85 research outputs found
Off-shell effects on particle production
We investigate the observable effects of off-shell propagation of nucleons in
heavy-ion collisions at SIS energies. Within a semi-classical BUU transport
model we find a strong enhancement of subthreshold particle production when
off-shell nucleons are propagated.Comment: 11 pages, 3 figure
Particle production in quantum transport theories
The particle production in the intermediate energy heavy ion collisions is
discussed in the framework of the nonequilibrium Green's functions formalism.
The evolution equations of the Green's functions for fermions allows for the
discussion of the off-shell fermion propagator and of the large momentum
component in the initial state. For the case of a homogeneous system numerical
calculations of the meson production rate are performed and compared with the
semiclassical production rate.Comment: 45 pages, figures included, uses FEYNMAN macro
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
Equilibration within a semiclassical off-shell transport approach
Equilibration times for nuclear matter configurations -- modelling
intermediate and high energy nucleus-nucleus collisions -- are evaluated within
the semiclassical off-shell transport approach developed recently. The
transport equations are solved for a finite box in coordinate space employing
periodic boundary conditions. The off-shell transport model is shown to give
proper off-shell equilibrium distributions in the limit for the
nucleon and -resonance spectral functions. We find that equilibration
times within the off-shell approach are only slightly enhanced as compared to
the on-shell limit for the momentum configurations considered.Comment: 19 pages, LaTeX, including 4 postscript figures, submitted to Nucl.
Phys.
Baryon flow at SIS energies
We calculate the baryon flow in the energy range from .25 to
in a relativistic transport model for and
collisions employing various models for the baryon self energies. We find that
to describe the flow data of the FOPI Collaboration the strength of the vector
potential has to be reduced at high relative momentum or at high density such
that the Schr\"odinger- equivalent potential at normal nuclear density
decreases above 1 GeV relative kinetic energy and approaches zero above 2 GeV.Comment: 20 pages, LATEX, 7 PostScript figure
Dilepton production from parton interactions in the early stage of relativistic heavy-ion collisions
We address the dilepton production from the parton interactions in the early
stage of relativistic heavy-ion collisions within the parton-hadron-string
dynamics (PHSD) off-shell transport approach. The description of partons in
PHSD is based on the dynamical quasiparticle model (DQPM) matched to reproduce
lattice QCD results in thermodynamic equilibrium. According to the DQPM the
constituents of the strongly interacting quark-gluon plasma (sQGP) are massive
and off-shell quasi-particles (quarks and gluons) with broad spectral
functions. In order to address the electromagnetic radiation of the sQGP, we
derive off-shell cross sections of q+qbar->gamma*, q+qbar->gamma*+g and
q+g->gamma*+q (qbar+g->gamma*+qbar) reactions taking into account the effective
propagators for quarks and gluons from the DQPM. Dilepton production in In+In
collisions at 158 AGeV and in Au+Au at sqrt(s)=200 GeV is calculated by
implementing these off-shell processes into the PHSD transport approach. By
comparing to the data of the NA60 and PHENIX Collaborations, we study the
relative importance of different dilepton production mechanisms and point out
the regions in phase space where partonic channels are dominant.Comment: Talk presented at the International Nuclear Physics Conference (INPC
2010) in Vancouver, Canada on July 4 - 9, 2010, to be published in
J.Phys.Conf.Se
Self-consistent Approach to Off-Shell Transport
The properties of two forms of the gradient expanded Kadanoff--Baym
equations, i.e. the Kadanoff--Baym and Botermans-Malfliet forms, suitable to
describe the transport dynamics of particles and resonances with broad spectral
widths, are discussed in context of conservation laws, the definition of a
kinetic entropy and the possibility of numerical realization. Recent results on
exact conservations of charge and energy-momentum within Kadanoff-Baym form of
quantum kinetics based on local coupling schemes are extended to two cases
relevant in many applications. These concern the interaction via a finite range
potential, and, relevant in nuclear and hadron physics, e.g. for the
pion--nucleon interaction, the case of derivative coupling.Comment: 35 pages, submitted to issue of Phys. Atom. Nucl. dedicated to S.T.
Belyaev on the occasion of his 80th birthday. Few references are adde
Towards a fully self-consistent spectral function of the nucleon in nuclear matter
We present a calculation of nuclear matter which goes beyond the usual
quasi-particle approximation in that it includes part of the off-shell
dependence of the self-energy in the self-consistent solution of the
single-particle spectrum. The spectral function is separated in contributions
for energies above and below the chemical potential. For holes we approximate
the spectral function for energies below the chemical potential by a
-function at the quasi-particle peak and retain the standard form for
energies above the chemical potential. For particles a similar procedure is
followed. The approximated spectral function is consistently used at all levels
of the calculation. Results for a model calculation are presented, the main
conclusion is that although several observables are affected by the inclusion
of the continuum contributions the physical consistency of the model does not
improve with the improved self-consistency of the solution method. This in
contrast to expectations based on the crucial role of self-consistency in the
proofs of conservation laws.Comment: 26 pages Revtex with 4 figures, submitted to Phys. Rev.
Spectral Function of Quarks in Quark Matter
We investigate the spectral function of light quarks in infinite quark matter
using a simple, albeit self-consistent model. The interactions between the
quarks are described by the SU(2) Nambu--Jona-Lasinio model. Currently mean
field effects are neglected and all calculations are performed in the chirally
restored phase at zero temperature. Relations between correlation functions and
collision rates are used to calculate the spectral function in an iterative
process.Comment: final version, published in PRC; 15 pages, RevTeX
Differential Flow of Protons in Au+Au Collisions at AGS Energies
We study the proton sideward and elliptic differential flow for Au+Au
collisions at AGS energies (2 -- 8 A cdot GeV) in a microscopic relativistic
transport model that includes all baryon resonances up to a mass of 2 GeV as
well as string degrees of freedom for the higher hadronic excitations. In order
to explore the sensitivity of the various differential flows to the nuclear
equation of state (EoS) we use three different parameterizations of the scalar-
and vector mean-fields, i.e. NL2 (soft), NL23 (medium) and NL3 (hard), with
their momentum dependence fitted to the experimental Schrodinger equivalent
potential (at normal nuclear matter density rho_0) up to kinetic energies of 1
GeV. We calculate the excitation function of sideward and elliptic flow within
these parameter sets for Au+Au collisions and compare with the recent data from
the E895 Collaboration as a function of rapidity, impact parameter and
transverse momentum, respectively. We find that the best description of the
differential data is provided by a rather 'stiff' EoS at 2 A cdot GeV (NL3)
while at higher bombarding energies (4--8 A cdot GeV) a 'medium' EoS leads to
the lowest chi^2 with respect to the data. However, the differences in the
transverse and elliptic flows (from the different parameter sets) become of
minor significance at 4--8 A cdot GeV. We attribute this insensitivity to a
similar reduction of the vector potential in all models and to the dominance of
string degrees of freedom at these bombarding energies.Comment: 18 pages, 7 figures, submitted for publicatio
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