7,341 research outputs found
Anti-de Sitter Supersymmetry
We give a pedagogical introduction to certain aspects of supersymmetric field
theories in anti-de Sitter space. Among them are the presence of masslike terms
in massless wave equations, irreducible unitary representations and the
phenomenon of multiplet shortening.Comment: Lectures presented by B. de Wit at the Winter School of Theoretical
Physics, Polanica, Poland, February 1999. 23 pp., LateX file, requires
packages latexsym, amsfonts, cl2emult.cl
A mid-IR interferometric survey with MIDI/VLTI: resolving the second-generation protoplanetary disks around post-AGB binaries
We present a mid-IR interferometric survey of the circumstellar environment
of a specific class of post-Asymptotic Giant Branch (post-AGB) binaries. For
this class the presence of a compact dusty disk has been postulated on the
basis of various spatially unresolved measurements. Our interferometric survey
was performed with the MIDI instrument on the VLTI. In total 19 different
systems were observed using variable baseline configurations. Combining all the
visibilities at a single wavelength at 10.7 micron, we fitted two parametric
models to the data: a uniform disk (UD) and a ring model mimicking a
temperature gradient. We compared our observables of the whole sample, with
synthetic data computed from a grid of radiative transfer models of passively
irradiated disks in hydrostatic equilibrium. These models are computed with a
Monte Carlo code that has been widely applied to describe the structure of
protoplanetary disks around young stellar objects (YSO). The spatially resolved
observations show that the majority of our targets cluster closely together in
the distance-independent size-colour diagram, and have extremely compact N-band
emission regions. The typical uniform disk diameter of the N-band emission
region is about 40 mass which corresponds to a typical brightness temperature
of 400-600~K. The resolved objects display very similar characteristics in the
interferometric observables and in the spectral energy distributions.
Therefore, the physical properties of the disks around our targets must be
similar. The grid of protoplanetary disk models covers very well the observed
objects. Much like for young stars, the spatially resolved N-band emission
region is determined by the hot inner rim of the disk. Continued comparisons
between post-AGB and protoplanetary disks will help to understand grain growth
and disk evolution processes,Comment: 30 pages, 21 figures, in press in Astronomy and Astrophysic
Regularisation, the BV method, and the antibracket cohomology
We review the Lagrangian Batalin--Vilkovisky method for gauge theories. This
includes gauge fixing, quantisation and regularisation. We emphasize the role
of cohomology of the antibracket operation. Our main example is gravity,
for which we also discuss the solutions for the cohomology in the space of
local integrals. This leads to the most general form for the action, for
anomalies and for background charges.Comment: 12 pages, LaTeX, Preprint-KUL-TF-94/2
Methodological standards in non-inferiority AIDS trials: moving from adherence to compliance: Response
A response to Parienti JJ, Verdon R and Massari V: Methodological standards in non-inferiority AIDS trials: moving from adherence to compliance. BMC Med Res Meth 2006, 6:4
University students and HIV in Namibia: an HIV prevalence survey and a knowledge and attitude survey
<p>Abstract</p> <p>Background</p> <p>With an overall adult HIV prevalence of 15.3%, Namibia is facing one of the largest HIV epidemics in Africa. Young people aged 20 to 34 years constitute one of the groups at highest risk of HIV infection in Namibia. However, little is known about the impact of HIV on this group and its access to healthcare. The purpose of this study was to estimate HIV prevalence, to assess the knowledge of and attitudes towards HIV/AIDS, and to assess access to healthcare among university students in Namibia.</p> <p>Methods</p> <p>We assessed HIV/AIDS knowledge and attitudes, HIV prevalence and access to healthcare among students at the Polytechnic of Namibia and the University of Namibia. HIV prevalence was tested through anonymous oral fluid-based tests.