14,908 research outputs found
Interstellar Scintillations of Polarization of Compact Sources
We demostrate that the measurement of fluctuations of polarization due to the
galactic interstellar scintillations may be used to study the structure of the
radiation field at compact radio sources. We develop a mathematical formalism
and demonstrate it on a simple analytical model in which the scale of the
polarization variation through the source is comparable to the source size. The
predicted amplitude of modulation of the polarized radiation flux is ~20% x
(pi_s) x (m_sc), where (pi_s) is the characteristic degree of polarization of
radiation at the source and (m_sc) is the typical modulation index due to
scattering, i.e., (m_sc)~1 for diffractive scintillations and (m_sc)<1 for
refractive scintillations.Comment: 5 pages, 2 figures, emilateapj.sty. Submitted to ApJ
Coherent Backscattering of Light with Nonlinear Atomic Scatterers
We study coherent backscattering of a monochromatic laser by a dilute gas of
cold two-level atoms in the weakly nonlinear regime. The nonlinear response of
the atoms results in a modification of both the average field propagation
(nonlinear refractive index) and the scattering events. Using a perturbative
approach, the nonlinear effects arise from inelastic two-photon scattering
processes. We present a detailed diagrammatic derivation of the elastic and
inelastic components of the backscattering signal both for scalar and vectorial
photons. Especially, we show that the coherent backscattering phenomenon
originates in some cases from the interference between three different
scattering amplitudes. This is in marked contrast with the linear regime where
it is due to the interference between two different scattering amplitudes. In
particular we show that, if elastically scattered photons are filtered out from
the photo-detection signal, the nonlinear backscattering enhancement factor
exceeds the linear barrier two, consistently with a three-amplitude
interference effect.Comment: 18 pages, 13 figures, submitted to Phys. Rev.
Growth of Perturbation in Gravitational Collapse and Accretion
When a self-gravitating spherical gas cloud collapses or accretes onto a
central mass, the inner region of the cloud develops a density profile
and the velocity approaches free-fall. We show that in
this region, nonspherical perturbations grow with decreasing radius. In the
linear regime, the tangential velocity perturbation increases as ,
while the Lagrangian density perturbation, , grows as
. Faster growth occurs if the central collapsed object maintains a
finite multiple moment, in which case increases as ,
where specifies the angular degree of the perturbation. These scaling
relations are different from those obtained for the collapse of a homogeneous
cloud. Our numerical calculations indicate that nonspherical perturbations are
damped in the subsonic region, and that they grow and approach the asymptotic
scalings in the supersonic region. The implications of our results to
asymmetric supernova collapse and to black hole accretion are briefly
discussed.Comment: 23 pages including 6 ps figures; Minor changes and update; To appear
in ApJ, 200
Surface-Enhanced Plasmon Splitting in a Liquid-Crystal-Coated Gold Nanoparticle
We show that, when a gold nanoparticle is coated by a thin layer of nematic
liquid crystal, the deformation produced by the nanoparticle surface can
enhance the splitting of the nanoparticle surface plasmon. We consider three
plausible liquid crystal director configurations in zero electric field: boojum
pair (north-south pole configuration), baseball (tetrahedral), and homogeneous.
From a calculation using the Discrete Dipole Approximation, we find that the
surface plasmon splitting is largest for the boojum pair, intermediate for the
homogeneous, and smallest for the baseball configuration. The boojum pair
results are in good agreement with experiment. We conclude that the
nanoparticle surface has a strong effect on the director orientation, but,
surprisingly, that this deformation can actually enhance the surface plasmon
splitting.Comment: 5 pages, 3 figures To be published in PR
Concurrent use of prescription drugs and herbal medicinal products in older adults: a systematic review protocol
Background: There has been a global increase in the use of herbal medicinal products (HMPs). About a quarter of
UK adults use HMPs, bought over the counter by self-prescription and often not disclosed to healthcare professionals.
Potential herb-drug interaction is a clinical concern, with older people at greater risk because of co-morbidities and
slower clearance of pharmacologically active compounds. While there is a good understanding of general herbal
medicine use by older people, less is known about the extent and implications of concurrent use with prescription
medicines. The aim of this systematic review is to assess the prevalence, patterns, safety issues and other factors
associated with concurrent prescription and herbal medicines use among older adults.
Methods/design: Systematic electronic searches of MEDLINE, PsychINFO, Excerpta Medica dataBASE (EMBASE),
Cumulative Index to Nursing and Allied Health Literature (CINAHL), Allied and Complementary Medicine Database
(AMED), Web of Science and Cochrane from inception till present for studies reporting the concurrent use of
prescription medicines with HMPs in older adults (â„65 years). Lateral searching via related citation (PubMed) and
checking reference lists of identified studies will be performed. Two reviewers will independently screen studies, extract
data and appraise methodological quality using the Joanna Briggs Institute checklist for prevalence data and the
Critical Appraisal Skills Programme (CASP) checklist. Qualitative and quantitative studies from all settings will be
included. Non-empirical papers, in vitro experiments and animal studies will be excluded. Primary outcomes are
prevalence and patterns of concurrent use, number and types of prescription and HMPs and adverse reactions
reported. Secondary outcomes are disclosure of HMP use to healthcare professionals and cost of HMPs. A narrative
synthesis of included studies will be performed to summarise the evidence.
Discussion: This review will synthesise and critically appraise current knowledge on the concurrent use of drugs and
HMPs by older adults and thus provide a better understanding of the issue. It will also identify any gaps in knowledge.
