16,790 research outputs found
Direct determination of the gauge coupling derivatives for the energy density in lattice QCD
By matching Wilson loop ratios on anisotropic lattices we measure the
coefficients \cs and \ct, which are required for the calculation of the
energy density. The results are compared to that of an indirect method of
determination. We find similar behaviour, the differences are attributed to
different discretization errors.Comment: Talk presented at LATTICE97(finite temperature), 3 pages, 5
Postscript figure
Unplugging the Universe: the neglected electromagnetic consequence of decoupling
This letter concentrates on the non-equilibrium evolution of magnetic field
structures at the onset of recombination, when the charged particle current
densities decay as neutrals are formed.
We consider the effect that a decaying magnetic flux has on the acceleration
of particles via the transient induced electric field. Since the residual
charged-particle number density is small as a result of decoupling, we shall
consider the magnetic and electric fields essentially to be imposed, neglecting
the feedback from any minority accelerated population.
We find that the electromagnetic treatment of this phase transition can
produce energetic electrons scattered throughout the Universe. Such particles
could have a significant effect on cosmic evolution in several ways: (i) their
presence could delay the effective end of the recombination era; (ii) they
could give rise to plasma concentrations that could enhance early gravitational
collapse of matter by opposing cosmic expansion to a greater degree than
neutral matter could; (iii) they could continue to be accelerated, and become
the seed for reionisation at the later epoch .Comment: 4 pages, no figure
Resolving velocity space dynamics in continuum gyrokinetics
Many plasmas of interest to the astrophysical and fusion communities are
weakly collisional. In such plasmas, small scales can develop in the
distribution of particle velocities, potentially affecting observable
quantities such as turbulent fluxes. Consequently, it is necessary to monitor
velocity space resolution in gyrokinetic simulations. In this paper, we present
a set of computationally efficient diagnostics for measuring velocity space
resolution in gyrokinetic simulations and apply them to a range of plasma
physics phenomena using the continuum gyrokinetic code GS2. For the cases
considered here, it is found that the use of a collisionality at or below
experimental values allows for the resolution of plasma dynamics with
relatively few velocity space grid points. Additionally, we describe
implementation of an adaptive collision frequency which can be used to improve
velocity space resolution in the collisionless regime, where results are
expected to be independent of collision frequency.Comment: 20 pages, 11 figures, submitted to Phys. Plasma
Pion and Kaon dissociation in hot quark medium
Pion and kaon dissociation in a medium of hot quark matter is studied in the
Nambu Jona-Lasinio model. The decay width of pion and kaon are found to be
large but finite at temperatures much higher than the so called critical
temperature of chiral or deconfinement transition, kaon decay width being
larger. Consequently, pions and even kaons (with a lower density compared to
pions) should coexist with quarks and gluons at such high temperatures. The
implication of the above result in the study of Quark-Gluon plasma is
discussed.Comment: Latex file with one postscript fil
ShapeFit and ShapeKick for Robust, Scalable Structure from Motion
We introduce a new method for location recovery from pair-wise directions
that leverages an efficient convex program that comes with exact recovery
guarantees, even in the presence of adversarial outliers. When pairwise
directions represent scaled relative positions between pairs of views
(estimated for instance with epipolar geometry) our method can be used for
location recovery, that is the determination of relative pose up to a single
unknown scale. For this task, our method yields performance comparable to the
state-of-the-art with an order of magnitude speed-up. Our proposed numerical
framework is flexible in that it accommodates other approaches to location
recovery and can be used to speed up other methods. These properties are
demonstrated by extensively testing against state-of-the-art methods for
location recovery on 13 large, irregular collections of images of real scenes
in addition to simulated data with ground truth
Model-Independent Semileptonic Form Factors Using Dispersion Relations
We present a method for parametrizing heavy meson semileptonic form factors
using dispersion relations, and from it produce a two-parameter description of
the B -> B elastic form factor. We use heavy quark symmetry to relate this
function to B -> D* l nu form factors, and extract
|V_cb|=0.0355^{+0.0029}_{-0.0025} from experimental data with a least squares
fit. Our method eliminates model-dependent uncertainties inherent in choosing a
parametrization for the extrapolation of the differential decay rate to
