651 research outputs found
Axion-photon Couplings in Invisible Axion Models
We reexamine the axion-photon couplings in various invisible axion models
motivated by the recent proposal of using optical interferometry at the ASST
facility in the SSCL to search for axion. We illustrate that the assignment of
charges for the fermion fields plays an important role in
determining the couplings. Several simple non-minimal invisible axion models
with suppressed and enhanced axion-photon couplings are constructed,
respectively. We also discuss the implications of possible new experiments to
detect solar axions by conversion to -rays in a static magnetic apparatus
tracking the sun.Comment: 14 pages, LaTeX fil
Attentive Learning of Sequential Handwriting Movements: A Neural Network Model
Defense Advanced research Projects Agency and the Office of Naval Research (N00014-95-1-0409, N00014-92-J-1309); National Science Foundation (IRI-97-20333); National Institutes of Health (I-R29-DC02952-01)
Analysis of human performance as a measure of mental fatigue
In our day to day, we often experience a sense of being tired
due to mental or physical workload. Along with that, there is also a
feeling of degrading performance, even after the completion of simple
tasks. These mental states however, are often not felt consciously or are
ignored. This is an attitude that may result in human error, failure,
and may lead to potential health problems together with a decrease in
quality of life. States of acute mental fatigue may be detected with the
close monitoring of certain indicators, such as productivity, performance
and health indicators. In this paper, a model and prototype are proposed
to detect and monitor acute acute fatigue, based on non-invasive Humancomputer
Interaction (HCI). This approach will enable the development
of better working environments, with an impact on the quality of life and
the work produced.This work was developed in the context of the project CAMCoF - Contextaware
Multimodal Communication Framework funded by ERDF - European
Regional Development Fund through the COMPETE Programme (operational
programme for competitiveness) and by National Funds through the FCT - Funda
ção para a CiĂȘncia e a Tecnologia (Portuguese Foundation for Science and
Technology) within project FCOMP-01-0124-FEDER-028980
Axion Radiation from Strings
This paper revisits the problem of the string decay contribution to the axion
cosmological energy density. We show that this contribution is proportional to
the average relative increase when axion strings decay of a certain quantity
which we define. We carry out numerical simulations of the
evolution and decay of circular and non-circular string loops, of bent strings
with ends held fixed, and of vortex-antivortex pairs in two dimensions. In the
case of string loops and of vortex-antivortex pairs, decreases by
approximately 20%. In the case of bent strings, remains constant
or increases slightly. Our results imply that the string decay contribution to
the axion energy density is of the same order of magnitude as the
well-understood contribution from vacuum realignment.Comment: 29 pages, 10 figure
Smoothed Particle Hydrodynamics with particle splitting, applied to self-gravitating collapse
We describe and demonstrate a method for increasing the resolution locally in
a Smoothed Particle Hydrodynamic (SPH) simulation, by splitting particles. We
show that in simulations of self-gravitating collapse (of the sort which are
presumed to occur in star formation) the method is stable, and affords great
savings in computer time and memory. When applied to the standard Boss &
Bodenheimer test -- which has been shown to depend critically on fulfilment of
the Jeans Condition -- the results are comparable both with those obtained
using Adaptive Mesh Refinement, and with those obtained using a standard
high-resolution SPH simulation, but they are achieved with considerably less
computational resource. Further development and testing is required before the
method can safely be applied to more general flows.Comment: 8 pages, 6 figure
Discrete symmetries, invisible axion and lepton number symmetry in an economic 3-3-1 model
We show that Peccei-Quinn and lepton number symmetries can be a natural
outcome in a 3-3-1 model with right-handed neutrinos after imposing a Z_11 x
Z_2 symmetry. This symmetry is suitably accommodated in this model when we
augmented its spectrum by including merely one singlet scalar field. We work
out the breaking of the Peccei-Quinn symmetry, yielding the axion, and study
the phenomenological consequences. The main result of this work is that the
solution to the strong CP problem can be implemented in a natural way, implying
an invisible axion phenomenologically unconstrained, free of domain wall
formation and constituting a good candidate for the cold dark matter.Comment: 17 pages, Revtex
Simulations of galactic dynamos
We review our current understanding of galactic dynamo theory, paying
particular attention to numerical simulations both of the mean-field equations
and the original three-dimensional equations relevant to describing the
magnetic field evolution for a turbulent flow. We emphasize the theoretical
difficulties in explaining non-axisymmetric magnetic fields in galaxies and
discuss the observational basis for such results in terms of rotation measure
analysis. Next, we discuss nonlinear theory, the role of magnetic helicity
conservation and magnetic helicity fluxes. This leads to the possibility that
galactic magnetic fields may be bi-helical, with opposite signs of helicity and
large and small length scales. We discuss their observational signatures and
close by discussing the possibilities of explaining the origin of primordial
magnetic fields.Comment: 28 pages, 15 figure, to appear in Lecture Notes in Physics "Magnetic
fields in diffuse media", Eds. E. de Gouveia Dal Pino and A. Lazaria
Factors associated with adherence to immunomodulator treatment in people with multiple sclerosis
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
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The micromorphology of glaciolacustrine varve sediments and their use for reconstructing palaeoglaciological and palaeoenvironmental change
Former glaciolacustrine systems are an important archive of palaeoglaciological, palaeoenvironmental and palaeoclimatic change. The annually laminated (varved) sediments that, under certain conditions, accumulate in former glacial lakes, offer a rare opportunity to reconstruct such changes (e.g. glacier advance and retreat cycles, glacier ablation trends, permafrost melt, nival events) at annual or even sub-annual temporal resolution. Data of this kind are desirable for their ability to guide and test numerical model simulations of glacier dynamics and palaeoclimatic change that occur over rapid time intervals, with implications for predicting future glacier response to climatic change, or the effects of weather and climate events on lake sedimentation. The most valuable records preserved in glaciolacustrine systems are continuous varved sequences formed in the distal parts of glacial lakes, where microscale lamination structures can accumulate relatively undisturbed. Technological advances, in the last few decades, have enabled improved characterisation of glaciolacustrine varve microfacies and the precise measurement of varve thickness at the micrometre scale. However, unlike in cognate fields (e.g. soil science), protocols for the robust and consistent description and interpretation of glaciolacustrine varve sediments are lacking. To fill this gap, and to provide a resource for future studies of glaciolacustrine varved sediments, this paper reviews the processes of sedimentation in glacial lake basins, and presents the defining microfacies characteristics of glacial varves using a descriptive protocol that uses consistent examination of grain size, sorting, structure, nature of contacts, development of plasmic fabrics and features such as dropgrains and intraclasts within individual laminations. These lamination types are then combined into lamination sets, whose structures can be interpreted as glaciolacustrine varves. Within this framework, we define five principal assemblages of glaciolacustrine varve microfacies which, if clearly identified in palaeoglaciolacustrine settings, enable more detailed palaeoenvironmental interpretations to be made. Finally, we discuss the utility and complexities of reconstructing the evolution of former glacial lake systems using varve microfacies and thickness datasets
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