Soft transverse expansion in Pb(158 AGeV) on Pb collisions: preequilibrium motion or 1st order phase transition?

Transverse expansion of centrally produced matter in Pb on Pb collisions at beam energies around 158 AGeV appears to be rather `soft'. Two possible reasons -- an extended preequilibrium stage and a first order phase transition from a quark-gluon-plasma into hadronic matter -- are discussed. The softening of transverse expansion caused by preequilibrium dynamics is estimated with the aid of the transport model RQMD which does not contain a first order phase transition. It is found that the anisotropy of transverse flow in non-central reactions is very different in the preequilibrium and hydrodynamic scenarios even if the latter are based on a strong 1st order transition.Comment: 14 pages LaTeX including 3 postscript figure

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

Pion dispersion relation at finite density and temperature

We study the behavior of the pion dispersion relation in a pion medium at finite density and temperature. We introduce a pion chemical potential to describe the finite pion number density and argue that such description is valid during the hadronic phase of a relativistic heavy-ion collision between chemical and thermal freeze-out. We make use of an effective Lagrangian that explicitly respects chiral symmetry through the enforcement of the chiral Ward identities. The pion dispersion relation is computed through the computation of the pion self-energy in a non-perturbative fashion by giving an approximate solution to the Schwinger-Dyson equation for this self-energy. The dispersion relation is described in terms of a density and temperature dependent mass and an index of refraction which is also temperature, density as well as momentum dependent. The index of refraction is larger than unity for all values of the momentum for finite $\mu$ and $T$. We conclude by exploring some of the possible consequences for the propagation of pions through the boundary between the medium and vacuum.Comment: 7 pages, 5 figures, 3 new references, published versio

Flow effects on the freeze-out phase-space density in heavy ion collisions

The strong longitudinal expansion of the reaction zone formed in relativistic heavy-ion collisions is found to significantly reduce the spatially averaged pion phase-space density, compared to naive estimates based on thermal distributions. This has important implications for data interpretation and leads to larger values for the extracted pion chemical potential at kinetic freeze-out.Comment: 5 pages, 3 figures included via epsfig, added discussion of different transverse density profiles, 1 new figur

Temperatures and Non-ideal Expansion in Ultrarelativistic Nucleus-Nucleus Collisions

The hadronic phase space distributions calculated with the transport model RQMD for central S(200 AGeV) on S and Pb(160AGeV) on Pb collisions are analyzed to study the deviations from ideal hydrodynamical evolution. After the preequilibrium stage, which lasts for approximately 4 (2) fm/c in Pb+Pb (S+S) the source stays in approximate kinetic equilibrium for about 2 fm/c at a temperature close to 140 MeV. The interactions of mesons last until around 14 (5) fm/c during which time strong transverse flow is generated. The interactions in the hadronic resonance gas are not sufficiently strong to maintain ideal fluid expansion. While pions acquire average transverse fluid velocities around 0.47-0.58 c, heavier particles like protons and kaons cannot keep up with the pionic fluid, since their average velocities are smaller by about 20 to 30 \%. Although kinetic equilibrium breaks down in the final dilute stage of $AA$ collisions, the system resembles a thermal system at a temperature of 130 MeV, if the free streaming of hadrons after freeze-out is suppressed. This freeze-out temperature is consistent with estimates based on mean free paths and expansion rates in a thermal fireball but lower than values derived from fits to measured particle ratios and transverse momentum spectra. The processes in RQMD to which the differences can be attributed to are the non-ideal expansion of the hadronic matter and the absence of chemical equilibrium at freeze-out.Comment: 12 pages + 3 postscript figures (uuencoded and included

Resolving the Antibaryon-Production Puzzle in High-Energy Heavy-Ion Collisions

We argue that the observed antiproton production in heavy-ion collisions at CERN-SpS energies can be understood if (contrary to most sequential scattering approaches) the backward direction in the process $p\bar p \leftrightarrow \bar{n}\pi$ (with $\bar{n}$=5-6) is consistently accounted for within a thermal framework. Employing the standard picture of subsequent chemical and thermal freezeout, which induces an over-saturation of pion number with associated chemical potentials of $\mu_\pi\simeq$~60-80 MeV, enhances the backward reaction substantially. The resulting rates and corresponding cross sections turn out to be large enough to maintain the abundance of antiprotons at chemical freezeout until the decoupling temperature, in accord with the measured $\bar{p}/p$ ratio in Pb(158AGeV)+Pb collisions.Comment: 4 pages ReVTeX incl. 2 eps-figs, minor changes (two figs added, rate eq. written more explicitly), version accepted for publication in PR

