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ABSTRACT: Transverse hadron spectra from proton–proton, proton–nucleus and nucleus–nucleus collisions from 2 <i?A</i?·GeV to 21.3 <i?A</i?·TeV are investigated within two independent transport approaches (HSD and UrQMD). For central Au+Au (Pb+Pb) collisions at energies above <i?E</i?lab ~ 5 <i?A</i?·GeV, the measured K± transverse mass spectra have a larger inverse slope parameter than expected from the default calculations. The additional pressure — as supported by lattice QCD calculations at finite quark chemical potential μ<i?q</i? and temperature <i?T</i? — might be generated by strong interactions in the early pre-hadronic/partonic phase of central Au+Au (Pb+Pb) collisions E.L. Bratkovskaya et al., <i?Phys. Rev. C</i? <b?69</b? (2004) 054907.
Acta Physica Hungarica 09/2005; 24(1):181-188.
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ABSTRACT: The Omega/Omega ratio originating from string decays is predicted to be larger than unity in proton-proton interactions at SPS energies ( E(lab) = 160 GeV). The antiomega dominance increases with decreasing beam energy. This surprising behavior is caused by the combinatorics of quark-antiquark production in small and low-mass strings. Since this behavior is not found in a statistical description of hadron production in proton-proton collisions, it may serve as a key observable to probe the hadronization mechanism in such collisions.
Physical Review Letters 06/2002; 88(20):202501. · 7.37 Impact Factor
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ABSTRACT: The Omega-bar/Omega ratio originating from string decays is predicted to be larger than unity in proton-proton interaction at SPS energies. The anti-omega dominance increases with decreasing beam energy. This surprising behavior is caused by the combinatorics of quark-antiquark production in small and low-mass strings. Since this behavior is not found in a statistical description of hadron production in proton-proton collisions, it may serve as a key observable to probe the hadronization mechanism in such collisions.
05/2002;
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D. Ardouin,
Sven Soff,
C. Spieles, S. A. Bass,
H. Stocker,
D. Gourio,
S. Schramm,
C. Greiner,
R. Lednicky,
V. L. Lyuboshits,
J. P. Coffin,
C Kuhn
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ABSTRACT: We present a new technique for observing the strange quark matter distillation process based on unlike particle correlations. A simulation is presented based on the scenario of a two-phase thermodynamical evolution model. Comment: 15 pages, 2 figures, 1 table
03/2002;
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ABSTRACT: The Omegabar/Omega ratio originating from string decays is predicted to be larger than unity in proton-proton interactions at SPS energies ( Elab = 160 GeV). The antiomega dominance increases with decreasing beam energy. This surprising behavior is caused by the combinatorics of quark-antiquark production in small and low-mass strings. Since this behavior is not found in a statistical description of hadron production in proton-proton collisions, it may serve as a key observable to probe the hadronization mechanism in such collisions.
Physical Review Letters - PHYS REV LETT. 01/2002; 88(20).
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ABSTRACT: We calculate the Gaussian radius parameters of the pion-emitting source in high-energy heavy-ion collisions, assuming a first-order phase transition from a thermalized quark-gluon plasma (QGP) to a gas of hadrons. Such a model leads to a very long-lived dissipative hadronic rescattering phase which dominates the properties of the two-pion correlation functions. The radii are found to depend only weakly on the thermalization time tau(i), the critical temperature Tc (and thus the latent heat), and the specific entropy of the QGP. The model calculations suggest a rapid increase of R(out)/R(side) as a function of KT if a thermalized QGP were formed.
Physical Review Letters 05/2001; 86(18):3981-4. · 7.37 Impact Factor
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ABSTRACT: The equilibration of hot and dense nuclear matter produced in the central cell of central Au+Au collisions at √s=200AGeV is studied within a microscopic transport model. The pressure in the cell becomes isotropic at t≈5fm/c after beginning of the collision. Within the next 15fm/c the expansion of matter in the cell proceeds almost isentropically with the entropy per baryon ratio S/A≅150, and the equation of state in the (P,ɛ) plane has a very simple form, P=0.15ɛ. Comparison with the statistical model of an ideal hadron gas indicates that the time t≈20fm/c may be too short to reach the fully equilibrated state. Particularly, the creation of long-lived resonance-rich matter in the cell decelerates the relaxation to chemical equilibrium. This resonance-abundant state can be detected experimentally after the thermal freeze out of particles.
Phys. Rev. C. 04/2001; 63(6).
