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- Kollisionen schwerer Ionen (8)
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#### Institute

- "Pressure equilibration" in ultrarelativistic heavy ion collisions (1997)
- We study the time scale for pressure equilibration in heavy ion collisions at AGS energies within the three-fluid hydrodynamical model and a microscopic cascade model (UrQMD). We find that kinetic equilibrium is reached in both models after a time of 5 fm/c (center-of-mass time). Thus, observables which are sensitive to the early stage of the reaction differ considerably from the expectations within the instant thermalization scenario (one-fluid hydrodynamical model).

- "Soft'' transverse expansion and flow in a multi-fluid model without phase transition (1997)
- Abstract: We study transverse expansion and directed flow in Au(11AGeV)Au reactions within a multi-fluid dynamical model. Although we do not employ an equation of state (EoS) with a first order phase transition, we find a slow increase of the transverse velocities of the nucleons with time. A similar behaviour can be observed for the directed nucleon flow. This is due to non-equilibrium e ects which also lead to less and slower conversion of longitudinal into transverse momentum. We also show that the proton rapidity distribution at CERN energies, as calculated within this model, agrees well with the preliminary NA44-data.

- Antiflow of nucleons at the softest point of the EoS (1999)
- Report-no: UFTP-492/1999 Journal-ref: Phys.Rev. C61 (2000) 024909 We investigate flow in semi-peripheral nuclear collisions at AGS and SPS energies within macroscopic as well as microscopic transport models. The hot and dense zone assumes the shape of an ellipsoid which is tilted by an angle Theta with respect to the beam axis. If matter is close to the softest point of the equation of state, this ellipsoid expands predominantly orthogonal to the direction given by Theta. This antiflow component is responsible for the previously predicted reduction of the directed transverse momentum around the softest point of the equation of state.

- Critical review of quark gluon plasma signatures (1999)
- Noneequilibrium models (three-fluid hydrodynamics and UrQMD) use to discuss the uniqueness of often proposed experimental signatures for quark matter formation in relativistic heavy ion collisions. It is demonstrated that these two models - although they do treat the most interesting early phase of the collisions quite differently(thermalizing QGP vs. coherent color fields with virtual particles) - both yields a reasonable agreement with a large variety of the available heavy ion data.

- Density perturbations in heavy- ion collisions below the critical point (2004)
- Universality arguments suggest that the chiral phase transition for two massless quark flavors is second-order at baryon-chemical potential µB = 0 [1], which then becomes a crossover for small quark masses.

- Direct emission of multiple strange baryons in ultrarelativistic heavy-ion collisions from the phase boundary (1999)
- We discuss a model for the space-time evolution of ultrarelativistic heavy-ion collisions which employs relativistic hydrodynamics within one region of the forward light-cone, and microscopic transport theory (i.e. UrQMD) in the complement. Our initial condition consists of a quark-gluon plasma which expands hydrodynamically and hadronizes. After hadronization the solution eventually changes from expansion in local equilibrium to free streaming, as determined selfconsistently by the interaction rates between the hadrons and the local expansion rate. We show that in such a scenario the inverse slopes of the mT -spectra of multiple strange baryons ( Xi,Omega) are practically una ected by the purely hadronic stage of the reaction, while the flow of p's and Lambda's increases. Moreover, we find that the rather soft transverse expansion at RHIC energies (due to a first-order phase transition) is not washed out by strong rescattering in the hadronic stage. The earlier kinetic freeze-out as compared to SPS-energies results in similar inverse slopes (of the mT -spectra of the hadrons in the final state) at RHIC and SPS energies.

- Direct photons in Pb+Pb at CERN-SPS from microscopic transport theory (1997)
- Direct photon production in central Pb+Pb collisions at CERN-SPS energy is calculated within the relativistic microscopic transport model UrQMD, and within distinctly di erent versions of relativistic hydrodynamics. We find that in UrQMD the local momentum distributions of the secondaries are strongly elongated along the beam axis initially. Therefore, the preequilibrium contribution dominates the photon spectrum at transverse momenta above H 1.5 GeV. The hydrodynamics prediction of a strong correlation between the temperature and radial expansion velocities on the one hand and the slope of the transverse momentum distribution of direct photons on the other hand thus is not recovered in UrQMD. The rapidity distribution of direct photons in UrQMD reveals that the initial conditions for the longitudinal expansion of the photon source (the meson fluid ) resemble rather boostinvariance than Landau-like flow.

- Distinguishing hadronic cascades from hydrodynamic models in Pb(160 AGeV)+Pb reactions by impact parameter variation (1998)
- We propose to study the impact parameter dependence of the anti-Lambda/anti-Proton ratio in Pb(160AGeV)+Pb reactions. The anti-Lambda/anti-Proton ratio is a sensible tool to distinguish between hadronic cascade models and hydrodynamical models, which incorporate a QGP phase transition.

- Entropy production in collisions of relativistic heavy ions : a signal for quark-gluon plasma phase transition? (1998)
- Entropy production in the compression stage of heavy ion collisions is discussed within three distinct macroscopic models (i.e. generalized RHTA, geometrical overlap model and three-fluid hydrodynamics). We find that within these models \sim 80% or more of the experimentally observed final-state entropy is created in the early stage. It is thus likely followed by a nearly isentropic expansion. We employ an equation of state with a first-order phase transition. For low net baryon density, the entropy density exhibits a jump at the phase boundary. However, the excitation function of the specific entropy per net baryon, S/A, does not reflect this jump. This is due to the fact that for final states (of the compression) in the mixed phase, the baryon density \rho_B increases with \sqrt{s}, but not the temperature T. Calculations within the three-fluid model show that a large fraction of the entropy is produced by nuclear shockwaves in the projectile and target. With increasing beam energy, this fraction of S/A decreases. At \sqrt{s}=20 AGeV it is on the order of the entropy of the newly produced particles around midrapidity. Hadron ratios are calculated for the entropy values produced initially at beam energies from 2 to 200 AGeV.

- Excitation function of entropy and pion production from AGS to SPS energies (1998)
- Entropy production in the initial compression stage of relativistic heavy-ion collisions from AGS to SPS energies is calculated within a three-fluid hydrodynamical model. The entropy per participating net baryon is found to increase smoothly and does not exhibit a jump or a plateau as in the 1-dimensional one-fluid shock model. Therefore, the excess of pions per participating net baryon in nucleus-nucleus collisions as compared to proton-proton reactions also increases smoothly with beam energy.