15,501 research outputs found
The uniqueness of the invariant polarisation-tensor field for spin-1 particles in storage rings
We argue that the invariant tensor field introduced in [1] is unique under
the condition that the invariant spin field is unique, and thereby complete
that part of the discussion in that paper.Comment: 8 page
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
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
Two-component radiation model of the sonoluminescing bubble
Based on the experimental data from Weninger, Putterman & Barber, Phys. Rev.
(E), 54, R2205 (1996), we offer an alternative interpretation of their
experimetal results. A model of sonoluminescing bubble which proposes that the
electromagnetic radiation originates from two sources: the isotropic black body
or bramsstrahlung emitting core and dipole radiation-emitting shell of
accelerated electrons driven by the liquid-bubble interface is outlined.Comment: 5 pages Revtex, submitted to Phys. Rev.
A New and Unifying Approach to Spin Dynamics and Beam Polarization in Storage Rings
With this paper we extend our studies [1] on polarized beams by distilling
tools from the theory of principal bundles. Four major theorems are presented,
one which ties invariant fields with the notion of normal form, one which
allows one to compare different invariant fields, and two that relate the
existence of invariant fields to the existence of certain invariant sets and
relations between them. We then apply the theory to the dynamics of spin-1/2
and spin-1 particles and their density matrices describing statistically the
particle-spin content of bunches. Our approach thus unifies the spin-vector
dynamics from the T-BMT equation with the spin-tensor dynamics and other
dynamics. This unifying aspect of our approach relates the examples elegantly
and uncovers relations between the various underlying dynamical systems in a
transparent way
An Informal Summary of a New Formalism for Classifying Spin-Orbit Systems Using Tools Distilled from the Theory of Bundles
We give an informal summary of ongoing work which uses tools distilled from
the theory of fibre bundles to classify and connect invariant fields associated
with spin motion in storage rings. We mention four major theorems. One ties
invariant fields with the notion of normal form, the second allows comparison
of different invariant fields and the two others tie the existence of invariant
fields to the existence of certain invariant sets. We explain how the theorems
apply to the spin dynamics of spin- and spin- particles. Our approach
elegantly unifies the spin-vector dynamics from the T-BMT equation with the
spin-tensor dynamics and other dynamics and suggests an avenue for addressing
the question of the existence of the invariant spin field.Comment: Based on a presentation at Spin2014, The 21st International Symposium
on Spin Physics, Beijing, China, October 2014. To be published in the
International Journal of Modern Physics, Conference Serie
Optimal Axes of Siberian Snakes for Polarized Proton Acceleration
Accelerating polarized proton beams and storing them for many turns can lead
to a loss of polarization when accelerating through energies where a spin
rotation frequency is in resonance with orbit oscillation frequencies.
First-order resonance effects can be avoided by installing Siberian Snakes in
the ring, devices which rotate the spin by 180 degrees around the snake axis
while not changing the beam's orbit significantly. For large rings, several
Siberian Snakes are required.
Here a criterion will be derived that allows to find an optimal choice of the
snake axes. Rings with super-period four are analyzed in detail, and the HERA
proton ring is used as an example for approximate four-fold symmetry. The
proposed arrangement of Siberian Snakes matches their effects so that all
spin-orbit coupling integrals vanish at all energies and therefore there is no
first-order spin-orbit coupling at all for this choice, which I call snakes
matching. It will be shown that in general at least eight Siberian Snakes are
needed and that there are exactly four possibilities to arrange their axes.
When the betatron phase advance between snakes is chosen suitably, four
Siberian Snakes can be sufficient.
To show that favorable choice of snakes have been found, polarized protons
are tracked for part of HERA-p's acceleration cycle which shows that
polarization is preserved best for the here proposed arrangement of Siberian
Snakes.Comment: 14 pages, 16 figure
Hemostatic function and progressing ischemic stroke: D-dimer predicts early clinical progression
<p><b>Background and Purpose:</b> Early clinical progression of ischemic stroke is common and is associated with increased risk of death and dependency. We hypothesized that activation of the coagulation system is an important contributor in some cases of deterioration. We aimed to characterize alterations in circulating hemostatic markers in patients with progressing stroke.</p>
<p><b>Methods:</b> Consecutive acute ischemic stroke admissions were recruited. Progressing stroke was defined by deterioration in components of the Scandinavian Stroke Scale. Hemostatic markers (coagulation factors VIIc, VIIIc, and IXc, prothrombin fragments 1+2 [F1+2], thrombin-antithrombin complexes [TAT], D- dimer, fibrinogen, von Willebrand factor [vWF] and tissue plasminogen activator) were measured within 24 hours of symptom recognition.</p>
<p><b>Results:</b> Fifty-four (25%) of the 219 patients met criteria for progressing stroke. F1+2 (median 1.28 versus 1.06 nmol/L, P=0.01), TAT (5.28 versus 4.07 mug/L, P lt 0.01), D-dimer ( 443 versus 194 ng/mL, P lt 0.001) and vWF (216 versus 198 IU/dL, P lt 0.05) levels were higher in these patients than in stable/improving patients. In logistic regression analysis, with all important clinical and laboratory variables included, only natural log D-dimer (odds ratio [OR]: 1.87; 95% confidence interval [CI]: 1.38 to 2.54; P=0.0001) and mean arterial blood pressure (OR: 1.26 per 10 mm Hg change; 95% CI: 1.05 to 1.51; P=0.01) remained independent predictors of progressing stroke.</p>
<p><b>Conclusions:</b> There is evidence of excess thrombin generation and fibrin turnover in patients with progressing ischemic stroke. Measurement of D-dimer levels can identify patients at high risk for stroke progression. Further research is required to determine whether such patients benefit from acute interventions aimed at modifying hemostatic function.</p>
Bubble Shape Oscillations and the Onset of Sonoluminescence
An air bubble trapped in water by an oscillating acoustic field undergoes
either radial or nonspherical pulsations depending on the strength of the
forcing pressure. Two different instability mechanisms (the Rayleigh--Taylor
instability and parametric instability) cause deviations from sphericity.
Distinguishing these mechanisms allows explanation of many features of recent
experiments on sonoluminescence, and suggests methods for finding
sonoluminescence in different parameter regimes.Comment: Phys. Rev. Lett., in pres
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