10,709 research outputs found
Searching for statistical equilibrium in a dynamical multifragmentation path
A method for identifying statistical equilibrium stages in dynamical
multifragmentation paths as provided by transport models, already successfully
tested for for the reaction ^{129}Xe+^{119}Sn at 32 MeV/u is applied here to a
higher energy reaction, ^{129}Xe+^{119}Sn at 50 MeV/u. The method evaluates
equilibrium from the point of view of the microcanonical multifragmentation
model (MMM) and reactions are simulated by means of the stochastic mean field
model (SMF). A unique solution, corresponding to the maximum population of the
system phase space, was identified suggesting that a huge part of the available
phase space is occupied even in the case of the 50 MeV/u reaction, in presence
of a considerable amount of radial collective flow. The specific equilibration
time and volume are identified and differences between the two systems are
discussed.Comment: 7 pages, 10 figures, accepted for publication in Physical Review
Isovector Vibrations in Nuclear Matter at Finite Temperature
We consider the propagation and damping of isovector excitations in heated
nuclear matter within the Landau Fermi-liquid theory. Results obtained for
nuclear matter are applied to calculate the Giant Dipole Resonance (GDR) at
finite temperature in heavy spherical nuclei within Steinwedel and Jensen
model.
The centroid energy of the GDR slightly decreases with increasing temperature
and the width increases as for temperatures MeV in agreement with
recent experimental data for GDR in Pb and Sn.
The validity of the method for other Fermi fluids is finally suggested.Comment: gzipped LaTeX file with text: 19 pages, 26 blocks; 3 gzipped *.ps
files with figures: 50 block
Multiphotons and Photon-Jets
We discuss an extension of the Standard Model with a new vector-boson
decaying predominantly into a multi-photon final state through intermediate
light degrees of freedom. The model has a distinctive phase in which the
photons are collimated. As such, they would fail the isolation requirements of
standard multi-photon searches, but group naturally into a novel object, the
photon-jet. Once defined, the photon-jet object facilitates more inclusive
searches for similar phenomena. We present a concrete model, discuss
photon-jets more generally, and outline some strategies that may prove useful
when searching for such objects.Comment: 4 pages, 2 figure
How a General-Purpose Commonsense Ontology can Improve Performance of Learning-Based Image Retrieval
The knowledge representation community has built general-purpose ontologies
which contain large amounts of commonsense knowledge over relevant aspects of
the world, including useful visual information, e.g.: "a ball is used by a
football player", "a tennis player is located at a tennis court". Current
state-of-the-art approaches for visual recognition do not exploit these
rule-based knowledge sources. Instead, they learn recognition models directly
from training examples. In this paper, we study how general-purpose
ontologies---specifically, MIT's ConceptNet ontology---can improve the
performance of state-of-the-art vision systems. As a testbed, we tackle the
problem of sentence-based image retrieval. Our retrieval approach incorporates
knowledge from ConceptNet on top of a large pool of object detectors derived
from a deep learning technique. In our experiments, we show that ConceptNet can
improve performance on a common benchmark dataset. Key to our performance is
the use of the ESPGAME dataset to select visually relevant relations from
ConceptNet. Consequently, a main conclusion of this work is that
general-purpose commonsense ontologies improve performance on visual reasoning
tasks when properly filtered to select meaningful visual relations.Comment: Accepted in IJCAI-1
Statistical analysis of a dynamical multifragmentation path
A microcanonical multifragmentation model (MMM) is used for investigating
whether equilibration really occurs in the dynamical evolution of two heavy ion
collisions simulated via a stochastic mean field approach (SMF). The standard
deviation function between the dynamically obtained freeze-out fragment
distributions corresponding to the reaction Xe+Sn at 32 MeV/u
and the MMM ones corresponding to a wide range of mass, excitation energy,
freeze-out volume and nuclear level density cut-off parameter shows a unique
minimum. A distinct statistically equilibrated stage is identified in the
dynamical evolution of the system.Comment: 5 pages, 3 figure
Robustness of a high-resolution central scheme for hydrodynamic simulations in full general relativity
A recent paper by Lucas-Serrano et al. indicates that a high-resolution
central (HRC) scheme is robust enough to yield accurate hydrodynamical
simulations of special relativistic flows in the presence of ultrarelativistic
speeds and strong shock waves. In this paper we apply this scheme in full
general relativity (involving {\it dynamical} spacetimes), and assess its
suitability by performing test simulations for oscillations of rapidly rotating
neutron stars and merger of binary neutron stars. It is demonstrated that this
HRC scheme can yield results as accurate as those by the so-called
high-resolution shock-capturing (HRSC) schemes based upon Riemann solvers.
Furthermore, the adopted HRC scheme has increased computational efficiency as
it avoids the costly solution of Riemann problems and has practical advantages
in the modeling of neutron star spacetimes. Namely, it allows simulations with
stiff equations of state by successfully dealing with very low-density
unphysical atmospheres. These facts not only suggest that such a HRC scheme may
be a desirable tool for hydrodynamical simulations in general relativity, but
also open the possibility to perform accurate magnetohydrodynamical simulations
in curved dynamic spacetimes.Comment: 4 pages, to be published in Phys. Rev. D (brief report
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