Remnants of Initial Anisotropic High Energy Density Domains in Nucleus-Nucleus Collisions

Anisotropic high energy density domains may be formed at early stages of ultrarelativistic heavy ion collisions, e.g. due to phase transition dynamics or non-equilibrium phenomena like (mini-)jets. Here we investigate hadronic observables resulting from an initially created anisotropic high energy density domain. Based on our studies using a transport model we find that the initial anisotropies are reflected in the freeze-out multiplicity distribution of both pions and kaons due to secondary hadronic rescattering. The anisotropy appears to be stronger for particles at high transverse momenta. The overall kaon multiplicity increases with large fluctuations of local energy densities, while no change has been found in the pion multiplicity.Comment: Submitted to PR

Elliptical flow -- a signature for early pressure in ultrarelativistic nucleus-nucleus collisions

Elliptical energy flow patterns in non-central Au(11.7AGeV) on Au reactions have been studied employing the RQMD model. The strength of these azimuthal asymmetries is calculated comparing the results in two different modes of RQMD (mean field and cascade). It is found that the elliptical flow which is readily observable with current experimental detectors may help to distinguish different reasonable expansion scenarios for baryon-dense matter. The final asymmetries are very sensitive to the pressure at maximum compression, because they involve a partial cancelation between early squeeze-out and subsequent flow in the reaction plane. This cancelation can be expected to occur in a broad energy region covered by the current heavy ion fixed-target programs at BNL and at CERN.Comment: 14 pages LaTeX including 3 postscript figure

Systematic Study of the Kaon to Pion Multiplicity Ratios in Heavy-Ion Collisions

We present a systematic study of the kaon to pion multiplicity ratios (K+/pi+ and K-/pi-) in heavy-ion collisions from AGS to RHIC energy using the Relativistic Quantum Molecular Dynamics (RQMD) model. The model satisfactorily describes the available experimental data on K+/pi+ and K-/pi-. Within the model, we find that the strong increase of the ratios with the number of participants is mainly due to hadronic rescattering of produced mesons with ingoing baryons and their resonances. The enhancement of K/pi in heavy-ion collisions with respect to elementary p+p interactions is larger at AGS energy than SPS energy, and decreases smoothly with bombarding energy. The total multiplicity ratios at RHIC energy are predicted by RQMD to be K+/pi+ = 0.19 and K-/pi- = 0.15.Comment: 10 pages, 8 figures, RevTeX style. A section is added to discuss effects of rope formatio

Evidence of early multi-strange hadron freeze-out in high energy nuclear collisions

Recently reported transverse momentum distributions of strange hadrons produced in Pb(158AGeV) on Pb collisions and corresponding results from the relativistic quantum molecular dynamics (RQMD) approach are examined. We argue that the experimental observations favor a scenario in which multi-strange hadrons are formed and decouple from the system rather early at large energy densities (around 1 GeV/fm$^3$). The systematics of the strange and non-strange particle spectra indicate that the observed transverse flow develops mainly in the late hadronic stages of these reactions.Comment: 4 pages, 4 figure

Enhancement of low-mass dileptons in SPS heavy-ion collisions: possible evidence for dropping rho meson mass in medium

Dilepton production in proton- and nucleus-induced reactions at SPS energies is studied in the relativistic transport model using initial conditions determined by the string dynamics from RQMD. It is found that both the CERES and HELIOS-3 data for dilepton spectra in proton-nucleus reactions can be well described by the conventional mechanism of Dalitz decay and direct vector meson decay. However, to provide a quantitative explanation of the observed dilepton spectra in central S+Au and S+W collisions requires contributions other than these direct decays. Introducing a decrease of vector meson masses in hot dense medium, we find that these heavy-ion data can also be satisfactorily explained. We also give predictions for Pb+Au collisions at 160 GeV/nucleon using current CERES mass resolution and acceptance.Comment: 8 pages, LaTeX, figures available from [email protected], contribution to QM'96, to appear in the proceeding

