed to Workshop on Heavy Ion Collisions at the LHC: Last Call for Predictions, Geneva, Switzerland, 14 May - 8 Jun 2007 v2: acknowledgment include

Heavy-ion collisions at the LHC - Last call for predictions

This writeup is a compilation of the predictions for the forthcoming Heavy Ion Program at the Large Hadron Collider, as presented at the CERN Theory Institute 'Heavy Ion Collisions at the LHC - Last Call for Predictions', held from 14th May to 10th June 2007.This is a manuscript of an article from Journal of Physics G 35 (2008): 054001, doi: 10.1088/0954-3899/35/5/054001. Posted with permission.</p