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ABSTRACT: The relaxation of hot nuclear matter to an equilibrated state in the central zone of heavy-ion collisions at energies from AGS to RHIC is studied within the microscopic UrQMD model. It is found that the system reaches the (quasi)equilibrium stage for the period of 10-15 fm/$c$. Within this time the matter in the cell expands nearly isentropically with the entropy to baryon ratio $S/A = 150 - 170$. Thermodynamic characteristics of the system at AGS and at SPS energies at the endpoints of this stage are very close to the parameters of chemical and thermal freeze-out extracted from the thermal fit to experimental data. Predictions are made for the full RHIC energy $\sqrt{s} = 200$ AGeV. The formation of a resonance-rich state at RHIC energies is discussed. Comment: Talk at the conference Quark Matter'2001, 4 pages, to appear in Nucl. Phys. A
04/2001;
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ABSTRACT: We study the local equilibrium in the central $V = 125$ fm$^3$ cell in heavy-ion collisions at energies from 10.7 AGeV (AGS) to 160 AGeV (SPS) calculated in the microscopic transport model. In the present paper the hadron yields and energy spectra in the cell are compared with those of infinite nuclear matter, as calculated within the same model. The agreement between the spectra in the two systems is established for times $t \geq 10$ fm/$c$ in the central cell. The cell results do not deviate noticeably from the infinite matter calculations with rising incident energy, in contrast to the apparent discrepancy with predictions of the statistical model (SM) of an ideal hadron gas. The entropy of this state is found to be very close to the maximum entropy, while hadron abundances and energy spectra differ significantly from those of the SM.
12/2000;
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ABSTRACT: The yields of strange particles are calculated with the UrQMD model for p,Pb(158 AGeV)Pb collisions and compared to experimental data. The yields are enhanced in central collisions if compared to proton induced or peripheral Pb+Pb collisions. The enhancement is due to secondary interactions. Nevertheless, only a reduction of the quark masses or equivalently an increase of the string tension provides an adequate description of the large observed enhancement factors (WA97 and NA49). Furthermore, the yields of unstable strange resonances as the Lambda_star(1520) resonance or the phi meson are considerably affected by hadronic rescattering of the decay products.
11/2000;
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L. V. Bravina,
E. E. Zabrodin,
M. Bleicher, S. A. Bass,
M. Brandstetter,
A. Faessler,
C. Fuchs,
W. Greiner,
M. I. Gorenstein,
S. Soff,
H. Stoecker
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ABSTRACT: The equilibration of hot and dense nuclear matter produced in the central region in central Au+Au collisions at $\sqrt{s}=200$ AGeV is studied within the microscopic transport model UrQMD. The pressure here becomes isotropic at $t \approx 5$ fm/c. Within the next 15 fm/c the expansion of the matter proceeds almost isentropically with the entropy per baryon ratio $S/A \approx 150$. During this period the equation of state in the $(P,\epsilon)$-plane has a very simple form, $P=0.15 \epsilon$. Comparison with the statistical model (SM) of an ideal hadron gas reveals that the time of $\approx 20$ fm/$c$ may be too short to attain the fully equilibrated state. Particularly, the fractions of resonances are overpopulated in contrast to the SM values. The creation of such a long-lived resonance-rich state slows down the relaxation to chemical equilibrium and can be detected experimentally.
11/2000;
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ABSTRACT: We compute bremsstrahlung arising from the acceleration of individual charged baryons and mesons during the time evolution of high-energy Au+Au collisions at the Relativistic Heavy Ion Collider using a microscopic transport model. We elucidate the connection between bremsstrahlung and charge stopping by colliding artificial pure proton on pure neutron nuclei. From the intensity of low energy bremsstrahlung, the time scale and the degree of stopping could be accurately extracted without measuring any hadronic observables. Comment: 25 pages using revtex with 9 embedded EPS figures, modified somewhat the discussion on the method in sect. II B, to appear in Phys. Rev. C
08/2000;
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ABSTRACT: The ultrarelativistic quantum molecular dynamics model (UrQMD) is used to study global observables in central reactions of Au+Au at √s=200A GeV at the Relativistic Heavy Ion Collider (RHIC). Strong stopping governed by massive particle production is predicted if secondary interactions are taken into account. The underlying string dynamics and the early hadronic decoupling implies only small transverse expansion rates. However, rescattering with mesons is found to act as a source of pressure leading to additional flow of baryons and kaons, while cooling down pions.
Phys. Rev. C. 07/2000; 62(2).
