156,321 research outputs found
Adiabatic evolution on a spatial-photonic Ising machine
Combinatorial optimization problems are crucial for widespread applications
but remain difficult to solve on a large scale with conventional hardware.
Novel optical platforms, known as coherent or photonic Ising machines, are
attracting considerable attention as accelerators on optimization tasks
formulable as Ising models. Annealing is a well-known technique based on
adiabatic evolution for finding optimal solutions in classical and quantum
systems made by atoms, electrons, or photons. Although various Ising machines
employ annealing in some form, adiabatic computing on optical settings has been
only partially investigated. Here, we realize the adiabatic evolution of
frustrated Ising models with 100 spins programmed by spatial light modulation.
We use holographic and optical control to change the spin couplings
adiabatically, and exploit experimental noise to explore the energy landscape.
Annealing enhances the convergence to the Ising ground state and allows to find
the problem solution with probability close to unity. Our results demonstrate a
photonic scheme for combinatorial optimization in analogy with adiabatic
quantum algorithms and enforced by optical vector-matrix multiplications and
scalable photonic technology.Comment: 9 pages, 4 figure
The Power Spectrum, Bias Evolution, and the Spatial Three-Point Correlation Function
We calculate perturbatively the normalized spatial skewness, , and full
three-point correlation function (3PCF), , induced by gravitational
instability of Gaussian primordial fluctuations for a biased tracer-mass
distribution in flat and open cold-dark-matter (CDM) models. We take into
account the dependence on the shape and evolution of the CDM power spectrum,
and allow the bias to be nonlinear and/or evolving in time, using an extension
of Fry's (1996) bias-evolution model. We derive a scale-dependent,
leading-order correction to the standard perturbative expression for in
the case of nonlinear biasing, as defined for the unsmoothed galaxy and
dark-matter fields, and find that this correction becomes large when probing
positive effective power-spectrum indices. This term implies that the inferred
nonlinear-bias parameter, as usually defined in terms of the smoothed density
fields, might depend on the chosen smoothing scale. In general, we find that
the dependence of on the biasing scheme can substantially outweigh that
on the adopted cosmology. We demonstrate that the normalized 3PCF, , is an
ill-behaved quantity, and instead investigate , the variance-normalized
3PCF. The configuration dependence of shows similarly strong
sensitivities to the bias scheme as , but also exhibits significant
dependence on the form of the CDM power spectrum. Though the degeneracy of
with respect to the cosmological parameters and constant linear- and
nonlinear-bias parameters can be broken by the full configuration dependence of
, neither statistic can distinguish well between evolving and non-evolving
bias scenarios. We show that this can be resolved, in principle, by considering
the redshift dependence of .Comment: 41 pages, including 12 Figures. To appear in The Astrophysical
Journal, Vol. 521, #
Colour Reconnection from Soft Gluon Evolution
We consider soft gluon evolution at the amplitude level to expose the
structure of colour reconnection from a perturbative point of view. Considering
the cluster hadronization model and an universal Ansatz for the soft anomalous
dimension we find strong support for geometric models considered earlier. We
also show how reconnection into baryonic systems arises, and how larger cluster
systems evolve. Our results provide the dynamic basis for a new class of colour
reconnection models for cluster hadronization.Comment: 24 pages, 15 figure
Modelling the evolution of cerebral aneurysms: biomechanics, mechanobiology and multiscale modelling
Intracranial aneurysms (IAs) are abnormal dilatations of the cerebral vasculature. Computational modelling may shed light on the aetiology of the disease and lead to improved criteria to assist diagnostic decisions. We briefly review models of aneurysm evolution to date and present a novel fluid-solid-growth (FSG) framework for patient-specific modelling of IA evolution. We illustrate its application to 4 clinical cases depicting an IA. The section of arterial geometry containing the IA is removed and replaced with a cylindrical section: this represents an idealised section of healthy artery upon which IA evolution is simulated. The utilisation of patient-specific geometries enables G&R to be explicitly linked to physiologically realistic spatial distributions and magnitudes of haemodynamic stimuli. In this study, we investigate the hypothesis that elastin degradation is driven by locally low wall shear stress (WSS). In 3 out of 4 cases, the evolved model IA geometry is qualitatively similar to the corresponding in vivo IA geometry. This suggests some tentative support for the hypothesis that low WSS plays a role in the mechanobiology of IA evolution
The Angular Three-Point Correlation Function in the Quasilinear Regime
We calculate the normalized angular three-point correlation function (3PCF),
, as well as the normalized angular skewness, , assuming the
small-angle approximation, for a biased mass distribution in flat and open
cold-dark-matter (CDM) models with Gaussian initial conditions. The
leading-order perturbative results incorporate the explicit dependence on the
cosmological parameters, the shape of the CDM transfer function, the linear
evolution of the power spectrum, the form of redshift distribution function,
and linear and nonlinear biasing, which may be evolving. Results are presented
for different redshift distributions, including that appropriate for the APM
Galaxy Survey, as well as for a survey with a mean redshift of (such as the VLA FIRST Survey). Qualitatively, many of the results found for
and are similar to those obtained in a related treatment of the
spatial skewness and 3PCF (Buchalter & Kamionkowski 1999), such as a
leading-order correction to the standard result for in the case of
nonlinear bias (as defined for unsmoothed density fields), and the sensitivity
of the configuration dependence of to both cosmological and biasing models.
