71 research outputs found
Hybrid Stars in an SU(3) Parity Doublet Model
We apply an extended version of the SU(3) parity model, containing quark
degrees of freedom, to study neutron stars. The model successfully reproduces
the main thermodynamic features of QCD which allows us to describe the
composition of dense matter. Chiral symmetry restoration is realized inside the
star and the chiral partners of the baryons appear, their masses becoming
degenerate. Furthermore, quark degrees of freedom appear in a transition to a
deconfined state. Performing an investigation of the macroscopic properties of
neutron stars, we show that observational constraints, like mass and thermal
evolution, are satisfied and new predictions can be made
Fully integrated transport approach to heavy ion reactions with an intermediate hydrodynamic stage
We present a coupled Boltzmann and hydrodynamics approach to relativistic
heavy ion reactions. This hybrid approach is based on the Ultra-relativistic
Quantum Molecular Dynamics (UrQMD) transport approach with an intermediate
hydrodynamical evolution for the hot and dense stage of the collision.
Event-by-event fluctuations are directly taken into account via the
non-equilibrium initial conditions generated by the initial collisions and
string fragmentations in the microscopic UrQMD model. After a (3+1)-dimensional
ideal hydrodynamic evolution, the hydrodynamical fields are mapped to hadrons
via the Cooper-Frye equation and the subsequent hadronic cascade calculation
within UrQMD proceeds to incorporate the important final state effects for a
realistic freeze-out. This implementation allows to compare pure microscopic
transport calculations with hydrodynamic calculations using exactly the same
initial conditions and freeze-out procedure. The effects of the change in the
underlying dynamics - ideal fluid dynamics vs. non-equilibrium transport theory
- will be explored. The freeze-out and initial state parameter dependences are
investigated for different observables. Furthermore, the time evolution of the
baryon density and particle yields are discussed. We find that the final pion
and proton multiplicities are lower in the hybrid model calculation due to the
isentropic hydrodynamic expansion while the yields for strange particles are
enhanced due to the local equilibrium in the hydrodynamic evolution. The
results of the different calculations for the mean transverse mass excitation
function, rapidity and transverse mass spectra for different particle species
at three different beam energies are discussed in the context of the available
data.Comment: 20 pages, 21 figures, 1 additional figure, minor corrections and
revised figures for clarity, version published in PR
(3+1)-Dimensional Hydrodynamic Expansion with a Critical Point from Realistic Initial Conditions
We investigate a (3+1)-dimensional hydrodynamic expansion of the hot and
dense system created in head-on collisions of Pb+Pb/Au+Au at beam energies from
GeV. An equation of state that incorporates a critical end point (CEP)
in line with the lattice data is used. The necessary initial conditions for the
hydrodynamic evolution are taken from a microscopic transport approach (UrQMD).
We compare the properties of the initial state and the full hydrodynamical
calculation with an isentropic expansion employing an initial state from a
simple overlap model. We find that the specific entropy () from both
initial conditions is very similar and only depends on the underlying equation
of state. Using the chiral (hadronic) equation of state we investigate the
expansion paths for both initial conditions. Defining a critical area around
the critical point, we show at what beam energies one can expect to have a
sizable fraction of the system close to the critical point. Finally, we
emphasise the importance of the equation of state of strongly interacting
matter, in the (experimental) search for the CEP.Comment: 8 pages, 8 figure
excitation function: Freeze-out and equation of state dependence
An integrated Boltzmann+hydrodynamics transport approach is applied to
investigate the dependence of the mean transverse mass on the freeze-out and
the equation of state over the energy range from GeV. This
transport approach based on the Ultra-relativistic Quantum Molecular Dynamics
(UrQMD) with an intermediate hydrodynamic stage allows for a systematic
comparison without adjusting parameters. We find that the multiplicities of
pions and protons are rather insensitive to different freeze-out prescriptions
and changes in the equation of state, but the yields are slightly reduced in
the hybrid model calculation compared to a pure transport calculation while the
(anti)kaon multiplicities are increased. The mean transverse mass excitation
functions of all three particle species are found to be sensitive to the
different freeze-out treatments as well as to the equation of state. We find
that the bag model equation of state with a strong first order phase transition
is in qualitative agreement with the experimentally observed step-like
behaviour in the excitation function. The hybrid model with a hadron
gas equation of state leads to a overestimation of the , especially at
higher energies. However, non-equilibrium effects seem also to be substantial
as is suggested by the comparison with standard UrQMD results.Comment: 12 pages, 8 figures, discussion extended, final version to appaer in
