96 research outputs found
Dynamical interpretation of chemical freeze-out in heavy ion collisions
It is demonstrated that there exists a direct correlation between chemical
freeze-out point and the softest point of the equation of state where the
pressure divided by the energy density, , has a minimum.
A dynamical model is given as an example where the passage of the softest point
coincides with the condition for chemical freeze-out, namely an average energy
per hadron 1 GeV. The sensitivity of the result to the equation of
state used is discussed.Comment: 10 pages, 2 figure
Directed Flow of Baryons in Heavy-Ion Collisions
The collective motion of nucleons from high-energy heavy-ion collisions is
analyzed within a relativistic two-fluid model for different equations of state
(EoS). As function of beam energy the theoretical slope parameter F_y of the
differential directed flow is in good agreement with experimental data, when
calculated for the QCD-consistent EoS described by the statistical mixed-phase
model. Within this model, which takes the deconfinement phase transition into
account, the excitation function of the directed flow turns out to be a
smooth function in the whole range from SIS till SPS energies. This function is
close to that for pure hadronic EoS and exhibits no minimum predicted earlier
for a two-phase bag-model EoS. Attention is also called to a possible formation
of nucleon antiflow (F_y < 0) at energies of the order of 100 A GeV.Comment: 7 pages, 5 figure
Theoretical analysis of a possible observation of the chiral magnetic effect in Au + Au collisions within the RHIC beam energy scan program
In terms of the hadron-string-dynamics (HSD) approach we investigate the
correlation function in the azimuthal angle of charged hadrons that is
expected to be sensitive to a signal of local strong parity violation. Our
analysis of Au+Au collisions is based on the recent STAR data within the RHIC
Beam-Energy-Scan (BES) program. The HSD model reasonably reproduces STAR data
for 7.7 GeV, while there are some deviations from the
experiment at the collision energy of 11.5 GeV and an increase of deviations
between theory and experiment at 39 GeV. For reference, the
results for 200 GeV are given as well. The role of the
retarded electromagnetic field is discussed and a compensation effect for the
action of its electric and magnetic components is pointed out. We conclude that
the recent RHIC BES data at 7.7 and 11.5 GeV can be understood
on the hadronic level without involving the idea of a strong parity violation;
however, at 40 GeV and above one needs to take into
consideration explicit partonic (quark-qluon) degrees-of-freedom for a proper
treatment of the dynamics.Comment: 6pages, 3 figures, 1 table; title changed by editor, references
update
Parton/hadron dynamics in heavy-ion collisions at FAIR energies
Recent STAR data for the directed flow of protons, antiprotons and charged pions obtained within the beam energy scan program are analyzed within the Parton-Hadron-String-Dynamics (PHSD/HSD) transport models. Both versions of the kinetic approach are used to clarify the role of partonic degrees of freedom. The PHSD results, simulating a partonic phase and its coexistence with a hadronic one, are roughly consistent with the STAR data. Generally, the semi-qualitative agreement between the measured data and model results supports the idea of a crossover type of quark-hadron transition which softens the nuclear EoS but shows no indication of a first-order phase transition. Furthermore, the directed flow of kaons and antikaons is evaluated in the PHSD/HSD approachesfrom √sNN ≈ 3 - 200 GeV which shows a high sensitivity to hadronic potentials in the FAIR/NICA energy regime √sNN ≤ 8 GeV
Decay anisotropy of e+e- sources from pN and pd collisions
A full calculation of lepton-pair angular characteristics is carried out for e^+e^- pairs created in pp, pn and pd collisions at intermediate energies. It is demonstrated that the proposed new observable, the dilepton decay anisotropy, quite sensitively changes for different sources and may be useful for their disentangling. The relevance of the dilepton decay anisotropy is shown in the context of a puzzling energy behavior for the ratio of the lepton yield from pd to pp reactions as observed at the BEVALAC
Electromagnetic field evolution in relativistic heavy-ion collisions
The hadron string dynamics (HSD) model is generalized to include the creation
and evolution of retarded electromagnetic fields as well as the influence of
the magnetic and electric fields on the quasiparticle propagation. The
time-space structure of the fields is analyzed in detail for non-central Au+Au
collisions at 200 GeV. It is shown that the created magnetic
field is highly inhomogeneous but in the central region of the overlapping
nuclei it changes relatively weakly in the transverse direction. For the impact
parameter 10 fm the maximal magnetic field - perpendicularly to the
reaction plane - is obtained of order 5 for a very short time
0.2 fm/c, which roughly corresponds to the time of a maximal overlap of
the colliding nuclei. We find that at any time the location of the maximum in
the distribution correlates with that of the energy density of the
created particles. In contrast, the electric field distribution, being also
highly inhomogeneous, has a minimum in the center of the overlap region.
Furthermore, the field characteristics are presented as a function of the
collision energy and the centrality of the collisions. To explore the effect of
the back reaction of the fields on hadronic observables a comparison of HSD
results with and without fields is exemplified. Our actual calculations show no
noticeable influence of the electromagnetic fields - created in heavy-ion
collisions - on the effect of the electric charge separation with respect to
the reaction plane.Comment: 17 pages, 22 figures, title changed by editor, accepted for PR
Lattice QCD Constraints on Hybrid and Quark Stars
A QCD-motivated dynamical-quasiparticle model with parameters adjusted to
reproduce the lattice-QCD equation of state is extrapolated from region of high
temperatures and moderate baryonic densities to the domain of high baryonic
densities and zero temperature. The resulting equation of state matched with
realistic hadronic equations of state predicts a phase transition into the
quark phase at higher densities than those reachable in neutron star interiors.
This excludes the possibility of the existence of hybrid (hadron-quark) stars.
Pure quark stars are possible and have low masses, small radii and very high
central densities. Similar results are obtained for a simple bag model with
massive quarks, fitted to reproduce the same lattice results. Self-bound quark
matter is also excluded within these models. Uncertainties in the present
extrapolation re discussed. Comparison with standard bag models is made.Comment: 13 p., 8 figs., 7 tables, Version accepted by Phys. Rev.
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