14,459 research outputs found
Hierarchy of Full Band Structure Models for Monte Carlo Simulation
This paper discusses the various hierarchy levels that are possible when the full band structure
is considered. At the highest level, the scatterings are treated using complete k-k' transition
rates, which entail extremely memory intensive computational applications. At the
lowest level, the scattering anisotropy is neglected and the scattering rate is considered to be a
constant average value on energy isosurfaces of the bandstructure. This model is more practical
for device simulation. In between the two extremes, it is possible to design intermediate
models which preserve some essential features of both. At all levels of the band structure
hierarchy of models, there are similar issues of numerical noise, related to the sampling of
real and momentum space that the Monte Carlo method necessarily performs with a relatively
small number of particles. We discuss here computationally efficient approaches based on the
assignment of variable weights to the simulated particles, in conjunction with careful gatherscatter
procedures to split particles of large weight and combine particles of small weight
Radiative transfer on hierarchial grids
We present new methods for radiative transfer on hierarchial grids. We
develop a new method for calculating the scattered flux that employs the grid
structure to speed up the computation. We describe a novel subiteration
algorithm that can be used to accelerate calculations with strong dust
temperature self-coupling. We compute two test models, a molecular cloud and a
circumstellar disc, and compare the accuracy and speed of the new algorithms
against existing methods. An adaptive model of the molecular cloud with less
than 8 % of the cells in the uniform grid produced results in good agreement
with the full resolution model. The relative RMS error of the surface
brightness <4 % at all wavelengths, and in regions of high column density the
relative RMS error was only 10^{-4}. Computation with the adaptive model was
faster by a factor of ~5. The new method for calculating the scattered flux is
faster by a factor of ~4 in large models with a deep hierarchy structure, when
images of the scattered light are computed towards several observing
directions. The efficiency of the subiteration algorithm is highly dependent on
the details of the model. In the circumstellar disc test the speed-up was a
factor of two, but much larger gains are possible. The algorithm is expected to
be most beneficial in models where a large number of small, dense regions are
embedded in an environment with a lower mean density.Comment: Accepted to A&A; 13 pages, 8 figures; (v2: minor typos corrected
The ALPS project: open source software for strongly correlated systems
We present the ALPS (Algorithms and Libraries for Physics Simulations)
project, an international open source software project to develop libraries and
application programs for the simulation of strongly correlated quantum lattice
models such as quantum magnets, lattice bosons, and strongly correlated fermion
systems. Development is centered on common XML and binary data formats, on
libraries to simplify and speed up code development, and on full-featured
simulation programs. The programs enable non-experts to start carrying out
numerical simulations by providing basic implementations of the important
algorithms for quantum lattice models: classical and quantum Monte Carlo (QMC)
using non-local updates, extended ensemble simulations, exact and full
diagonalization (ED), as well as the density matrix renormalization group
(DMRG). The software is available from our web server at
http://alps.comp-phys.org.Comment: For full software and introductory turorials see
http://alps.comp-phys.or
Fibril elongation mechanisms of HET-s prion-forming domain: Topological evidence for growth polarity
The prion-forming C-terminal domain of the fungal prion HET-s forms
infectious amyloid fibrils at physiological pH. The conformational switch from
the non-prion soluble form to the prion fibrillar form is believed to have a
functional role, since HET-s in its prion form participates in a recognition
process of different fungal strains. Based on the knowledge of the
high-resolution structure of HET-s(218-289) (the prion forming-domain) in its
fibrillar form, we here present a numerical simulation of the fibril growth
process which emphasizes the role of the topological properties of the
fibrillar structure. An accurate thermodynamic analysis of the way an
intervening HET-s chain is recruited to the tip of the growing fibril suggests
that elongation proceeds through a dock and lock mechanism. First, the chain
docks onto the fibril by forming the longest -strands. Then, the
re-arrangement in the fibrillar form of all the rest of molecule takes place.
Interestingly, we predict also that one side of the HET-s fibril is more
suitable for substaining its growth with respect to the other. The resulting
strong polarity of fibril growth is a consequence of the complex topology of
HET-s fibrillar structure, since the central loop of the intervening chain
plays a crucially different role in favouring or not the attachment of the
C-terminus tail to the fibril, depending on the growth side.Comment: 16 pages, 10 figure
The Long-Baseline Neutrino Experiment: Exploring Fundamental Symmetries of the Universe
The preponderance of matter over antimatter in the early Universe, the
dynamics of the supernova bursts that produced the heavy elements necessary for
life and whether protons eventually decay --- these mysteries at the forefront
of particle physics and astrophysics are key to understanding the early
evolution of our Universe, its current state and its eventual fate. The
Long-Baseline Neutrino Experiment (LBNE) represents an extensively developed
plan for a world-class experiment dedicated to addressing these questions. LBNE
is conceived around three central components: (1) a new, high-intensity
neutrino source generated from a megawatt-class proton accelerator at Fermi
National Accelerator Laboratory, (2) a near neutrino detector just downstream
of the source, and (3) a massive liquid argon time-projection chamber deployed
as a far detector deep underground at the Sanford Underground Research
Facility. This facility, located at the site of the former Homestake Mine in
Lead, South Dakota, is approximately 1,300 km from the neutrino source at
Fermilab -- a distance (baseline) that delivers optimal sensitivity to neutrino
charge-parity symmetry violation and mass ordering effects. This ambitious yet
cost-effective design incorporates scalability and flexibility and can
