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

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

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

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

Scaling of transverse energies and multiplicities with atomic number and energy in ultrarelativistic nuclear collisions

We compute how the initial energy density and produced gluon, quark and antiquark numbers scale with atomic number and beam energy in ultrarelativistic heavy ion collisions. The computation is based on the argument that the effect of all momentum scales can be estimated by performing the computation at one transverse momentum scale, the saturation momentum. The initial numbers are converted to final ones by assuming kinetic thermalisation and adiabatic expansion. The main emphasis of the study is at LHC and RHIC energies but it is observed that even at SPS energies this approach leads to results which are not unreasonable: what is usually described as a completely soft nonperturbative process can also be described in terms of gluons and quarks. The key element is the use of the saturation scale.Comment: 13 pages, includes 7 eps-figure

Baryon-to-entropy ratio in very high energy nuclear collisions

We compute as a function of rapidity $y$ the baryon number carried by quarks and antiquarks with $p_T > p_0 \approx$ 2 GeV produced in Pb+Pb collisions at TeV energies. The computation is carried out in lowest order QCD perturbation theory using structure functions compatible with HERA results. At $p_0=2$ GeV the initial gluon density is both transversally saturated and thermalised in the sense that the energy/gluon equals to that of an ideal gas with the same energy density. Even at these high energies the initial net baryon number density at $y=0$ at $\tau=0.1$ fm will be more than the normal nuclear matter density but the baryon-to-entropy ratio is only $(B-\bar B)/S\sim 1/5000$. Further evolution of the system is discussed and the final baryon-to-entropy ratio is estimated.Comment: 19 pages, including 10 ps-figure

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