By establishing safety issues associated with concurrent use, it will also inform strategies that can help practitioners to
identify and manage older people at potential risk of herb-drug interactions.
Systematic review registration: PROSPERO CRD4201400909
Missing and Quenched Gamow Teller Strength
Gamow-Teller strength functions in full spaces are calculated with
sufficient accuracy to ensure that all the states in the resonance region have
been populated. Many of the resulting peaks are weak enough to become
unobservable. The quenching factor necessary to bring into agreement the low
lying observed states with shell model predictions is shown to be due to
nuclear correlations. To within experimental uncertainties it is the same that
is found in one particle transfer and (e,e') reactions. Perfect consistency
between the observed peaks and the calculation is
achieved by assuming an observation threshold of 0.75\% of the total strength,
a value that seems typical in several experimentsComment: 11 pages, 6 figures avalaible upon request, RevTeX, FTUAM-94/0
The composition of cosmic rays near the Bend (10 to the 15th power eV) from a study of muons in air showers at sea level
The distribution of muons near shower cores was studied at sea level at Fermilab using the E594 neutrino detector to sample the muon with E testing 3 GeV. These data are compared with detailed Monte Carlo simulations to derive conclusions about the composition of cosmic rays near the bend in the all particle spectrum. Monte Carlo simulations generating extensive air showers (EAS) with primary energy in excess of 50 TeV are described. Each shower record contains details of the electron lateral distribution and the muon and hadron lateral distributions as a function of energy, at the observation level of 100g/cm. The number of detected electrons and muons in each case was determined by a Poisson fluctuation of the number incident. The resultant predicted distribution of muons, electrons, the rate events are compared to those observed. Preliminary results on the rate favor a heavy primary dominated cosmic ray spectrum in energy range 50 to 1000 TeV
Radio Scintillation due to Discontinuities in the Interstellar Plasma Density
We develop the theory of interstellar scintillation as caused by an irregular
plasma having a power-law spatial density spectrum with a spectral exponent of
4 corresponding to a medium with abrupt changes in its density. An ``outer
scale'' is included in the model representing the typical scale over which the
density of the medium remains uniform. Such a spectrum could be used to model
plasma shock fronts in supernova remnants or other plasma discontinuities. We
investigate and develop equations for the decorrelation bandwidth of
diffractive scintillations and the refractive scintillation index and compare
our results with pulsar measurements. We consider both a medium concentrated in
a thin layer and an extended irregular medium. We conclude that the
discontinuity model gives satisfactory agreement for many diffractive
measurements, in particular the VLBI meaurements of the structure function
exponent between 5/3 and 2. However, it gives less satisfactory agreement for
the refractive scintillation index than does the Kolmogorov turbulence
spectrum. The comparison suggests that the medium consists of a pervasive
background distribution of turbulence embedded with randomly placed discrete
plasma structures such as shocks or HII regions. This can be modeled by a
composite spectrum following the Kolmogorov form at high wavenumbers and
steepening at lower wavenumbers corresponding to the typical (inverse) size of
the discrete structures. Such a model can also explain the extreme scattering
events. However, lines of sight through the enhanced scattering prevalent at
low galactic latitudes are accurately described by the Kolmogorov spectrum in
an extended medium and do not appear to have a similar low-wavenumber
steepening.Comment: Accpeted for ApJ vol 531, March 200
Branching rules of semi-simple Lie algebras using affine extensions
We present a closed formula for the branching coefficients of an embedding p
in g of two finite-dimensional semi-simple Lie algebras. The formula is based
on the untwisted affine extension of p. It leads to an alternative proof of a
simple algorithm for the computation of branching rules which is an analog of
the Racah-Speiser algorithm for tensor products. We present some simple
applications and describe how integral representations for branching
coefficients can be obtained. In the last part we comment on the relation of
our approach to the theory of NIM-reps of the fusion rings of WZW models with
chiral algebra g_k. In fact, it turns out that for these models each embedding
p in g induces a NIM-rep at level k to infinity. In cases where these NIM-reps
can be be extended to finite level, we obtain a Verlinde-like formula for
branching coefficients.Comment: 11 pages, LaTeX, v2: one reference added, v3: Clarified proof of
Theorem 2, completely rewrote and extended Section 5 (relation to CFT), added
various references. Accepted for publication in J. Phys.
Missing at random assumption made more plausible: evidence from the 1958 British birth cohort
Objective:
Non-response is unavoidable in longitudinal surveys. The consequences are lower statistical power
and the potential for bias. We implemented a systematic data-driven approach to identify predictors
of non-response in the National Child Development Study (NCDS; 1958 British birth cohort). Such
variables can help make the missing at random assumption more plausible, which has implications for
the handling of missing data. /
Study Design and Setting:
We identified predictors of non-response using data from the 11 sweeps (birth to age 55) of the NCDS
(n = 17,415), employing parametric regressions and the LASSO for variable selection. /
Results:
Disadvantaged socio-economic background in childhood, worse mental health and lower cognitive
ability in early life, and lack of civic and social participation in adulthood were consistently associated
with non-response. Using this information, along with other data from NCDS, we were able to replicate
the âpopulation distributionâ of educational attainment and marital status (derived from external
data), and the original distributions of key early life characteristics. /
Conclusion:
The identified predictors of non-response have the potential to improve the plausibility of the missing
at random assumption. They can be straightforwardly used as âauxiliary variablesâ in analyses with
principled methods to reduce bias due to missing data
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