16,808 research outputs found
Quasiperiodic spin-orbit motion and spin tunes in storage rings
We present an in-depth analysis of the concept of spin precession frequency
for integrable orbital motion in storage rings. Spin motion on the periodic
closed orbit of a storage ring can be analyzed in terms of the Floquet theorem
for equations of motion with periodic parameters and a spin precession
frequency emerges in a Floquet exponent as an additional frequency of the
system. To define a spin precession frequency on nonperiodic synchro-betatron
orbits we exploit the important concept of quasiperiodicity. This allows a
generalization of the Floquet theorem so that a spin precession frequency can
be defined in this case too. This frequency appears in a Floquet-like exponent
as an additional frequency in the system in analogy with the case of motion on
the closed orbit. These circumstances lead naturally to the definition of the
uniform precession rate and a definition of spin tune. A spin tune is a uniform
precession rate obtained when certain conditions are fulfilled. Having defined
spin tune we define spin-orbit resonance on synchro--betatron orbits and
examine its consequences. We give conditions for the existence of uniform
precession rates and spin tunes (e.g. where small divisors are controlled by
applying a Diophantine condition) and illustrate the various aspects of our
description with several examples. The formalism also suggests the use of
spectral analysis to ``measure'' spin tune during computer simulations of spin
motion on synchro-betatron orbits.Comment: 62 pages, 1 figure. A slight extension of the published versio
From linear to non-linear scales: analytical and numerical predictions for the weak lensing convergence
Weak lensing convergence can be used directly to map and probe the dark mass
distribution in the universe. Building on earlier studies, we recall how the
statistics of the convergence field are related to the statistics of the
underlying mass distribution, in particular to the many-body density
correlations. We describe two model-independent approximations which provide
two simple methods to compute the probability distribution function, pdf, of
the convergence. We apply one of these to the case where the density field can
be described by a log-normal pdf. Next, we discuss two hierarchical models for
the high-order correlations which allow one to perform exact calculations and
evaluate the previous approximations in such specific cases. Finally, we apply
these methods to a very simple model for the evolution of the density field
from linear to highly non-linear scales. Comparisons with the results obtained
from numerical simulations, obtained from a number of different realizations,
show excellent agreement with our theoretical predictions. We have probed
various angular scales in the numerical work and considered sources at 14
different redshifts in each of two different cosmological scenarios, an open
cosmology and a flat cosmology with non-zero cosmological constant. Our
simulation technique employs computations of the full 3-d shear matrices along
the line of sight from the source redshift to the observer and is complementary
to more popular ray-tracing algorithms. Our results therefore provide a
valuable cross-check for such complementary simulation techniques, as well as
for our simple analytical model, from the linear to the highly non-linear
regime.Comment: 20 pages, final version published in MNRA
Design, fabrication, and structural testing of a lightweight shadow shield for deep-space application
Two full-scale, lightweight, double-sheeted shadow shields were developed as the primary element of a deep-space thermal protection system for liquid-hydrogen propellant tankage. The thermal and mechanical considerations used in s, the method of fabrication, and the environmental testing results on a prototype shield are discussed. Testing consisted of a transient cooldown period, a prolonged cold soak, and a transient warmup. The mechanical and thermal analyses used in the shield design are sufficient to produce a lightweight rugged shadow shield assembly that is structurally adequate for its intended application
Contractile stresses in cohesive cell layers on finite-thickness substrates
Using a minimal model of cells or cohesive cell layers as continuum active
elastic media, we examine the effect of substrate thickness and stiffness on
traction forces exerted by strongly adhering cells. We obtain a simple
expression for the length scale controlling the spatial variation of stresses
in terms of cell and substrate parameters that describes the crossover between
the thin and thick substrate limits. Our model is an important step towards a
unified theoretical description of the dependence of traction forces on cell or
colony size, acto-myosin contractility, substrate depth and stiffness, and
strength of focal adhesions, and makes experimentally testable predictions.Comment: 5 pages, 3 figure
