3,444 research outputs found
Collective polarization exchanges in collisions of photon clouds
The one-loop "vacuum" Heisenberg-Euler coupling of four electromagnetic
fields can lead to interesting collective effects in the collision of two
photon clouds, on a time scale orders of magnitude faster than one estimates
from the cross-section and density. We estimate the characteristic time for
macroscopic transformation of positive to negative helicity in clouds that are
initially totally polarized and for depolarization of a polarized beam
traversing an unpolarized cloud.Comment: Recapitulates much that is in hep-ph/0402127, with new results in the
last section, and the first section drastically reduced in view of the
previous work of Kotkin and Serbo. Typo corrected in eq. 1
Quantum Hall Transition in the Classical Limit
We study the quantum Hall transition using the density-density correlation
function. We show that in the limit h->0 the electron density moves along the
percolating trajectories, undergoing normal diffusion. The localization
exponent coincides with its percolation value \nu=4/3. The framework provides a
natural way to study the renormalization group flow from percolation to quantum
Hall transition. We also confirm numerically that the critical conductivity of
a classical limit of quantum Hall transition is \sigma_{xx} = \sqrt{3}/4.Comment: 8 pages, 4 figures; substantial changes include the critical
conductivity calculatio
Dark Energy, Induced Gravity and Broken Scale Invariance
We study the cosmological evolution of an induced gravity model with a
self-interacting scalar field and in the presence of matter and
radiation. Such model leads to Einstein Gravity plus a cosmological constant as
a stable attractor among homogeneous cosmologies and is therefore a viable
dark-energy (DE) model for a wide range of scalar field initial conditions and
values for its positive coupling to the Ricci curvature .Comment: 6 pages, 5 figures, 1 table: final version accepted for publication
in PL
Fiber Optic Gyro-Based Attitude Determination for High-Performance Target Tracking
Small satellite-enabled terrestrial target tracking applications from low-Earth orbit are demanding stringent pointing performance, prompting the need for developing high-precision attitude estimation and control systems that adhere to cost and mass constraints. The attitude determination and control system onboard the Space Flight Laboratory’s NEMO-class satellite platforms uses an extended Kalman filter and low-cadence (1Hz) star-tracker measurements to constrain the attitude and rate estimation errors to within 0.05° and 0.04°/s (2-σ), respectively. In addition, the pointing error of this satellite platform is constrained to below 0.3° (2-σ) for ground target tracking applications. However, in order to meet the stability requirements of future missions that require precise target-tracking capabilities, a combination of star tracker and high frequency gyro-measurements is preferred. Leveraging high-grade miniaturized and commercially-accessible fiber optic gyroscopes (FOGs) with sampling frequencies of ≥ 2Hz, a high-performance attitude determination and control system suitable for target tracking micro- and nano-satellites is under development at the Space Flight Laboratory of Toronto, Canada. This paper discusses the design of an attitude estimation filter tailored to constrain the ground target pointing error of NEMO-class satellites to well below 0.3° (3-σ). To evaluate the performance of this filter, precision target tracking simulations were conducted, and the results demonstrated significant improvement in some state estimates when a combination of three-orthogonally mounted FOGs operating at high cadence (5Hz) and a single star tracker operating at 1Hz were implemented
Student teachers’ future time perspective and teacher identity:A longitudinal study about students who will become primary school teachers
This three-wave study examined associations between a motivational construct future time perspective (FTP) and teachers’ identity (TI) in a sample of Dutch student teachers (N = 368). Additionally, gender and educational level were included as factors affecting FTP and TI. Random intercept cross-lagged panel models indicated that FTP and TI were moderately and positively correlated at the between-person level. At the within-person level, positive cross-lagged effects from wave 2 FTP on wave 3 TI were uncovered. Male student teachers and those that are academically educated reported weaker FTP and TI relations compared to their counterparts. Implications for practice are discussed.</p
Neutrino Masses and A TeV Scale Seesaw Mechanism
A simple extension of the Standard Model providing TeV scale seesaw mechanism
is presented. Beside the Standard Model particles and right-handed Majorana
neutrinos, the model contains a singly charged scalar, an extra Higgs doublet
and three vector like singly charged fermions. In our model, Dirac neutrino
mass matrix raises only at the loop level. Small but non-zero Majorana neutrino
masses come from integrating out heavy Majorana neutrinos, which can be at the
TeV scale. The phenomenologies of the model are investigated, including scalar
mass spectrum, neutrino masses and mixings, lepton flavor violations, heavy
neutrino magnetic moments as well as possible collider signatures of the model
at the LHC.Comment: 13 pages, 4 figures. references adde
Transverse Lepton Polarization in Polarized W Decays
Calculations of transverse polarization of leptons in the decay with polarized 's are presented. Planned accelerators will produce
enough 's for observation of the Standard Model contributions to this
polarization. One loop corrections to the polarization are given; these are too
small to be seen at presently available sources. The exchange of Majorons
will contribute to these polarizations; these may provide limits on the
couplings of these particles to leptons.Comment: 8 pages set in RevTex III and 4 uucompressed figures. This revised
version studies polarization effects due to the exchange of charged Majoron
doublet
Impact and application of electron shuttles on the redox (bio)transformation of contaminants : a review
During the last two decades, extensive research has explored the catalytic effects of different organic molecules with redox mediating properties on the anaerobic (bio)transformation of a wide variety of organic and inorganic compounds. The accumulated evidence points at a major role of electron shuttles in the redox conversion of several distinct contaminants, both by chemical and biological mechanisms. Many microorganisms are capable of reducing redox mediators linked to the anaerobic oxidation of organic and inorganic substrates. Electron shuttles can also be chemically reduced by electron donors commonly found in anaerobic environments (e.g. sulfide and ferrous iron). Reduced electron shuttles can transfer electrons to several distinct electron-withdrawing compounds, such as azo dyes, polyhalogenated compounds, nitroaromatics and oxidized metalloids, among others. Moreover, reduced molecules with redox properties can support the microbial reduction of electron acceptors, such as nitrate, arsenate and perchlorate. The aim of this review paper is to summarize the results of reductive (bio)transformation processes catalyzed by electron shuttles and to indicate which aspects should be further investigated to enhance the applicability of redox mediators on the (bio)transformation of contaminants.F.P. van der Zee thanks the Portuguese Fundaicao para a Ciencia e a Tecnologia for financial support (Grant SFRH/BPD/39086/2007). F. J. Cervantes greatly acknowledges a grant from Council of Science and Technology of Mexico (Grant SEP-CONACYT-C02-55045)
Discontinuities without discontinuity: The Weakly-enforced Slip Method
Tectonic faults are commonly modelled as Volterra or Somigliana dislocations
in an elastic medium. Various solution methods exist for this problem. However,
the methods used in practice are often limiting, motivated by reasons of
computational efficiency rather than geophysical accuracy. A typical
geophysical application involves inverse problems for which many different
fault configurations need to be examined, each adding to the computational
load. In practice, this precludes conventional finite-element methods, which
suffer a large computational overhead on account of geometric changes. This
paper presents a new non-conforming finite-element method based on weak
imposition of the displacement discontinuity. The weak imposition of the
discontinuity enables the application of approximation spaces that are
independent of the dislocation geometry, thus enabling optimal reuse of
computational components. Such reuse of computational components renders
finite-element modeling a viable option for inverse problems in geophysical
applications. A detailed analysis of the approximation properties of the new
formulation is provided. The analysis is supported by numerical experiments in
2D and 3D.Comment: Submitted for publication in CMAM
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