38,541 research outputs found
Photodisintegration of three- and four- nucleon systems
Three- and four-nucleon photodisintegration processes are quite efficiently
treated by means of effective two-body integral equations in momentum space. We
recall some aspects of their derivation, present previous and most recent
results obtained within this framework, and discuss general features, trends
and effects observed in these investigations: At low energies final-state
interaction plays an important role. Even more pronounced is the effect of
meson exchange currents. A considerable potential dependence shows up in the
low-energy peak region. The different peak heights are found to be closely
correlated with the corresponding binding energies. Above the peak region only
the difference between potentials with or without p-wave contributions remains
relevant. In the differential cross sections the electric quadrupole
contributions have to be taken into account. The remarkable agreement between
theory and experiment in - radiative capture is achieved only when
incorporating this contribution, together with most of the above-mentioned
effects. In the final part of this report we briefly review also methods
developed, and results achieved in three- and four- nucleon
electrodisintegration. We, in particular, compare them with a recent access to
this problem, based on the construction of nucleon-nucleus potentials via
Marchenko inversion theory.Comment: 20 pages LaTeX and 22 postscript figures included, uses epsfig.sty
and espcrc1.sty. Invited talk at the XVth International Conference on
Few-Body Problems in Physics (22-26 July, 1997, Groningen, The Netherlands).
To be published in the conference proceedings in Nucl. Phys.
Thermal Diagnostics with the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory: A Validated Method for Differential Emission Measure Inversions
We present a new method for performing differential emission measure (DEM)
inversions on narrow-band EUV images from the Atmospheric Imaging Assembly
(AIA) onboard the Solar Dynamics Observatory (SDO). The method yields positive
definite DEM solutions by solving a linear program. This method has been
validated against a diverse set of thermal models of varying complexity and
realism. These include (1) idealized gaussian DEM distributions, (2) 3D models
of NOAA Active Region 11158 comprising quasi-steady loop atmospheres in a
non-linear force-free field, and (3) thermodynamic models from a
fully-compressible, 3D MHD simulation of AR corona formation following magnetic
flux emergence. We then present results from the application of the method to
AIA observations of Active Region 11158, comparing the region's thermal
structure on two successive solar rotations. Additionally, we show how the DEM
inversion method can be adapted to simultaneously invert AIA and XRT data, and
how supplementing AIA data with the latter improves the inversion result. The
speed of the method allows for routine production of DEM maps, thus
facilitating science studies that require tracking of the thermal structure of
the solar corona in time and space.Comment: 21 pages, 18 figures, accepted for publication in Ap
Classification of Possible Finite-Time Singularities by Functional Renormalization
Starting from a representation of the early time evolution of a dynamical
system in terms of the polynomial expression of some observable f (t) as a
function of the time variable in some interval 0 < t < T, we investigate how to
extrapolate/forecast in some optimal stability sense the future evolution of
f(t) for time t>T. Using the functional renormalization of Yukalov and Gluzman,
we offer a general classification of the possible regimes that can be defined
based on the sole knowledge of the coefficients of a second-order polynomial
representation of the dynamics. In particular, we investigate the conditions
for the occurence of finite-time singularities from the structure of the time
series, and quantify the critical time and the functional nature of the
singularity when present. We also describe the regimes when a smooth extremum
replaces the singularity and determine its position and amplitude. This extends
previous works by (1) quantifying the stability of the functional
renormalization method more accurately, (2) introducing new global constraints
in terms of moments and (3) going beyond the ``mean-field'' approximation.Comment: Latex document of 18 pages + 7 ps figure
Toward ab initio density functional theory for nuclei
We survey approaches to nonrelativistic density functional theory (DFT) for
nuclei using progress toward ab initio DFT for Coulomb systems as a guide. Ab
initio DFT starts with a microscopic Hamiltonian and is naturally formulated
using orbital-based functionals, which generalize the conventional
local-density-plus-gradients form. The orbitals satisfy single-particle
equations with multiplicative (local) potentials. The DFT functionals can be
developed starting from internucleon forces using wave-function based methods
or by Legendre transform via effective actions. We describe known and
unresolved issues for applying these formulations to the nuclear many-body
problem and discuss how ab initio approaches can help improve empirical energy
density functionals.Comment: 69 pages, 16 figures, many revisions based on feedback. To appear in
Progress in Particle and Nuclear Physic
Velocity relaxation of a particle in a confined compressible fluid
The velocity relaxation of an impulsively forced spherical particle in a
fluid confined by two parallel plane walls is studied using a direct numerical
simulation approach. During the relaxation process, the momentum of the
particle is transmitted in the ambient fluid by viscous diffusion and sound
wave propagation, and the fluid flow accompanied by each mechanism has a
different character and affects the particle motion differently. Because of the
bounding walls, viscous diffusion is hampered, and the accompanying shear flow
is gradually diminished. However, the sound wave is repeatedly reflected and
spreads diffusely. As a result, the particle motion is governed by the sound
wave and backtracks differently in a bulk fluid. The time when the backtracking
of the particle occurs changes non-monotonically with respect to the
compressibility factor and is minimized at the characteristic compressibility
factor. This factor depends on the wall spacing, and the dependence is
different at small and large wall spacing regions based on the different
mechanisms causing the backtracking.Comment: 8 pages, 9 figure
Collisional excitation of doubly and triply deuterated ammonia NDH and ND by H
The availability of collisional rate coefficients is a prerequisite for an
accurate interpretation of astrophysical observations, since the observed media
often harbour densities where molecules are populated under non--LTE
conditions. In the current study, we present calculations of rate coefficients
suitable to describe the various spin isomers of multiply deuterated ammonia,
namely the NDH and ND isotopologues. These calculations are based on
the most accurate NH--H potential energy surface available, which has
been modified to describe the geometrical changes induced by the nuclear
substitutions. The dynamical calculations are performed within the
close--coupling formalism and are carried out in order to provide rate
coefficients up to a temperature of = 50K. For the various
isotopologues/symmetries, we provide rate coefficients for the energy levels
below 100 cm. Subsequently, these new rate coefficients are used
in astrophysical models aimed at reproducing the NHD, NDH and ND
observations previously reported towards the prestellar cores B1b and 16293E.
We thus update the estimates of the corresponding column densities and find a
reasonable agreement with the previous models. In particular, the
ortho--to--para ratios of NHD and NHD are found to be consistent with
the statistical ratios
Automated Code Generation for Lattice Quantum Chromodynamics and beyond
We present here our ongoing work on a Domain Specific Language which aims to
simplify Monte-Carlo simulations and measurements in the domain of Lattice
Quantum Chromodynamics. The tool-chain, called Qiral, is used to produce
high-performance OpenMP C code from LaTeX sources. We discuss conceptual issues
and details of implementation and optimization. The comparison of the
performance of the generated code to the well-established simulation software
is also made
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