84 research outputs found
Effective Field Theory and Finite Density Systems
This review gives an overview of effective field theory (EFT) as applied at
finite density, with a focus on nuclear many-body systems. Uniform systems with
short-range interactions illustrate the ingredients and virtues of many-body
EFT and then the varied frontiers of EFT for finite nuclei and nuclear matter
are surveyed.Comment: 27 pages, 5 figure
Parallelization of Kinetic Theory Simulations
Numerical studies of shock waves in large scale systems via kinetic
simulations with millions of particles are too computationally demanding to be
processed in serial. In this work we focus on optimizing the parallel
performance of a kinetic Monte Carlo code for astrophysical simulations such as
core-collapse supernovae. Our goal is to attain a flexible program that scales
well with the architecture of modern supercomputers. This approach requires a
hybrid model of programming that combines a message passing interface (MPI)
with a multithreading model (OpenMP) in C++. We report on our approach to
implement the hybrid design into the kinetic code and show first results which
demonstrate a significant gain in performance when many processors are applied.Comment: 10 pages, 3 figures, conference proceeding
Analysis of Boltzmann-Langevin Dynamics in Nuclear Matter
The Boltzmann-Langevin dynamics of harmonic modes in nuclear matter is
analyzed within linear-response theory, both with an elementary treatment and
by using the frequency-dependent response function. It is shown how the source
terms agitating the modes can be obtained from the basic BL correlation kernel
by a simple projection onto the associated dual basis states, which are
proportional to the RPA amplitudes and can be expressed explicitly. The source
terms for the correlated agitation of any two such modes can then be extracted
directly, without consideration of the other modes. This facilitates the
analysis of collective modes in unstable matter and makes it possible to asses
the accuracy of an approximate projection technique employed previously.Comment: 13 latex pages, 4 PS figure
On the elliptical flow in asymmetric collisions and nuclear equation of state
We here present the results of elliptical flow for the collision of different
asymmetric nuclei (10Ne20 +13 Al27, 18Ar40 +21 Sc45, 30Zn64 +28 Ni58, 36Kr86
+41 Nb93) by using the Quantum Molecular Dynamics (QMD) model. General features
of elliptical flow are investigated with the help of theoretical simulations.
The simulations are performed at different beam energies between 40 and 105
MeV/nucleon. A significant change can be seen from in-plane to out-of-plane
elliptical flow of different fragments with incident energy. A comparison with
experimental data is also made. Further, we predict, for the first time that,
elliptical flow for different kind of fragments follow power law dependence ?
C(Atot)? for asymmetric systems
Entangled-State Cycles of Atomic Collective-Spin States
We study quantum trajectories of collective atomic spin states of
effective two-level atoms driven with laser and cavity fields. We show that
interesting ``entangled-state cycles'' arise probabilistically when the (Raman)
transition rates between the two atomic levels are set equal. For odd (even)
, there are () possible cycles. During each cycle the
-qubit state switches, with each cavity photon emission, between the states
, where is a Dicke state in a rotated
collective basis. The quantum number (), which distinguishes the
particular cycle, is determined by the photon counting record and varies
randomly from one trajectory to the next. For even it is also possible,
under the same conditions, to prepare probabilistically (but in steady state)
the Dicke state , i.e., an -qubit state with excitations,
which is of particular interest in the context of multipartite entanglement.Comment: 10 pages, 9 figure
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
Vortex nucleation as a case study of symmetry breaking in quantum systems
Mean-field methods are a very powerful tool for investigating weakly
interacting many-body systems in many branches of physics. In particular, they
describe with excellent accuracy trapped Bose-Einstein condensates. A generic,
but difficult question concerns the relation between the symmetry properties of
the true many-body state and its mean-field approximation. Here, we address
this question by considering, theoretically, vortex nucleation in a rotating
Bose-Einstein condensate. A slow sweep of the rotation frequency changes the
state of the system from being at rest to the one containing one vortex. Within
the mean-field framework, the jump in symmetry occurs through a turbulent phase
around a certain critical frequency. The exact many-body ground state at the
critical frequency exhibits strong correlations and entanglement. We believe
that this constitutes a paradigm example of symmetry breaking in - or change of
the order parameter of - quantum many-body systems in the course of adiabatic
evolution.Comment: Minor change
Vortex arrays in neutral trapped Fermi gases through the BCSâBEC crossover
Vortex arrays in type-II superconductors reflect the translational symmetry of an infinite system. There are cases, however, such as ultracold trapped Fermi gases and the crust of neutron stars, where finite-size effects make it complex to account for the geometrical arrangement of vortices. Here, we self-consistently generate these arrays of vortices at zero and finite temperature through a microscopic description of the non-homogeneous superfluid based on a differential equation for the local order parameter, obtained by coarse graining the Bogoliubovâde Gennes (BdG) equations. In this way, the strength of the inter-particle interaction is varied along the BCSâBEC crossover, from largely overlapping Cooper pairs in the BardeenâCooperâSchrieffer (BCS) limit to dilute composite bosons in the BoseâEinstein condensed (BEC) limit. Detailed comparison with two landmark experiments on ultracold Fermi gases, aimed at revealing the presence of the superfluid phase, brings out several features that make them relevant for other systems in nature as well
Liquid-gas phase transition in nuclear multifragmentation
The equation of state of nuclear matter suggests that at suitable beam
energies the disassembling hot system formed in heavy ion collisions will pass
through a liquid-gas coexistence region. Searching for the signatures of the
phase transition has been a very important focal point of experimental
endeavours in heavy ion collisions, in the last fifteen years. Simultaneously
theoretical models have been developed to provide information about the
equation of state and reaction mechanisms consistent with the experimental
observables. This article is a review of this endeavour.Comment: 63 pages, 27 figures, submitted to Adv. Nucl. Phys. Some typos
corrected, minor text change
Heavy Meson Production in Proton-Nucleus Reactions with Empirical Spectral Functions
We study the production of and mesons in reactions on the basis of empirical spectral functions. The high
momentum, high removal energy part of the spectral function is found to be
negligible in all cases close to the absolute threshold. Furthermore, the
two-step process () dominates the cross section at threshold energies in line with
earlier calculations based on the folding model.Comment: 18 pages, LaTeX, plus 14 postscript figures, submitted to Z. Phys.
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