accommodate a variety of upgrades and contributions. With its exceptional
combination of experimental configuration, technical capabilities, and
potential for transformative discoveries, LBNE promises to be a vital facility
for the field of particle physics worldwide, providing physicists from around
the globe with opportunities to collaborate in a twenty to thirty year program
of exciting science. In this document we provide a comprehensive overview of
LBNE's scientific objectives, its place in the landscape of neutrino physics
worldwide, the technologies it will incorporate and the capabilities it will
possess.Comment: Major update of previous version. This is the reference document for
LBNE science program and current status. Chapters 1, 3, and 9 provide a
comprehensive overview of LBNE's scientific objectives, its place in the
landscape of neutrino physics worldwide, the technologies it will incorporate
and the capabilities it will possess. 288 pages, 116 figure
Monte Carlo Study of Supernova Neutrino Spectra Formation
The neutrino flux and spectra formation in a supernova core is studied by
using a Monte Carlo code. The dominant opacity contribution for nu_mu and
nu_tau is elastic scattering on nucleons. In addition we switch on or off a
variety of processes which allow for the exchange of energy or the creation and
destruction of neutrino pairs, notably nucleon bremsstrahlung, the e^+ e^- pair
annihilation process and nu_e-bar nu_e -> nu_{mu,tau} nu_{mu,tau}-bar, recoil
and weak magnetism in elastic nucleon scattering, elastic scattering on
electrons and positrons and elastic scattering on electron neutrinos and
anti-neutrinos. The least important processes are neutrino-neutrino scattering
and e^+ e^- annihilation. The formation of the spectra and fluxes of nu_mu is
dominated by the nucleonic processes, i.e. bremsstrahlung and elastic
scattering with recoil, but also nu_e nu_e-bar annihilation and nu_mu e^\pm
scattering contribute significantly. When all processes are included, the
spectral shape of the emitted neutrino flux is always ``pinched,'' i.e. the
width of the spectrum is smaller than that of a thermal spectrum with the same
average energy. In all of our cases we find that the average nu_mu-bar energy
exceeds the average nu_e-bar energy by only a small amount, 10% being a typical
number. Weak magnetism effects cause the opacity of nu_mu to differ slightly
from that of nu_mu-bar, translating into differences of the luminosities and
average energies of a few percent. Depending on the density, temperature, and
composition profile, the flavor-dependent luminosities L_{nu_e}$, L_{nu_e-bar},
and L_{nu_mu} can mutually differ from each other by up to a factor of two in
either direction.Comment: 33 pages, 16 eps-figs, submitted to ApJ. Sections added: weak
magnetism, discussion of different analytic fits to the spectra and detailed
spectral shap
A statistical study of the luminosity gap in galaxy groups
The luminosity gap between the two brightest members of galaxy groups and
clusters is thought to offer a strong test for the models of galaxy formation
and evolution. This study focuses on the statistics of the luminosity gap in
galaxy groups, in particular fossil groups, e.g. large luminosity gap, in an
analogy with the same in a cosmological simulation. We use spectroscopic legacy
data of seventh data release (DR7) of SDSS, to extract a volume limited sample
of galaxy groups utilizing modified friends-of-friends (mFoF) algorithm.
Attention is paid to galaxy groups with the brightest group galaxy (BGG) more
luminous than \Mr = -22. An initial sample of 620 groups in which 109 optical
fossil groups, where the luminosity gap exceeds 2 magnitude, were identified.
We compare the statistics of the luminosity gap in galaxy groups at low mass
range from the SDSS with the same in the Millennium simulations where galaxies
are modeled semi-analytically. We show that the BGGs residing in galaxy groups
with large luminosity gap, i.e. fossil groups, are on average brighter and live
in lower mass halos with respect to their counter parts in non-fossil systems.
Although low mass galaxy groups are thought to have recently formed, we show
that in galaxy groups with 15 galaxies brighter than ,
evolutionary process are most likely to be responsible for the large luminosity
gap. We also examine a new probe of finding fossil group. In addition we extend
the recently introduced observational probe based on the luminosity gap, the
butterfly diagram, to galaxy groups and study the probe as a function of halo
mass. This probe can, in conjunction with the luminosity function, help to fine
tune the semi-analytic models of galaxies employed in the cosmological
simulations.Comment: 11 pages, 11 figures, accepted to PASP journa
Probing seed black holes using future gravitational-wave detectors
Identifying the properties of the first generation of seeds of massive black
holes is key to understanding the merger history and growth of galaxies.
Mergers between ~100 solar mass seed black holes generate gravitational waves
in the 0.1-10Hz band that lies between the sensitivity bands of existing
ground-based detectors and the planned space-based gravitational wave detector,
the Laser Interferometer Space Antenna (LISA). However, there are proposals for
more advanced detectors that will bridge this gap, including the third
generation ground-based Einstein Telescope and the space-based detector DECIGO.
In this paper we demonstrate that such future detectors should be able to
detect gravitational waves produced by the coalescence of the first generation
of light seed black-hole binaries and provide information on the evolution of
structure in that era. These observations will be complementary to those that
LISA will make of subsequent mergers between more massive black holes. We
compute the sensitivity of various future detectors to seed black-hole mergers,
and use this to explore the number and properties of the events that each
detector might see in three years of observation. For this calculation, we make
use of galaxy merger trees and two different seed black hole mass distributions
in order to construct the astrophysical population of events. We also consider
the accuracy with which networks of future ground-based detectors will be able
to measure the parameters of seed black hole mergers, in particular the
luminosity distance to the source. We show that distance precisions of ~30% are
achievable, which should be sufficient for us to say with confidence that the
sources are at high redshift.Comment: 14 pages, 6 figures, 2 tables, accepted for proceedings of 13th GWDAW
meetin
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