We show that since angular CFs are sensitive to clustering over a range of
redshifts, the various evolutionary dependences included in our predictions
imply that measurements of in a deep survey might better discriminate
between models with different histories, such as evolving vs. non-evolving
bias, that can have similar spatial CFs at low redshift. Our calculations
employ a derived equation---valid for open, closed, and flat models---for
obtaining the angular bispectrum from the spatial bispectrum in the small-angle
approximation.Comment: 45 pages, including 11 Figures, submitted to the Astrophysical
Journa
Extremely red galaxies: dust attenuation and classification
We re-address the classification criterion for extremely red galaxies (ERGs)
of Pozzetti and Mannucci (2000 -- PM00), which aims to separate, in the Ic-K
(or Rc-K) vs. J-K colour--colour diagram, passively evolving, old (> 1 Gyr)
stellar populations in a dust-free environment, associated with ellipticals
(Es), from dusty starburst galaxies (DSGs), both at 1 < z < 2. We explore a
category of objects not considered previously, i.e., galaxies forming in this
redshift range on short (0.1 Gyr) timescales and observed also in their early,
dusty post-starburst phase. We also investigate the impact of structure of the
dusty medium and dust amount on the observed optical/near-IR colours of high-z
DSGs/DPSGs, through multiple-scattering radiative transfer calculations for a
dust/stars configuration and an extinction function calibrated with nearby
dusty starbursts. As a main result, we find that dusty post-starburst galaxies
(DPSGs), with ages between 0.2 and 1 Gyr, at 1.3 < z < 2 mix with Es at 1 < z <
2 for a large range in dust amount. This ``intrusion'' is a source of concern
for the present two-colour classification of ERGs. On the other hand, we
confirm, in agreement with PM00, that DSGs are well separated from Es, both at
1 < z < 2, in the Ic-K vs. J-K colour--colour diagram, whatever the structure
(two-phase clumpy or homogeneous) of their dusty medium and their dust amount
are. This result holds under the new hypothesis of high-z Es being as dusty as
nearby ones. Thus the interpretation of the optical/near-IR colours of high-z
Es may suffer from a multiple degeneracy among age, metallicity, dust and
redshift. We also find that DPSGs at z around 1 mix with DSGs at 1 < z < 2, as
a function of dust amount and structure of the dusty medium. All these results
help explaining the complexity of the ERG classification... (Abridged)Comment: 17 pages, 19 figures, accepted for publication in MNRA
The Chiral Phase Transition in Dissipative Dynamics
Numerical simulations of the chiral phase transition in the (3+1)dimensional
O(4)-model are presented. The evolutions of the chiral field follow purely
dissipative dynamics, starting from random chirally symmetric initial
configurations down to the true vacuum with spontaneously broken symmetry. The
model stabilizes topological textures which are formed together with domains of
disoriented chiral condensate (DCC) during the roll-down phase. The classically
evolving field acts as source for the emission of pions and mesons.
The exponents of power laws for the growth of angular correlations and for
emission rates are extracted. Fluctuations in the abundance ratios for neutral
and charged pions are compared with those for uncorrelated sources as potential
signature for the chiral phase transition after heavy-ion collisions. It is
found that the presence of stabilizing textures (baryons and antibaryons)
prevents sufficiently rapid growth of DCC-domain size, so observability of
anomalous tails in the abundance ratios is unlikely. However, the transient
formation of growing DCC domains causes sizable broadening of the distributions
as compared to the statistical widths of generic sources.Comment: 28 pages, 8 figure
Nonsingular and accelerated expanding universe from effective Yang-Mills theory
The energy-momentum tensor coming from one-parameter effective Yang- Mills
theory is here used to describe the matter-energy content of the homogeneous
and isotropic Friedmann cosmology in its early stages. The behavior of all
solutions is examined. Particularly, it is shown that only solutions
corresponding to an open model allow the universe to evolve into an accelerated
expansion. This result appears as a possible mechanism for an inflationary
phase produced by a vector field. Further, depending on the value of some
parameters characterizing the system, the resulting models are classified as
singular or nonsingular.Comment: 15 pages, 7 figures, some discussions were simplified and new remarks
were introduce
DCC Dynamics in (2+1)D-O(3) model
The dynamics of symmetry-breaking after a quench is numerically simulated on
a lattice for the (2+1)-dimensional O(3) model. In addition to the standard
sigma-model with temperature-dependent Phi^4-potential the energy functional
includes a four-derivative current-current coupling to stabilize the size of
the emerging extended topological textures. The total winding number can be
conserved by constraint. As a model for the chiral phase transition during the
cooling phase after a hadronic collision this allows to investigate the
interference of 'baryon-antibaryon' production with the developing disoriented
aligned domains. The growth of angular correlations, condensate, average
orientation is studied in dependence of texture size, quench rate, symmetry
breaking. The classical dissipative dynamics determines the rate of energy
emitted from the relaxing source for each component of the 3-vector field which
provides a possible signature for domains of Disoriented Chiral Condensate. We
find that the 'pions' are emitted in two distinct pulses; for sufficiently
small lattice size the second one carries the DCC signal, but it is strongly
suppressed as compared to simultaneous 'sigma'-meson emission. We compare the
resulting anomalies in the distributions of DCC pions with probabilities
derived within the commonly used coherent state formalism.Comment: 27 pages, 17 figures; several minor insertions in the text; two
references adde
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