71 research outputs found
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 and . 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
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 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
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
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 (with =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 ~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 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
- …