Bayesian performance comparison of text classifiers
How can we know whether one classifier is really better than the other? In the area of text classification, since the publication of Yang and Liu's seminal SIGIR-1999 paper, it has become a standard practice for researchers to apply null-hypothesis significance testing (NHST) on their experimental results in order to establish the superiority of a classifier. However, such a frequentist approach has a number of inherent deficiencies and limitations, e.g., the inability to accept the null hypothesis (that the two classifiers perform equally well), the difficulty to compare commonly-used multivariate performance measures like F1 scores instead of accuracy, and so on. In this paper, we propose a novel Bayesian approach to the performance comparison of text classifiers, and argue its advantages over the traditional frequentist approach based on t-test etc. In contrast to the existing probabilistic model for F1 scores which is unpaired, our proposed model takes the correlation between classifiers into account and thus achieves greater statistical power. Using several typical text classification algorithms and a benchmark dataset, we demonstrate that the our approach provides rich information about the difference between two classifiers' performances
Evidence for O(2) universality at the finite temperature transition for lattice QCD with 2 flavours of massless staggered quarks
We simulate lattice QCD with 2 flavours of massless quarks on lattices of
temporal extent N_t=8, to study the finite temperature transition from hadronic
matter to a quark-gluon plasma. A modified action which incorporates an
irrelevant chiral 4-fermion interaction is used, which allows simulations at
zero quark mass. We obtain excellent fits of the chiral condensates to the
magnetizations of a 3-dimensional O(2) spin model on lattices small enough to
model the finite size effects. This gives predictions for correlation lengths
and chiral susceptibilities from the corresponding spin-model quantities. These
are in good agreement with our measurements over the relevant range of
parameters. Binder cumulants are measured, but the errors are too large to draw
definite conclusions. From the properties of the O(2) spin model on the
relatively small lattices with which we fit our `data', we can see why earlier
attempts to fit staggered lattice data to leading-order infinite-volume scaling
functions, as well as finite size scaling studies, failed and led to erroneous
conclusions.Comment: 27 pages, Latex with 10 postscript figures. Some of the discussions
have been expanded to satisfy a referee. Typographical errors were correcte
Critical Exponent for the Density of Percolating Flux
This paper is a study of some of the critical properties of a simple model
for flux. The model is motivated by gauge theory and is equivalent to the Ising
model in three dimensions. The phase with condensed flux is studied. This is
the ordered phase of the Ising model and the high temperature, deconfined phase
of the gauge theory. The flux picture will be used in this phase. Near the
transition, the density is low enough so that flux variables remain useful.
There is a finite density of finite flux clusters on both sides of the phase
transition. In the deconfined phase, there is also an infinite, percolating
network of flux with a density that vanishes as . On
both sides of the critical point, the nonanalyticity in the total flux density
is characterized by the exponent . The main result of this paper is
a calculation of the critical exponent for the percolating network. The
exponent for the density of the percolating cluster is . The specific heat exponent and the crossover exponent
can be computed in the -expansion. Since , the variation in the separate densities is much more rapid than
that of the total. Flux is moving from the infinite cluster to the finite
clusters much more rapidly than the total density is decreasing.Comment: 20 pages, no figures, Latex/Revtex 3, UCD-93-2
Systems comparison of direct and relay link data return modes for advanced planetary missions
Advanced planetary missions using direct and relay link data return mode
Multiple Thresholds in a Model System of Noisy Ion Channels
Voltage-activated ion channels vary randomly between open and closed states,
influenced by the membrane potential and other factors. Signal transduction is
enhanced by noise in a simple ion channel model. The enhancement occurs in a
finite range of signals; the range can be extended using populations of
channels. The range increases more rapidly in multiple-threshold channel
populations than in single-threshold populations. The diversity of ion channels
may thus be present as a strategy to reduce the metabolic costs of handling a
broad class of electrochemical signals.Comment: REVTeX 4, 5 pages, 4 figures; added paragrap
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