478 research outputs found
DESENVOLVIMENTOECONÔMICO E POLÍTICA INDUSTRIAL E TECNOLÓGICA: A VISÃO DO PROFESSOR FABIO ERBER
Radiation in Lorentz violating electrodynamics
Synchrotron radiation is analyzed in the classical effective Lorentz
invariance violating model of Myers-Pospelov. Within the full far-field
approximation we compute the electric and magnetic fields, the angular
distribution of the power spectrum and the total emitted power in the m-th
harmonic, as well as the polarization. We find the appearance of rather
unexpected and large amplifying factors, which go together with the otherwise
negligible naive expansion parameter. This opens up the possibility of further
exploring Lorentz invariance violations by synchrotron radiation measurements
in astrophysical sources where these amplifying factors are important.Comment: Presented at the Second Mexican Meeting on Theoretical and
Experimental Physics, El Colegio Nacional, Mexico City, 6-10 September 200
Asymptotic conditions of motion for radiating charged particles
Approximate asymptotic conditions on the motion of compact, electrically
charged particles are derived within the framework of general relativity using
the Einstein- Infeld-Hoffmann (EIH) surface integral method. While
superficially similar to the Abraham-Lorentz and Lorentz-Dirac (ALD) equations
of motion, these conditions differ from them in several fundamental ways. They
are not equations of motion in the usual sense but rather a set of conditions
which these motions must obey in the asymptotic future of an initial value
surface. In addition to being asymptotic, these conditions of motion are
approximate and apply, as do the original EIH equations, only to slowly moving
systems. Also, they do not admit the run- away solutions of these other
equations. As in the original EIH work, they are integrability conditions
gotten from integrating the empty-space (i.e., source free) Einstein-Maxwell
equations of general relativity over closed two-surfaces surrounding the
sources of the fields governed by these equations. No additional ad hoc
assumptions, such as the form of a force law or the introduction of inertial
reaction terms, needed to derive the ALD equations are required for this
purpose. Nor is there a need for any of the infinite mass renormalizations that
are required in deriving these other equations.Comment: 15 page
Differentiation of benign and malignant vertebral fractures using a convolutional neural network to extract CT-based texture features.
PURPOSE
To assess the diagnostic performance of three-dimensional (3D) CT-based texture features (TFs) using a convolutional neural network (CNN)-based framework to differentiate benign (osteoporotic) and malignant vertebral fractures (VFs).
METHODS
A total of 409 patients who underwent routine thoracolumbar spine CT at two institutions were included. VFs were categorized as benign or malignant using either biopsy or imaging follow-up of at least three months as standard of reference. Automated detection, labelling, and segmentation of the vertebrae were performed using a CNN-based framework ( https://anduin.bonescreen.de ). Eight TFs were extracted: Varianceglobal, Skewnessglobal, energy, entropy, short-run emphasis (SRE), long-run emphasis (LRE), run-length non-uniformity (RLN), and run percentage (RP). Multivariate regression models adjusted for age and sex were used to compare TFs between benign and malignant VFs.
RESULTS
Skewnessglobal showed a significant difference between the two groups when analyzing fractured vertebrae from T1 to L6 (benign fracture group: 0.70 [0.64-0.76]; malignant fracture group: 0.59 [0.56-0.63]; and p = 0.017), suggesting a higher skewness in benign VFs compared to malignant VFs.
