1,058 research outputs found
Lensing of ultra-high energy cosmic rays in turbulent magnetic fields
We consider the propagation of ultra high energy cosmic rays through
turbulent magnetic fields and study the transition between the regimes of
single and multiple images of point-like sources. The transition occurs at
energies around , where is the distance traversed by the
CR's with electric charge in the turbulent magnetic field of root mean
square strength and coherence length . We find that above only sources located in a fraction of a few % of the sky can reach large
amplifications of its principal image or start developing multiple images. New
images appear in pairs with huge magnifications, and they remain amplified over
a significant range of energies. At decreasing energies the fraction of the sky
in which sources can develop multiple images increases, reaching about 50% for
. The magnification peaks become however increasingly narrower and for
their integrated effect becomes less noticeable. If a uniform
magnetic field component is also present it would further narrow down the
peaks, shrinking the energy range in which they can be relevant. Below some kind of scintillation regime is reached, where many demagnified
images of a source are present but with overall total magnification of order
unity. We also search for lensing signatures in the AGASA data studying
two-dimensional correlations in angle and energy and find some interesting
hints.Comment: 30 pages, 16 figures, final version with minor change
R-matrix theory of driven electromagnetic cavities
Resonances of cylindrical symmetric microwave cavities are analyzed in
R-matrix theory which transforms the input channel conditions to the output
channels. Single and interfering double resonances are studied and compared
with experimental results, obtained with superconducting microwave cavities.
Because of the equivalence of the two-dimensional Helmholtz and the stationary
Schroedinger equations, the results present insight into the resonance
structure of regular and chaotic quantum billiards.Comment: Revtex 4.
Classical Infinite-Range-Interaction Heisenberg Ferromagnetic Model: Metastability and Sensitivity to Initial Conditions
A N-sized inertial classical Heisenberg ferromagnet, which consists in a
modification of the well-known standard model, where the spins are replaced by
classical rotators, is studied in the limit of infinite-range interactions. The
usual canonical-ensemble mean-field solution of the inertial classical
-vector ferromagnet (for which recovers the particular Heisenberg
model considered herein) is briefly reviewed, showing the well-known
second-order phase transition. This Heisenberg model is studied numerically
within the microcanonical ensemble, through molecular dynamics.Comment: 18 pages text, and 7 EPS figure
The clustering of ultra-high energy cosmic rays and their sources
The sky distribution of cosmic rays with energies above the 'GZK cutoff'
holds important clues to their origin. The AGASA data, although consistent with
isotropy, shows evidence for small-angle clustering, and it has been argued
that such clusters are aligned with BL Lacertae objects, implicating these as
sources. It has also been suggested that clusters can arise if the cosmic rays
come from the decays of very massive relic particles in the Galactic halo, due
to the expected clumping of cold dark matter. We examine these claims and show
that both are in fact not justified.Comment: 13 pages, 8 figures, version in press at Phys. Rev.
Low-Energy Universality in Atomic and Nuclear Physics
An effective field theory developed for systems interacting through
short-range interactions can be applied to systems of cold atoms with a large
scattering length and to nucleons at low energies. It is therefore the ideal
tool to analyze the universal properties associated with the Efimov effect in
three- and four-body systems. In this "progress report", we will discuss recent
results obtained within this framework and report on progress regarding the
inclusion of higher order corrections associated with the finite range of the
underlying interaction.Comment: Commissioned article for Few-Body Systems, 47 pp, 16 fig
Critical perspectives on ‘consumer involvement’ in health research: epistemological dissonance and the know-do gap
Researchers in the area of health and social care (both in Australia and internationally) are encouraged to involve consumers throughout the research process, often on ethical, political and methodological grounds, or simply as ‘good practice’. This paper presents findings from a qualitative study in the UK of researchers’ experiences and views of consumer involvement in health research.
Two main themes are presented in the paper. Firstly, we explore the ‘know-do gap’ which relates to the tensions between researchers’ perceptions of the potential benefits of, and their actual practices in relation to, consumer involvement. Secondly, we focus on one of the reasons for this ‘know-do gap’, namely epistemological dissonance. Findings are linked to issues around consumerism in research, lay/professional knowledges, the (re)production of professional and consumer identities and the maintenance of boundaries between consumers and researchers
Templates for Convex Cone Problems with Applications to Sparse Signal Recovery
This paper develops a general framework for solving a variety of convex cone
problems that frequently arise in signal processing, machine learning,
statistics, and other fields. The approach works as follows: first, determine a
conic formulation of the problem; second, determine its dual; third, apply
smoothing; and fourth, solve using an optimal first-order method. A merit of
this approach is its flexibility: for example, all compressed sensing problems
can be solved via this approach. These include models with objective
functionals such as the total-variation norm, ||Wx||_1 where W is arbitrary, or
a combination thereof. In addition, the paper also introduces a number of
technical contributions such as a novel continuation scheme, a novel approach
for controlling the step size, and some new results showing that the smooth and
unsmoothed problems are sometimes formally equivalent. Combined with our
framework, these lead to novel, stable and computationally efficient
algorithms. For instance, our general implementation is competitive with
state-of-the-art methods for solving intensively studied problems such as the
LASSO. Further, numerical experiments show that one can solve the Dantzig
selector problem, for which no efficient large-scale solvers exist, in a few
hundred iterations. Finally, the paper is accompanied with a software release.
This software is not a single, monolithic solver; rather, it is a suite of
programs and routines designed to serve as building blocks for constructing
complete algorithms.Comment: The TFOCS software is available at http://tfocs.stanford.edu This
version has updated reference
Asteroseismology of Eclipsing Binary Stars in the Kepler Era
Eclipsing binary stars have long served as benchmark systems to measure
fundamental stellar properties. In the past few decades, asteroseismology - the
study of stellar pulsations - has emerged as a new powerful tool to study the
structure and evolution of stars across the HR diagram. Pulsating stars in
eclipsing binary systems are particularly valuable since fundamental properties
(such as radii and masses) can determined using two independent techniques.
Furthermore, independently measured properties from binary orbits can be used
to improve asteroseismic modeling for pulsating stars in which mode
identifications are not straightforward. This contribution provides a review of
asteroseismic detections in eclipsing binary stars, with a focus on space-based
missions such as CoRoT and Kepler, and empirical tests of asteroseismic scaling
relations for stochastic ("solar-like") oscillations.Comment: 28 pages, 12 figures, 2 tables; Proceedings of the AAS topical
conference "Giants of Eclipse" (AASTCS-3), July 28 - August 2 2013, Monterey,
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