</p> <p>Results</p> <p>Half (n = 2790/5568) of the university students and 45% (n = 2807/6302) of the Polytechnic students participated in the knowledge and attitudes surveys. HIV/AIDS knowledge was reasonable, except for misperceptions about transmission. Awareness of one's own HIV status and risks was low. In all, 55% (n = 3055/5568) of university students and 58% (n = 3680/6302) of Polytechnic students participated in the HIV prevalence survey; 54 (1.8%) university students and 103 (2.8%) Polytechnic students tested HIV positive. Campus clinics were not the major providers of healthcare to the students.</p> <p>Conclusions</p> <p>Meaningful strategies addressing the gap between knowledge, attitude and young people's perception of risk of HIV acquisition should be implemented. HIV prevalence among Namibian university students appears relatively low. Voluntary counselling and testing should be stimulated. Efforts should be made to increase access to healthcare through the campus clinics.</p
Gauge theories of spacetime symmetries
Gauge theories of conformal spacetime symmetries are presented which merge
features of Yang-Mills theory and general relativity in a new way. The models
are local but nonpolynomial in the gauge fields, with a nonpolynomial structure
that can be elegantly written in terms of a metric (or vielbein) composed of
the gauge fields. General relativity itself emerges from the construction as a
gauge theory of spacetime translations. The role of the models within a general
classification of consistent interactions of gauge fields is discussed as well.Comment: 8 pages, revtex; v2: minor improvements of text and formulas; v3:
typo in formula after eq. (35) correcte
Precise characterization of nanometer-scale systems using interferometric scattering microscopy and Bayesian analysis
Interferometric scattering microscopy (iSCAT) can image the dynamics of
nanometer-scale systems. The typical approach to analyzing interferometric
images involves intensive processing, which discards data and limits the
precision of measurements. We demonstrate an alternative approach: modeling the
interferometric point spread function (iPSF) and fitting this model to data
within a Bayesian framework. This approach yields best-fit parameters,
including the particle's three-dimensional position and polarizability, as well
as uncertainties and correlations between these parameters. Building on recent
work, we develop a model that is parameterized for rapid fitting. The model is
designed to work with Hamiltonian Monte Carlo techniques that leverage
automatic differentiation. We validate this approach by fitting the model to
interferometric images of colloidal nanoparticles. We apply the method to track
a diffusing particle in three dimensions, to directly infer the diffusion
coefficient of a nanoparticle without calculating a mean-square displacement,
and to quantify the ejection of DNA from an individual lambda phage virus,
demonstrating that the approach can be used to infer both static and dynamic
properties of nanoscale systems
Self-Dual N=8 Supergravity as Closed N=2(4) Strings
As open N=2 or 4 strings describe self-dual N=4 super Yang-Mills in 2+2
dimensions, the corresponding closed (heterotic) strings describe self-dual
ungauged (gauged) N=8 supergravity. These theories are conveniently formulated
in a chiral superspace with general supercoordinate and local OSp(8|2) gauge
invariances. The super-light-cone and covariant-component actions are analyzed.
Because only half the Lorentz group is gauged, the gravity field equation is
just the vanishing of the torsion.Comment: 17 pg., (uuencoded dvi file; revision: forgot 1 stupid term in the
last equation) ITP-SB-92-3
N = 3 chiral supergravity compatible with the reality condition and higher N chiral Lagrangian density
We obtain N = 3 chiral supergravity (SUGRA) compatible with the reality
condition by applying the prescription of constructing the chiral Lagrangian
density from the usual SUGRA. The chiral Lagrangian density in
first-order form, which leads to the Ashtekar's canonical formulation, is
determined so that it reproduces the second-order Lagrangian density of the
usual SUGRA especially by adding appropriate four-fermion contact terms. We
show that the four-fermion contact terms added in the first-order chiral
Lagrangian density are the non-minimal terms required from the invariance under
first-order supersymmetry transformations. We also discuss the case of higher N
theories, especially for N = 4 and N = 8.Comment: 20 pages, Latex, some more discussions and new references added, some
typos corrected, accepted for publication in Physical Review
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