180 research outputs found
Hadron production in relativistic nuclear collisions: thermal hadron source or hadronizing quark-gluon plasma?
Measured hadron yields from relativistic nuclear collisions can be equally
well understood in two physically distinct models, namely a static thermal
hadronic source vs.~a time-dependent, nonequilibrium hadronization off a
quark-gluon plasma droplet. Due to the time-dependent particle evaporation off
the hadronic surface in the latter approach the hadron ratios change (by
factors of ) in time. Final particle yields reflect time averages
over the actual thermodynamic properties of the system at a certain stage of
the evolution. Calculated hadron, strangelet and (anti-)cluster yields as well
as freeze-out times are presented for different systems. Due to strangeness
distillation the system moves rapidly out of the T, plane into the
-sector. Strangeness to baryon ratios f_s=1-2 prevail during a
considerable fraction (50%) of the time evolution (i.e. -droplets or
even -droplets form the system at the late stage: The possibility of
observing this time evolution via HBT correlations is discussed). The observed
hadron ratios require and . If
the present model is fit to the extrapolated hadron yields, metastable
hypermatter can only be produced with a probability for .Comment: Submitted to Z. Phys.
Distillation of Strangelets for low initial mu/T
We calculate the evolution of quark-gluon-plasma droplets during the
hadronization in a thermodynamical model. It is speculated that cooling as well
as strangeness enrichment allow for the formation of strangelets even at very
high initial entropy per baryon and low initial
baryon numbers of . It is shown that the
droplet with vanishing initial chemical potential of strange quarks and a very
moderate chemical potential of up/down quarks immediately charges up with
strangeness. Baryon densities of and strange chemical
potentials of ~MeV are reached if strangelets are stable. The
importance of net--baryon and net--strangeness fluctuations for the possible
strangelet formation at RHIC and LHC is emphasized
Can Momentum Correlations Proof Kinetic Equilibration in Heavy Ion Collisions at 160 AGeV?
We perform an event-by-event analysis of the transverse momentum distribution
of final state particles in central Pb(160AGeV)+Pb collisions within a
microscopic non-equilibrium transport model (UrQMD). Strong influence of
rescattering is found. The extracted momentum distributions show less
fluctuations in A+A collisions than in p+p reactions. This is in contrast to
simplified p+p extrapolations and random walk models.Comment: 9 pages, 3 eps figures, submitted to Phys. Lett.
Unlike particle correlations and the strange quark matter distillation process
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 tabl
Dissociation rates of J/psi's with comoving mesons - thermal vs. nonequilibrium scenario
We study J/psi dissociation processes 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 UrQMD, taking into account the production of
charmonium states through hard parton-parton interactions and subsequent
rescattering with hadrons. The thermal gas and microscopic transport scenarios
are shown to be very dissimilar. Estimates of J/psi survival probabilities
based on thermal models of comover interactions in heavy ion collisions are
therefore not reliable.Comment: 12 pages, 6 figure
Creation of strange matter at low initial m/T
We demonstrate that the creation of strange matter is conceivable in the midrapidity region of heavy ion collisions at Brookhaven RHIC and CERN LHC. A finite net-baryon density, abundant (anti)strangeness production, as well as strong net-baryon and net-strangeness fluctuations, provide suitable initial conditions for the formation of strangelets or metastable exotic multistrange ( baryonic) objects. Even at very high initial entropy per baryon SyAinit ¯ 500 and low initial baryon numbers of Ainit B ¯ 30 a quark-gluon-plasma droplet can immediately charge up with strangeness and accumulate net-baryon number. PACS numbers: 25.75.Dw, 12.38.Mh, 24.85.
Direct photons in Pb+Pb at CERN-SPS from microscopic transport theory
Direct photon production in central Pb+Pb collisions at CERN-SPS energy is
calculated within the relativistic microscopic transport model UrQMD, and
within distinctly different 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 pre-equilibrium
contribution dominates the photon spectrum at transverse momenta above 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.Comment: 14 pages, RevTex, 5 Encapsulated-PostScript Figure
``Pressure Equilibration'' in Ultrarelativistic Heavy Ion Collisions
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).Comment: to be published in GSI annual scientific report 1997, psfig style
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