556 research outputs found
On the temperature dependence of the symmetry energy
We perform large-scale shell model Monte Carlo (SMMC) calculations for many
nuclei in the mass range A=56-65 in the complete pfg_{9/2}d_{5/2} model space
using an effective quadrupole-quadrupole+pairing residual interaction. Our
calculations are performed at finite temperatures between T=0.33-2 MeV. Our
main focus is the temperature dependence of the symmetry energy which we
determine from the energy differences between various isobaric pairs with the
same pairing structure and at different temperatures. Our SMMC studies are
consistent with an increase of the symmetry energy with temperature. We also
investigate possible consequences for core-collapse supernovae events
Bohmian Mechanics at Space-Time Singularities. II. Spacelike Singularities
We develop an extension of Bohmian mechanics by defining Bohm-like
trajectories for quantum particles in a curved background space-time containing
a spacelike singularity. As an example of such a metric we use the
Schwarzschild metric, which contains two spacelike singularities, one in the
past and one in the future. Since the particle world lines are everywhere
timelike or lightlike, particles can be annihilated but not created at a future
spacelike singularity, and created but not annihilated at a past spacelike
singularity. It is argued that in the presence of future (past) spacelike
singularities, there is a unique natural Bohm-like evolution law directed to
the future (past). This law differs from the one in non-singular space-times
mainly in two ways: it involves Fock space since the particle number is not
conserved, and the wave function is replaced by a density matrix. In
particular, we determine the evolution equation for the density matrix, a
pure-to-mixed evolution equation of a quasi-Lindblad form. We have to leave
open whether a curvature cut-off needs to be introduced for this equation to be
well defined.Comment: 42 pages LaTeX, no figures; v2 shortened and minor change
Near-threshold Production in Heavy-ion Collisions
Within a hadronic transport model we study in detail contributions to kaon
yields and momentum spectra from various baryon (resonance)-baryon (resonance)
and interactions in heavy-ion collisions at beam energies near the
free-space kaon production threshold. It is found that the finite lifetime of
baryon resonances affects significantly the shape of kaon spectra, and the high
energy parts of the kaon spectra are dominated by kaons from processes. resonances are found to contribute
about 10\% to the kaon yield. Effects of boosting the Fermi momentum
distributions of the two colliding nuclei into their center of mass frame,
centrality of the reaction as well as the nuclear equation of state on kaon
yields and spectra are also discussed. Model calculations on ,
and spectra for the reaction of Au+Au at GeV are
compared with the experimental data from the KaoS collaboration.Comment: 18 pages, 11 figures available upon request. TAMU preprint #940403
The Role of the Resonance for Kaon Production in Heavy Ion Collisions
The long mean free path of mesons in nuclear matter makes this particle
a suitable messenger for the dynamics of nucleus-nucleus reactions at
intermediate energies (100 MeV to 3 GeV per nucleon). A prerequisite for this
is the knowledge of the elementary production cross sections . Here these cross sections are studied for the first time with the
explicite inclusion of the relevant baryon resonances up to 2 GeV as
intermediate states. The baryon resonances -- and -- are taken into account coherently in the
calculations of the process. (We refer to this
model as the `resonance model'.) Also vector meson
exchange is included. It is shown that the total cross sections for different
channels of the reactions, i.e. , , () and differ not only by absolute values but also by their
energy dependence. This shape differences are due to the mixture of the isospin
with isospin nucleon resonances. However,
this resonance does not give a contribution to the reactions. So the shapes of the total cross sections for different isospin projections are the same. In spite
of this, such cross sections averaged over different isospin projections in the
same multipletComment: ( Replaced with corrections of printing errors in the Table. ) 18
pages, Latex file with 6 figures, 2 figures are included in the text. A
compressed uuencode file for 4 figures is appended. Also available upon
reques
Partial Wave Analysis of
BES data on are presented. The
contribution peaks strongly near threshold. It is fitted with a
broad resonance with mass MeV, width MeV. A broad resonance peaking at 2020 MeV is also required
with width MeV. There is further evidence for a component
peaking at 2.55 GeV. The non- contribution is close to phase
space; it peaks at 2.6 GeV and is very different from .Comment: 15 pages, 6 figures, 1 table, Submitted to PL
Green function techniques in the treatment of quantum transport at the molecular scale
The theoretical investigation of charge (and spin) transport at nanometer
length scales requires the use of advanced and powerful techniques able to deal
with the dynamical properties of the relevant physical systems, to explicitly
include out-of-equilibrium situations typical for electrical/heat transport as
well as to take into account interaction effects in a systematic way.
Equilibrium Green function techniques and their extension to non-equilibrium
situations via the Keldysh formalism build one of the pillars of current
state-of-the-art approaches to quantum transport which have been implemented in
both model Hamiltonian formulations and first-principle methodologies. We offer
a tutorial overview of the applications of Green functions to deal with some
fundamental aspects of charge transport at the nanoscale, mainly focusing on
applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references,
submitted to Springer series "Lecture Notes in Physics
Menus for Feeding Black Holes
Black holes are the ultimate prisons of the Universe, regions of spacetime
where the enormous gravity prohibits matter or even light to escape to
infinity. Yet, matter falling toward the black holes may shine spectacularly,
generating the strongest source of radiation. These sources provide us with
astrophysical laboratories of extreme physical conditions that cannot be
realized on Earth. This chapter offers a review of the basic menus for feeding
matter onto black holes and discusses their observational implications.Comment: 27 pages. Accepted for publication in Space Science Reviews. Also to
appear in hard cover in the Space Sciences Series of ISSI "The Physics of
Accretion onto Black Holes" (Springer Publisher
On the selection of AGN neutrino source candidates for a source stacking analysis with neutrino telescopes
The sensitivity of a search for sources of TeV neutrinos can be improved by
grouping potential sources together into generic classes in a procedure that is
known as source stacking. In this paper, we define catalogs of Active Galactic
Nuclei (AGN) and use them to perform a source stacking analysis. The grouping
of AGN into classes is done in two steps: first, AGN classes are defined, then,
sources to be stacked are selected assuming that a potential neutrino flux is
linearly correlated with the photon luminosity in a certain energy band (radio,
IR, optical, keV, GeV, TeV). Lacking any secure detailed knowledge on neutrino
production in AGN, this correlation is motivated by hadronic AGN models, as
briefly reviewed in this paper.
The source stacking search for neutrinos from generic AGN classes is
illustrated using the data collected by the AMANDA-II high energy neutrino
detector during the year 2000. No significant excess for any of the suggested
groups was found.Comment: 43 pages, 12 figures, accepted by Astroparticle Physic
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