threshold.Comment: uses lanlmac(harvmac) and epsf, 12 pages, 1 eps figure included (Talk
by BG at the 6-th International Symposium on Heavy Flavour Physics, Pisa,
Italy, 6--10 June, 1995
Dynamic Energy Management
We present a unified method, based on convex optimization, for managing the
power produced and consumed by a network of devices over time. We start with
the simple setting of optimizing power flows in a static network, and then
proceed to the case of optimizing dynamic power flows, i.e., power flows that
change with time over a horizon. We leverage this to develop a real-time
control strategy, model predictive control, which at each time step solves a
dynamic power flow optimization problem, using forecasts of future quantities
such as demands, capacities, or prices, to choose the current power flow
values. Finally, we consider a useful extension of model predictive control
that explicitly accounts for uncertainty in the forecasts. We mirror our
framework with an object-oriented software implementation, an open-source
Python library for planning and controlling power flows at any scale. We
demonstrate our method with various examples. Appendices give more detail about
the package, and describe some basic but very effective methods for
constructing forecasts from historical data.Comment: 63 pages, 15 figures, accompanying open source librar
The Kepler Light Curves of AGN: A Detailed Analysis
We present a comprehensive analysis of 21 light curves of Type 1 AGN from the
Kepler spacecraft. First, we describe the necessity and development of a
customized pipeline for treating Kepler data of stochastically variable sources
like AGN. We then present the light curves, power spectral density functions
(PSDs), and flux histograms. The light curves display an astonishing variety of
behaviors, many of which would not be detected in ground-based studies,
including switching between distinct flux levels. Six objects exhibit PSD
flattening at characteristic timescales which roughly correlate with black hole
mass. These timescales are consistent with orbital timescales or freefall
accretion timescales. We check for correlations of variability and
high-frequency PSD slope with accretion rate, black hole mass, redshift and
luminosity. We find that bolometric luminosity is anticorrelated with both
variability and steepness of the PSD slope. We do not find evidence of the
linear rms-flux relationships or lognormal flux distributions found in X-ray
AGN light curves, indicating that reprocessing is not a significant contributor
to optical variability at the 0.1-10% level.Comment: 39 pages including 2 appendices. Accepted for Publication in the
Astrophysical Journal, with higher resolution figure
Statistics of opinion domains of the majority-vote model on a square lattice
The existence of juxtaposed regions of distinct cultures in spite of the fact
that people's beliefs have a tendency to become more similar to each other's as
the individuals interact repeatedly is a puzzling phenomenon in the social
sciences. Here we study an extreme version of the frequency-dependent bias
model of social influence in which an individual adopts the opinion shared by
the majority of the members of its extended neighborhood, which includes the
individual itself. This is a variant of the majority-vote model in which the
individual retains its opinion in case there is a tie among the neighbors'
opinions. We assume that the individuals are fixed in the sites of a square
lattice of linear size and that they interact with their nearest neighbors
only.
Within a mean-field framework, we derive the equations of motion for the
density of individuals adopting a particular opinion in the single-site and
pair approximations. Although the single-site approximation predicts a single
opinion domain that takes over the entire lattice, the pair approximation
yields a qualitatively correct picture with the coexistence of different
opinion domains and a strong dependence on the initial conditions. Extensive
Monte Carlo simulations indicate the existence of a rich distribution of
opinion domains or clusters, the number of which grows with whereas the
size of the largest cluster grows with . The analysis of the sizes of
the opinion domains shows that they obey a power-law distribution for not too
large sizes but that they are exponentially distributed in the limit of very
large clusters. In addition, similarly to other well-known social influence
model -- Axelrod's model -- we found that these opinion domains are unstable to
the effect of a thermal-like noise
Diagrammatic approach to coherent backscattering of laser light by cold atoms: Double scattering revisited
We present a diagrammatic derivation of the coherent backscattering spectrum
from two two-level atoms using the pump-probe approach, wherein the multiple
scattering signal is deduced from single-atom responses, and provide a physical
interpretation of the single-atom building blocks.Comment: 16 pages, 7 figure
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