350 research outputs found
Correlations and Equilibration in Relativistic Quantum Systems
In this article we study the time evolution of an interacting field
theoretical system, i.e. \phi^4-field theory in 2+1 space-time dimensions, on
the basis of the Kadanoff-Baym equations for a spatially homogeneous system
including the self-consistent tadpole and sunset self-energies. We find that
equilibration is achieved only by inclusion of the sunset self-energy.
Simultaneously, the time evolution of the scalar particle spectral function is
studied for various initial states. We also compare associated solutions of the
corresponding Boltzmann equation to the full Kadanoff-Baym theory. This
comparison shows that a consistent inclusion of the spectral function has a
significant impact on the equilibration rates only if the width of the spectral
function becomes larger than 1/3 of the particle mass. Furthermore, based on
these findings, the conventional transport of particles in the on-shell
quasiparticle limit is extended to particles of finite life time by means of a
dynamical spectral function A(X,\vec{p},M^2). The off-shell propagation is
implemented in the Hadron-String-Dynamics (HSD) transport code and applied to
the dynamics of nucleus-nucleus collisions.Comment: 20 pages, 7 figures to appear in "Nonequilibrium at short time scales
- Formation of correlations", edited by K. Morawetz, Springer, Berlin (2003),
p16
Heavy quark scattering and quenching in a QCD medium at finite temperature and chemical potential
The heavy quark collisional scattering on partons of the quark gluon plasma
(QGP) is studied in a QCD medium at finite temperature and chemical potential.
We evaluate the effects of finite parton masses and widths, finite temperature
and quark chemical potential on the different elastic cross
sections for dynamical quasi-particles (on- and off-shell particles in the QGP
medium as described by the dynamical quasi-particles model "DQPM") using the
leading order Born diagrams. Our results show clearly the decrease of the
and total elastic cross sections when the temperature and the quark
chemical potential increase. These effects are amplified for finite at
temperatures lower than the corresponding critical temperature .
Using these cross sections we, furthermore, estimate the energy loss and
longitudinal and transverse momentum transfers of a heavy quark propagating in
a finite temperature and chemical potential medium. Accordingly, we have shown
that the transport properties of heavy quarks are sensitive to the temperature
and chemical potential variations. Our results provide some basic ingredients
for the study of charm physics in heavy-ion collisions at Beam Energy Scan
(BES) at RHIC and CBM experiment at FAIR.Comment: 19 pages, 28 figure
Constraint Correlation Dynamics of SU(N) Gauge Theories
A constraint correlation dynamics up to 4-point Green functions is proposed
for SU(N) gauge theories which reduces the N-body quantum field problem to the
two-body level. The resulting set of nonlinear coupled equations fulfills all
conservation laws including fermion number, linear and angular momenta as well
as the total energy. Apart from the conservation laws in the space-time degrees
of freedom the Gauss law is conserved as a quantum expectation value
identically for all times. The same holds for the Ward identities as generated
by commutators of Gauss operators. The constraint dynamical equations are
highly non-perturbative and thus applicable also in the strong coupling regime,
as e.g. low-energy QCD problems.Comment: 26 pages, LATEX, UGI-94-0
Towards the dynamical study of heavy-flavor quarks in the Quark-Gluon-Plasma
Within the aim of a dynamical study of on- and off-shell heavy quarks Q in
the quark gluon plasma (QGP) - as produced in relativistic nucleus-nucleus
collisions - we study the heavy quark collisional scattering on partons of the
QGP. The elastic cross sections are evaluated for perturbative
partons (massless on-shell particles) and for dynamical quasi-particles
(massive off-shell particles as described by the dynamical quasi-particles
model "DQPM") using the leading order Born diagrams. We demonstrate that the
finite width of the quasi-particles in the DQPM has little influence on the
cross sections except close to thresholds. We, furthermore,
calculate the heavy quark relaxation time as a function of temperature T within
the different approaches using these cross sections.Comment: 4 pages, 5 figures, International Conference on Strangeness in Quark
Matter 2013 (SQM 2013
Modelling of compound nucleus formation in fusion of heavy nuclei
A new model that includes the time-dependent dynamics of the single-particle
(s.p.) motion in conjunction with the macroscopic evolution of the system is
proposed for describing the compound nucleus (CN) formation in fusion of heavy
nuclei. The diabaticity initially keeps the entrance system around its contact
configuration, but the gradual transition from the diabatic to the adiabatic
potential energy surface (PES) leads to fusion or quasifission. Direct
measurements of the probability for CN formation are crucial to discriminate
between the current models.Comment: 4 pages,2 figures,1 table, Submitted to PR
Collisional processes of on-shell and off-shell heavy quarks in vacuum and in the Quark-Gluon-Plasma
We study the heavy quark scattering on partons of the quark gluon plasma
(QGP) being especially interested in the collisional (elastic) scattering
