Multiplicities for LHC Nuclear Collisions Using HERA Structure Functions

We compute in QCD perturbation theory the transverse energy carried by gluons, quarks and antiquarks with $p_T\ge p_0\approx 2$ GeV in Pb+Pb collisions at $\sqrt{s}=5500$ $A$GeV by using structure functions compatible with the small-$x$ increase observed at HERA. This gives a perturbative estimate for the energy and entropy density of the bulk system at times $\tau\sim 0.1$ fm. The predicted initial gluon entropy density gives a lower limit of about 2200...3400 for the final charged multiplicity. Sources of further entropy increase are discussed.Comment: HU-TFT-94-6, 7 pages, 3 PostScript figures included in the end of the tex-fil

Multiplicities and Transverse Energies in Central AA Collisions at RHIC and LHC from pQCD, Saturation and Hydrodynamics

We compute the particle multiplicities and transverse energies at central and nearly central AA collisions at RHIC and LHC. The initial state is computed from perturbative QCD supplemented by the conjecture of saturation of produced partons. The expansion stage is described in terms of hydrodynamics assuming longitudinal boost invariance and azimuthal symmetry. Transverse flow effects, a realistic list of hadrons and resonance decays are included. Comparison with the data of the multiplicities at $\sqrt s=56$ AGeV and 130 AGeV from RHIC is done and predictions for the full RHIC energy and LHC energy are made for the multiplicities and transverse energies. The reduction from the initially released minijet transverse energy to the $E_T$ in the final state is less than in the one-dimensional case but still dramatic: a factor of 2.7 at RHIC, and 3.6 at the LHC.Comment: The results for LHC have been correcte

Pre-thermalization dynamics: initial conditions for QGP at the LHC and RHIC from perturbative QCD

I discuss how the initial conditions for QGP-production in ultrarelativistic heavy ion collisions at the LHC and RHIC can be computed from perturbative QCD.Comment: 11 pages, including 2 figures. A lecture at the International School on the Physics of Quark Gluon Plasma, June 3-6, 1997, Hiroshim

Initial state of the QGP from perturbative QCD + saturation

The production of the initial state of the QGP in very high-energy $AA$ collisions is discussed within the framework of perturbative QCD and saturation. The next-to-leading order computation of the transverse energy of minijets is reviewed. Saturation of parton production, conjectured to occur at a dynamically determinable perturbative scale, leads to estimates of the initial densities. The final state multiplicities are predicted by assuming an isentropic hydrodynamical further evolution. Comparison with RHIC data is shown.Comment: 10 pages, 8 figures. Invited talk at the International Conference on Statistical QCD, August 2001, Bielefel

On the sensitivity of the dijet asymmetry to the physics of jet quenching

The appearance of monojets is among the most striking signature of jet quenching in the context of ultrarelativistic heavy-ion collisions. Experimentally, the disappearance of jets has been quantified by the ATLAS and CMS collaborations in terms of the dijet asymmetry observable A_J. While the experimental findings initially gave rise to claims that the measured A_J would challenge the radiative energy loss paradigm, the results of a systematic investigation of A_J in different models for the medium evolution and for the shower-medium interaction presented here suggest that the observed properties of A_J arise fairly generically and independent of specific model assumptions for a large class of reasonable models. This would imply that rather than posing a challenge to any particular model, the observable prompts the question what model dynamics is not compatible with the data.Comment: 8 pages, 6 figures, added computations of jet R_AA and R=0.2 result

High Energy Nuclear Collisions

Highlights of the results from ultrarelativistic heavy ion collisions at CERN-SPS are reviewed. In particular, I discuss how the experimental results indicate that a collective strongly interacting system has been produced, and what are the implications towards the Quark Gluon Plasma. The physical ideas behind measuring certain observables are introduced. The future program of high energy nuclear collisions at BNL-RHIC and CERN-LHC/ALICE is also briefly discussed.Comment: Plenary talk at the International Europhysics Conference on High Energy Physics, EPS-HEP99, Tampere, Finland, July 1999; 21 page

Minijets in ultrarelativistic heavy ion collisions at future colliders

The role of minijet production as initial conditions for QGP production at $\tau\sim 0.1 fm/c$ in nuclear collisions at the LHC and RHIC energies is discussed.Comment: 15 pages, including 2 figures, an invited article to Comments on Nuclear and Particle Physic

Centrality dependence of multiplicity, transverse energy, and elliptic flow from hydrodynamics

The centrality dependence of the charged multiplicity, transverse energy, and elliptic flow coefficient is studied in a hydrodynamic model, using a variety of different initializations which model the initial energy or entropy production process as a hard or soft process, respectively. While the charged multiplicity depends strongly on the chosen initialization, the p_t-integrated elliptic flow for charged particles as a function of charged particle multiplicity and the p_t-differential elliptic flow for charged particles in minimum bias events turn out to be almost independent of the initial energy density profile.Comment: 11 pages RevTex, including 10 postscript figures. Slightly modified discussion of Figs. 5 and 6, updated references. This version to appear in Nuclear Physics