We compare multiplicities as well as rapidity and transverse momentum distributions of protons, pions and kaons calculated within presently available transport approaches for heavy ion collisions around 1 AGeV. For this purpose, three reactions have been selected: Au+Au at 1 and 1.48 AGeV and Ni+Ni at 1.93 AGeV.
We investigate hadron production as well as transverse hadron spectra from proton-proton, proton-nucleus, and nucleus-nucleus collisions from 2A GeV to 21.3A TeV within two independent transport approaches, i.e., hadron-string dynamics (HSD) and ultrarelativistic quantum molecular dynamics (UrQMD) that are based on quark, diquark, string, and hadronic degrees of freedom. The comparison to experimental data on transverse mass spectra from pp, pA, and C + C (or Si+ Si) reactions shows the reliability of the transport models for light systems. For central Au+ Au ͑Pb+ Pb͒ collisions at bombarding energies above ϳ5A GeV, furthermore, the measured K ± transverse mass spectra have a larger inverse slope parameter than expected from the default calculations. We investigate various scenarios to explore their potential effects on the K ± spectra. In particular the initial state Cronin effect is found to play a substantial role at top Super Proton Synchrotron (SPS) and Relativistic Heavy Ion Collider (RHIC) energies. However, the maximum in the K + / + ratio at 20-30 A GeV is missed by 40% and the approximately constant slope of the K ± spectra at SPS energies is not reproduced either. Our systematic analysis suggests that the additional pressure-as expected from lattice QCD calculations at finite quark chemical potential q and temperature T-should be generated by strong interactions in the early prehadronic/ partonic phase of central Au+ Au ͑Pb+ Pb͒ collisions.
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 ∼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 ρ B increases with √ 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 √ 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.
We investigate the excitation function of directed flow, which can provide a clear signature of the creation of the QGP and demonstrate that the minimum of the directed flow does not correspond to the softest point of the EoS for isentropic expansion. A novel technique measuring the compactness is introduced to determine the QGP transition in relativistic-heavy ion collisions: The QGP transition will lead to higher compression and therefore to higher compactness of the source in coordinate space. This effect can be observed by pion interferometry. We propose to measure the compactness of the source in the appropriate principal axis frame of the compactness tensor in coordinate space. MotivationThe primary goal for the investigation of heavy-ion collisions is to test the equation of state (EoS) of hot and dense matter far off the ground state, especially with view on possible phase transitions, e.g. to the Quark-Gluon-Plasma (QGP) [1]. Indeed, collective flow phenomena are sensitive indicators for thermodynamically abnormal matter [2]. In the case of a first-order phase transition to a QGP, an isentropic expansion proceeds through a stage of phase coexistence which should lead to signatures in the observables. The first ideas to investigate this phenomenon occured in the mid-seventies [3]. In this paper we investigate the excitation function of directed flow, as well as the compactness in heavy-ion collisions. ModelTo investigate quantitatively the experimental observables, we perform 1-fluid and 3-fluid (3+1)-dimensional relativistic hydrodynamic calculations. That is, we solve numerically the continuity equations for the energy-momentum tensor, ∂ µ T µν = 0, and the net baryon current, ∂ µ N µ B = 0. Detailed discussions of (3+1)-d numerical solutions for hydrodynamical compression and expansion can be found e.g. in [4]. We shall employ two * Invited speaker at CRIS 2000, 3rd Catania Relativistic Ion Studies,
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