processes of heavy quarks on quarks and gluons. We calculate the different
cross sections for perturbative partons (massless on-shell particles in the
vacuum) and for dynamical quasi-particles (off-shell particles in the QGP
medium as described by the dynamical quasi-particles model "DQPM") using the
leading order Born diagrams. Our results show clearly the effect of a finite
parton mass and width on the perturbative elastic cross sections which depend on temperature , energy density ,
the invariant energy and the scattering angle . Our detailed
comparisons demonstrate that the finite width of the quasi-particles in the
DQPM - which encodes the multiple partonic scattering - has little influence on
the cross section for as well as
scattering except close to thresholds. Thus when studying the dynamics of
energetic heavy quarks in a QGP medium the spectral width of the
degrees-of-freedom may be discarded. We have, furthermore, compared the cross
sections from the DQPM with corresponding results from hard-thermal-loop (HTL)
approaches. The HTL inspired models - essentially fixing the regulators by
elementary vacuum cross sections and decay amplitudes instead of properties of
the QGP at finite temperature - provide quite different results especially
w.r.t. the temperature dependence of the and cross sections (in all
settings). Accordingly, the transport properties of heavy quarks will be very
different as a function of temperature when compared to DQPM results.Comment: 28 pages, 32 figure
Transport coefficients of heavy quarks around at finite quark chemical potential
The interactions of heavy quarks with the partonic environment at finite
temperature and finite quark chemical potential are investigated in
terms of transport coefficients within the Dynamical Quasi-Particle model
(DQPM) designed to reproduce the lattice-QCD results (including the partonic
equation of state) in thermodynamic equilibrium. These results are confronted
with those of nuclear many-body calculations close to the critical temperature
. The hadronic and partonic spatial diffusion coefficients join smoothly
and show a pronounced minimum around , at as well as at finite
. Close and above its absolute value matches the lQCD calculations
for . The smooth transition of the heavy quark transport coefficients
from the hadronic to the partonic medium corresponds to a cross over in line
with lattice calculations, and differs substantially from perturbative QCD
(pQCD) calculations which show a large discontinuity at . This indicates
that in the vicinity of dynamically dressed massive partons and not
massless pQCD partons are the effective degrees-of-freedom in the quark-gluon
plasma.Comment: 4 pages, 4 figure
The QGP phase in relativistic heavy-ion collisions
The dynamics of partons, hadrons and strings in relativistic nucleus-nucleus
collisions is analyzed within the novel Parton-Hadron-String Dynamics (PHSD)
transport approach, which is based on a dynamical quasiparticle model for
partons (DQPM) matched to reproduce recent lattice-QCD results - including the
partonic equation of state - in thermodynamic equilibrium. The transition from
partonic to hadronic degrees of freedom is described by covariant transition
rates for the fusion of quark-antiquark pairs or three quarks (antiquarks),
respectively, obeying flavor current-conservation, color neutrality as well as
energy-momentum conservation. The PHSD approach is applied to nucleus-nucleus
collisions from low SIS to RHIC energies. The traces of partonic interactions
are found in particular in the elliptic flow of hadrons as well as in their
transverse mass spectra.Comment: To be published by Springer in Proceedings of the International
Symposium on `Exciting Physics', Makutsi-Range, South Africa, 13-20 November,
201
Open charm and charmonium production at relativistic energies
We calculate open charm and charmonium production in reactions at
= 200 GeV within the hadron-string dynamics (HSD) transport approach
employing open charm cross sections from and reactions that are
fitted to results from PYTHIA and scaled in magnitude to the available
experimental data. Charmonium dissociation with nucleons and formed mesons to
open charm ( pairs) is included dynamically. The 'comover'
dissociation cross sections are described by a simple phase-space model
including a single free parameter, i.e. an interaction strength , that
is fitted to the suppression data for collisions at SPS
energies. As a novel feature we implement the backward channels for charmonium
reproduction by channels employing detailed balance. From our
dynamical calculations we find that the charmonium recreation is comparable to
the dissociation by 'comoving' mesons. This leads to the final result that the
total suppression at = 200 GeV as a function of centrality
is slightly less than the suppression seen at SPS energies by the NA50
Collaboration, where the 'comover' dissociation is substantial and the backward
channels play no role. Furthermore, even in case that all directly produced
mesons dissociate immediately (or are not formed as a mesonic state),
a sizeable amount of charmonia is found asymptotically due to the + meson channels in central collisions of at =
200 GeV which, however, is lower than the yield expected from binary
scaling of collisions.Comment: 42 pages, including 14 eps figures, discussions extended and
references added, to be published in Phys. Rev.
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