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ABSTRACT: The centrality dependence of the antiproton per participant ratio is studied in Pb(160 AGeV)+Pb reactions. Antiproton production in collisions of heavy nuclei at the CERN/SPS seems considerably enhanced as compared to conventional hadronic physics, given by the antiproton production rates in $pp$ and antiproton annihilation in $\bar{p}p$ reactions. This enhancement is consistent with the observation of strong in-medium effects in other hadronic observables and may be an indication of partial restoration of chiral symmetry.
04/2000;
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ABSTRACT: We introduce a combined macroscopic/microscopic transport approach employing relativistic hydrodynamics for the early, dense, deconfined stage of the reaction and a microscopic non-equilibrium model for the later hadronic stage where the equilibrium assumptions are not valid anymore. Within this approach we study the dynamics of hot, bulk QCD matter, which is expected to be created in ultra-relativistic heavy ion collisions at the SPS, the RHIC and the LHC. Our approach is capable of self-consistently calculating the freeze-out of the hadronic system, while accounting for the collective flow on the hadronization hypersurface generated by the QGP expansion. In particular, we perform a detailed analysis of the reaction dynamics, hadronic freeze-out, and transverse flow. Comment: 55 pages, 15 figures
01/2000;
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ABSTRACT: We study J/ reaction kinetics in hadronic environments. The validity of a thermal meson gas ansatz is tested by confronting it with an alternative, nonequilibrium scenario. Heavy-ion collisions are simulated in the framework of the microscopic transport model, the ultrarelativistic quantum molecular dynamics model, taking into account the production of charmonium states through hard parton-parton interactions and subsequent rescattering with secondary hadrons assuming constant dissociation cross sections. The thermal gas and microscopic transport scenarios are shown to be very dissimilar. Estimates of J/ survival probabilities based on thermal models of co-mover interactions in heavy-ion collisions are therefore questionable.
Journal of Physics G Nuclear and Particle Physics 11/1999; 25(12):2351. · 4.18 Impact Factor
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ABSTRACT: The Ultra-relativistic Quantum Molecular Dynamics model (UrQMD) is used to study global observables in central reactions of Au+Au at $\sqrt{s}=200$ AGeV (RHIC). Strong stopping governed by massive particle production is predicted if secondary interactions are taken into account. The underlying string dynamics and the early hadronic decoupling implies only small transverse expansion rates. However, rescattering with mesons is found to act as a source of pressure leading to additional flow of baryons and kaons, while cooling down pions.
11/1999;
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M. Bleicher,
E. Zabrodin,
C. Spieles, S. A. Bass,
C. Ernst,
S. Soff,
L. Bravina,
M. Belkacem,
H Weber,
H. Stöcker,
W. Greiner
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ABSTRACT: Hadron-hadron collisions at high energies are investigated in the Ultra-relativistic-Quantum-Molecular-Dynamics approach (UrQMD). This microscopic transport model is designed to study pp, pA and A+A collisions. It describes the phenomenology of hadronic interactions at low and intermediate energies ($\sqrt s <5$ GeV) in terms of interactions between known hadrons and their resonances. At high energies, $\sqrt s >5$ GeV, the excitation of color strings and their subsequent fragmentation into hadrons dominates the multiple production of particles in the UrQMD model. The model shows a fair overall agreement with a large body of experimental h-h data over a wide range of h-h center-of-mass energies. Hadronic reaction data with higher precision would be useful to support the use of the UrQMD model for relativistic heavy ion collisions.
10/1999;
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ABSTRACT: Charmonium production and absorption in heavy ion collisions is studied with the ultrarelativisitic quantum molecular dynamics model. We compare the scenario of universal and time-independent color-octet dissociation cross sections with one of distinct color-singlet J/ψ, ψ′, and χc states, evolving from small, color transparent configurations to their asymptotic sizes. Both purely hadronic scenarios are compared with the pA and AB data at SPS energies. The predicted rapidity dependence of J/ψ suppression can be used to discriminate between the two scenarios experimentally. The importance of interactions with secondary hadrons and the applicability of thermal reaction kinetics to J/ψ absorption are investigated. We discuss the effect of nuclear stopping and the role of leading hadrons. The dependence of the ψ′/J/ψ ratio on the model assumptions and the possible influence of refeeding processes are also studied.
Phys. Rev. C. 09/1999; 60(5).
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ABSTRACT: Dilepton spectra are calculated within the microscopic transport model UrQMD and compared to data from the CERES experiment. The invariant mass spectra in the region 300 MeV < M < 600 MeV depend strongly on the mass dependence of the $\rho$ meson decay width which is not sufficiently determined by the Vector Meson Dominance model. A consistent explanation of both the recent Pb+Au data and the proton induced data can be given without additional medium effects.
08/1999;