J. Phys.
Chemical equilibration due to heavy Hagedorn states
A scenario of heavy resonances, called massive Hagedorn states, is proposed
which exhibits a fast ( fm/c) chemical equilibration of (strange)
baryons and anti-baryons at the QCD critical temperature . For
relativistic heavy ion collisions this scenario predicts that hadronization is
followed by a brief expansion phase during which the equilibration rate is
higher than the expansion rate, so that baryons and antibaryons reach chemical
equilibrium before chemical freeze-out occurs.Comment: 9 pages, 2 figures. Invited talk given at 8th International
Conference on Strangeness in Quark Matter (SQM2004), Cape Town, South Africa,
15-20 September 200
An effective chiral Hadron-Quark Equation of State
We construct an effective model for the QCD equation of state, taking into
account chiral symmetry restoration as well as the deconfinement phase
transition. The correct asymptotic degrees of freedom at the high and low
temperature limits are included (quarks hadrons). The model
shows a rapid crossover for both order parameters, as is expected from lattice
calculations. We then compare the thermodynamic properties of the model at
which turn out to be in qualitative agreement with lattice data,
while apparent quantitative differences can be attributed to hadronic
contributions and excluded volume corrections. Furthermore we discuss the
effects of a repulsive vector type quark interaction at finite baryon number
densities on the resulting phase diagram of the model. Our current model is
able to reproduce a first-order liquid gas phase transition as expected, but
does not show any signs of a first order deconfinement or chiral phase
transition. Both transitions rather appear as a very wide crossover in which
heavily medium modified hadron coexist with free quarks.Comment: 19 pages, 13 figures Version accepted by J. Phys.
Impact of resonance decays on critical point signals in net-proton fluctuations
The non-monotonic beam energy dependence of the higher cumulants of
net-proton fluctuations is a widely studied signature of the conjectured
presence of a critical point in the QCD phase diagram. In this work we study
the effect of resonance decays on critical fluctuations. We show that resonance
effects reduce the signatures of critical fluctuations, but that for reasonable
parameter choices critical effects in the net-proton cumulants survive. The
relative role of resonance decays has a weak dependence on the order of the
cumulants studied with a slightly stronger suppression of critical effects for
higher-order cumulants
Particlization in hybrid models
In hybrid models, which combine hydrodynamical and transport approaches to
describe different stages of heavy-ion collisions, conversion of fluid to
individual particles, particlization, is a non-trivial technical problem. We
describe in detail how to find the particlization hypersurface in a 3+1
dimensional model, and how to sample the particle distributions evaluated using
the Cooper-Frye procedure to create an ensemble of particles as an initial
state for the transport stage. We also discuss the role and magnitude of the
negative contributions in the Cooper-Frye procedure.Comment: 18 pages, 28 figures, EPJA: Topical issue on "Relativistic Hydro- and
Thermodynamics"; version accepted for publication, typos and error in Eq.(1)
corrected, the purpose of sampling and change from UrQMD to fluid clarified,
added discussion why attempts to cancel negative contributions of Cooper-Frye
are not applicable her
Constraining the initial state granularity with bulk observables in Au+Au collisions at GeV
In this paper we conduct a systematic study of the granularity of the initial
state of hot and dense QCD matter produced in ultra-relativistic heavy-ion
collisions and its influence on bulk observables like particle yields,
spectra and elliptic flow. For our investigation we use a hybrid transport
model, based on (3+1)d hydrodynamics and a microscopic Boltzmann transport
approach. The initial conditions are generated by a non-equilibrium hadronic
transport approach and the size of their fluctuations can be adjusted by
defining a Gaussian smoothing parameter . The dependence of the
hydrodynamic evolution on the choices of
and is explored by means of a Gaussian emulator.
To generate particle yields and elliptic flow that are compatible with
experimental data the initial state parameters are constrained to be
fm and fm. In addition, the influence of changes in the
equation of state is studied and the results of our event-by-event calculations
are compared to a calculation with averaged initial conditions. We conclude
that even though the initial state parameters can be constrained by yields and
elliptic flow, the granularity needs to be constrained by other correlation and
fluctuation observables.Comment: 14 pages, 8 figures, updated references, version to appear in J.
Phys.
QCD and strongly coupled gauge theories : challenges and perspectives
We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe
- …