Strangeness in ultrarelativistic nucleus-nucleus collisions

I discuss strangeness production in nucleus-nucleus reactions at ultrarelativistic energies (up to 200 AGeV). In these reactions matter may be created with densities and temperatures in the transition region between quark-gluon plasma (QGP) and hadron gas. Strange anti-baryon enhancement at 200 AGeV and probably even more so at 10 AGeV signals importance of interactions beyond hadron gas dynamics. The systematics of strangeness production indicates that energy and baryon density are key variables while the size of the production volume plays no visible role. Analysis of strangeness appears useful to explore thermalization, flow and the post-equilibrium stage in ultrarelativistic nucleus-nucleus collisions.Comment: 13 pages LaTeX including 6 postscript figures; needs style files espcrc1,floatfig,epsfig. Invited talk presented at 6th International Conference on Nucleus-Nucleus Collisions at Gatlinburg, June 2-6, 1997. To be published in Proceedings in Nuclear Physics

Transverse flow and hadro-chemistry in Au+Au collisions at \sqrt{s_{NN}}=200 GeV

We present a hydrodynamic assessment of preliminary particle spectra observed in Au+Au collisions at \sqrt{s_{NN}}=200 GeV. The hadronic part of the underlying equation of state is based on explicit conservation of (measured) particle ratios throughout the resonance gas stage after chemical freezeout by employing chemical potentials for stable mesons, nucleons and anti-nucleons. We find that under these conditions the data (in particular the proton spectra) favor a low freeze-out temperature of around 100 MeV. Furthermore we show that through inclusion of a moderate pre-hydrodynamic transverse flow field the shape of the spectra improves with respect to the data. The effect of the initial transverse boost on elliptic flow and the freeze-out geometry of the system is also elucidated.Comment: as published: more data included in Fig. 1, discussions throughout the text improved, 6 pages, 4 figure

The freeze-out mechanism and phase-space density in ultrarelativistic heavy-ion collisions

We explore the consequences of a freeze-out criterion for heavy-ion collisions, based on pion escape probabilities from the hot and dense but rapidly expanding collision region. The influence of the expansion and the scattering rate on the escape probability is studied. The temperature dependence of this scattering rate favors a low freeze-out temperature of ~100 MeV. In general, our results support freeze-out along finite four-volumes rather than sharp three-dimensional hypersurfaces, with high-pt particles decoupling earlier from smaller volumes. We compare our approach to the proposed universal freeze-out criteria using the pion phase-space density and its mean free path.Comment: 8 pages, 2 figures, although conclusions are unchanged, the paper has been re-written and the title has been changed for the sake of better presentatio

1+1 Dimensional Hydrodynamics for High-energy Heavy-ion Collisions

A 1+1 dimensional hydrodynamical model in the light-cone coordinates is used to describe central heavy-ion collisions at ultrarelativistic bombarding energies. Deviations from Bjorken's scaling are taken into account by choosing finite-size profiles for the initial energy density. The sensitivity of fluid dynamical evolution to the equation of state and the parameters of initial state is investigated. Experimental constraints on the total energy of produced particles are used to reduce the number of model parameters. Spectra of secondary particles are calculated assuming that the transition from the hydrodynamical stage to the collisionless expansion of matter occurs at a certain freeze-out temperature. An important role of resonances in the formation of observed hadronic spectra is demonstrated. The calculated rapidity distributions of pions, kaons and antiprotons in central Au+Au collisions at the c.m. energy 200 GeV per NN pair are compared with experimental data of the BRAHMS Collaboration. Parameters of the initial state are reconstructed for different choices of the equation of state. The best fit of these data is obtained for a soft equation of state and Gaussian-like initial profiles of the energy density, intermediate between the Landau and Bjorken limits.Comment: 43 pages, 27 figure