Excitation Function of Energy Density and Partonic Degrees of Freedom in Relativistic Heavy Ion Collisions

We estimate the energy density pile-up at mid-rapidity in central Pb+Pb collisions from 2 - 200 GeV/nucleon. The energy density is decomposed into hadronic and partonic contributions. A detailed analysis of the collision dynamics in the framework of a microscopic transport model shows the importance of partonic degrees of freedom and rescattering of leading (di)quarks in the early phase of the reaction for lab-energies > 30 GeV/nucleon. In Pb+Pb collisions at 160 GeV/nucleon the energy density reaches up to 4 GeV/fm^3, 95% of which are contained in partonic degrees of freedom.Comment: 10 pages, 4 figure

Fusion of strings vs. percolation and the transition to the quark-gluon plasma

In most of the models of hadronic collisions the number of exchanged colour strings grows with energy and atomic numbers of the projectile and target. At high string densities interaction between them should melt them into the quark-gluon plasma state. It is shown that under certain assumptions about the the string interaction, a phase transition to the quark gluon plasma indeed takes place in the system of many colour strings. It may be of the first or second order (percolation), depending on the particular mechanism of the interaction. The critical string density is about unity in both cases. The critical density may have been already reached in central Pb-Pb collisions at 158 A GeV.Comment: 16 pages, 3 Postscript figure

Thermal analysis of hadron multiplicities from relativistic quantum molecular dynamics

Some questions arising in the application of the thermal model to hadron production in heavy ion collisions are studied. We do so by applying the thermal model of hadron production to particle yields calculated by the microscopic transport model RQMD(v2.3). We study the bias of incomplete information about the final hadronic state on the extraction of thermal parameters.It is found that the subset of particles measured typically in the experiments looks more thermal than the complete set of stable particles. The hadrons which show the largest deviations from thermal behaviour in RQMD(v2.3) are the multistrange baryons and antibaryons. We also looked at the influence of rapidity cuts on the extraction of thermal parameters and found that they lead to different thermal parameters and larger disagreement between the RQMD yields and the thermal model.Comment: 12 pages, 2 figures, uses REVTEX, only misprint and stylistic corrections, to appear in Physical Review

Highly Sensitive Centrality Dependence of Elliptic Flow -- A Novel Signature of the Phase Transition in QCD

Elliptic flow of the hot, dense system which has been created in nucleus-nucleus collisions develops as a response to the initial azimuthal asymmetry of the reaction region. Here it is suggested that the magnitude of this response shows a ``kinky'' dependence on the centrality of collisions for which the system passes through a first-order or rapid transition between quark-gluon plasma and hadronic matter. We have studied the system Pb(158AGeV) on Pb employing a recent version of the transport theoretical approach RQMD and find the conjecture confirmed. The novel phase transition signature may be observable in present and forthcoming experiments at CERN-SPS and at RHIC, the BNL collider.Comment: Version as published in PRL 82 (1999) 2048, title chang

On the Equation of State of Nuclear Matter in 158A GeV Pb+Pb Collisions

Within a hydrodynamical approach we investigate the sensitivity of single inclusive momentum spectra of hadrons in 158A GeV Pb+Pb collisions to three different equations of state of nuclear matter. Two of the equations of state are based on lattice QCD results and include a phase transition to a quark-gluon plasma. The third equation of state has been extracted from the microscopic transport code RQMD under the assumption of complete local thermalization. All three equations of state provide reasonable fits to data taken by the NA44 and NA49 Collaborations. The initial conditions before the evolution of the fireballs and the space-time evolution pictures differ dramatically for the three equations of state when the same freeze-out temperature is used in all calculations. However, the softest of the equations of state results in transverse mass spectra that are too steep in the central rapidity region. We conclude that the transverse particle momenta are determined by the effective softness of the equation of state during the fireball expansion.Comment: 4 pages, including 4 figures and 2 tables. For a PostScript file of the manuscript, you can also goto http://t2.lanl.gov/schlei/eprint.htm