CONCLUSION
Three-dimensional CT-based global TF skewness assessed using a CNN-based framework showed significant difference between benign and malignant thoracolumbar VFs and may therefore contribute to the clinical diagnostic work-up of patients with VFs
Limits on models of the ultrahigh energy cosmic rays based on topological defects
An erratum exists for this article. Please see the description link below for details.Using the propagation of ultrahigh energy nucleons, photons, and electrons in the universal radiation backgrounds, we obtain limits on the luminosity of topological defect scenarios for the origin of the highest energy cosmic rays. The limits are set as a function of the mass of the X particles emitted by the cosmic strings or other defects, the cosmological evolution of the topological defects, and the strength of the extragalactic magnetic fields. The existing data on the cosmic ray spectrum and on the isotropic 100 MeV gamma-ray background limit significantly the parameter space in which topological defects can generate the flux of the highest energy cosmic rays, and rule out models with the standard X-particle mass of 10¹⁶GeV and higher.R. J. Protheroe and Todor Stane
Cosmic Physics: The High Energy Frontier
Cosmic rays have been observed up to energies times larger than those
of the best particle accelerators. Studies of astrophysical particles (hadrons,
neutrinos and photons) at their highest observed energies have implications for
fundamental physics as well as astrophysics. Thus, the cosmic high energy
frontier is the nexus to new particle physics. This overview discusses recent
advances being made in the physics and astrophysics of cosmic rays and cosmic
gamma-rays at the highest observed energies as well as the related physics and
astrophysics of very high energy cosmic neutrinos. These topics touch on
questions of grand unification, violation of Lorentz invariance, as well as
Planck scale physics and quantum gravity.Comment: Topical Review Paper to be published in the Journal of Physics G, 50
page
Corrections to flat-space particle dynamics arising from space granularity
The construction of effective Hamiltonians describing corrections to flat
space particle dynamics arising from the granularity of space at very short
distances is discussed in the framework of an heuristic approach to the
semiclassical limit of loop quantum gravity. After some general motivation of
the subject, a brief non-specialist introduction to the basic tools employed in
the loop approach is presented. The heuristical semiclassical limit is
subsequently defined and the application to the case of photons and spin 1/2
fermions is described. The resulting modified Maxwell and Dirac Hamiltonians,
leading in particular to Planck scale corrections in the energy-momentum
relations, are presented. Alternative interpretations of the results and their
limitations, together with other approaches are briefly discussed along the
text. Three topics related to the above methods are reviewed: (1) The
determination of bounds to the Lorentz violating parameters in the fermionic
sector, obtained from clock comparison experiments.(2) The calculation of
radiative corrections in preferred frames associated to space granularity in
the framework of a Yukawa model for the interactions and (3) The calculation of
synchrotron radiation in the framework of the Myers-Pospelov effective theories
describing Lorentz invariance violations, as well as a generalized approach to
radiation in Planck scale modified electrodynamics. The above exploratory
results show that quantum gravity phenomenology provides observational guidance
in the construction of quantum gravity theories and opens up the possibility of
probing Planck scale physics.Comment: 49 pages, 6 figures and 4 tables. Extended version of the talk given
at the 339-th WE-Heraeus-Seminar: Special Relativity, will it survive the
next 100 years?, Potsdam, february 200
Non-monotonic variation with salt concentration of the second virial coefficient in protein solutions
The osmotic virial coefficient of globular protein solutions is
calculated as a function of added salt concentration at fixed pH by computer
simulations of the ``primitive model''. The salt and counter-ions as well as a
discrete charge pattern on the protein surface are explicitly incorporated. For
parameters roughly corresponding to lysozyme, we find that first
decreases with added salt concentration up to a threshold concentration, then
increases to a maximum, and then decreases again upon further raising the ionic
strength. Our studies demonstrate that the existence of a discrete charge
pattern on the protein surface profoundly influences the effective interactions
and that non-linear Poisson Boltzmann and Derjaguin-Landau-Verwey-Overbeek
(DLVO) theory fail for large ionic strength. The observed non-monotonicity of
is compared to experiments. Implications for protein crystallization are
discussed.Comment: 43 pages, including 17 figure
Olfactory Interference during Inhibitory Backward Pairing in Honey Bees
Background: Restrained worker honey bees are a valuable model for studying the behavioral and neural bases of olfactory plasticity. The proboscis extension response (PER; the proboscis is the mouthpart of honey bees) is released in response to sucrose stimulation. If sucrose stimulation is preceded one or a few times by an odor (forward pairing), the bee will form a memory for this association, and subsequent presentations of the odor alone are sufficient to elicit the PER. However, backward pairing between the two stimuli (sucrose, then odor) has not been studied to any great extent in bees, although the vertebrate literature indicates that it elicits a form of inhibitory plasticity. Methodology/Principal Findings: If hungry bees are fed with sucrose, they will release a long lasting PER; however, this PER can be interrupted if an odor is presented 15 seconds (but not 7 or 30 seconds) after the sucrose (backward pairing). We refer to this previously unreported process as olfactory interference. Bees receiving this 15 second backward pairing show reduced performance after a subsequent single forward pairing (excitatory conditioning) trial. Analysis of the results supported a relationship between olfactory interference and a form of backward pairing-induced inhibitory learning/ memory. Injecting the drug cimetidine into the deutocerebrum impaired olfactory interference. Conclusions/Significance: Olfactory interference depends on the associative link between odor and PER, rather than between odor and sucrose. Furthermore, pairing an odor with sucrose can lead either to association of this odor to PER or t
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