22,564 research outputs found
Determinants of Mortality and Disease Progression of Kaposi Sarcoma in a Primary care ART Programme in Khayelitsha, South Africa
CROI 200
Brownian Motion in Robertson-Walker Space-Times from electromagnetic Vacuum Fluctuations
We consider classical particles coupled to the quantized electromagnetic
field in the background of a spatially flat Robertson-Walker universe. We find
that these particles typically undergo Brownian motion and acquire a non-zero
mean squared velocity which depends upon the scale factor of the universe. This
Brownian motion can be interpreted as due to non-cancellation of
anti-correlated vacuum fluctuations in the time dependent background
space-time. We consider several types of coupling to the electromagnetic field,
including particles with net electric charge, a magnetic dipole moment, and
electric polarizability. We also investigate several different model scale
factors.Comment: 29 pages, 7 figure
Robust designs for Poisson regression models
We consider the problem of how to construct robust designs for Poisson regression models. An analytical expression is derived for robust designs for first-order Poisson regression models where uncertainty exists in the prior parameter estimates. Given certain constraints in the methodology, it may be necessary to extend the robust designs for implementation in practical experiments. With these extensions, our methodology constructs designs which perform similarly, in terms of estimation, to current techniques, and offers the solution in a more timely manner. We further apply this analytic result to cases where uncertainty exists in the linear predictor. The application of this methodology to practical design problems such as screening experiments is explored. Given the minimal prior knowledge that is usually available when conducting such experiments, it is recommended to derive designs robust across a variety of systems. However, incorporating such uncertainty into the design process can be a computationally intense exercise. Hence, our analytic approach is explored as an alternative
One-Loop Renormalization of a Self-Interacting Scalar Field in Nonsimply Connected Spacetimes
Using the effective potential, we study the one-loop renormalization of a
massive self-interacting scalar field at finite temperature in flat manifolds
with one or more compactified spatial dimensions. We prove that, owing to the
compactification and finite temperature, the renormalized physical parameters
of the theory (mass and coupling constant) acquire thermal and topological
contributions. In the case of one compactified spatial dimension at finite
temperature, we find that the corrections to the mass are positive, but those
to the coupling constant are negative. We discuss the possibility of
triviality, i.e. that the renormalized coupling constant goes to zero at some
temperature or at some radius of the compactified spatial dimension.Comment: 16 pages, plain LATE
Previously Claimed(/Unclaimed) X-ray Emission Lines in High Resolution Afterglow Spectra
We review the significance determination for emission lines in the Chandra
HETGS spectrum for GRB020813, and we report on a search for additional lines in
high resolution Chandra spectra. No previously unclaimed features are found. We
also discuss the significance of lines sets reportedly discovered using XMM
data for GRB011211 and GRB030227. We find that these features are likely of
modest, though not negligible, significance.Comment: 4 pages, 1 figures, to appear in Santa Fe GRB Conference Proceedings,
200
Wigner Distribution Function Approach to Dissipative Problems in Quantum Mechanics with emphasis on Decoherence and Measurement Theory
We first review the usefulness of the Wigner distribution functions (WDF),
associated with Lindblad and pre-master equations, for analyzing a host of
problems in Quantum Optics where dissipation plays a major role, an arena where
weak coupling and long-time approximations are valid. However, we also show
their limitations for the discussion of decoherence, which is generally a
short-time phenomenon with decay rates typically much smaller than typical
dissipative decay rates. We discuss two approaches to the problem both of which
use a quantum Langevin equation (QLE) as a starting-point: (a) use of a reduced
WDF but in the context of an exact master equation (b) use of a WDF for the
complete system corresponding to entanglement at all times
Chandra Observations of the Optically Dark GRB030528
The X-ray-rich GRB030528 was detected by the HETE satellite and its
localization was rapidly disseminated. However, early optical observations
failed to detect a counterpart source. In a 2-epoch ToO observation with
Chandra, we discovered a fading X-ray source likely counterpart to GRB030528.
The source brightness was typical of X-ray afterglows observed at similar
epochs. Other observers detected an IR source at a location consistent with the
X-ray source. The X-ray spectrum is not consistent with a large absorbing
column.Comment: 4 pages, 1 figures, to appear in Santa Fe GRB Conference Proceedings,
200
What we learned from the Dust Bowl: lessons in science, policy, and adaptation
This article provides a review and synthesis of scholarly knowledge of Depression-era droughts on the North American Great Plains, a time and place known colloquially as the Dust Bowl era or the Dirty Thirties. Recent events, including the 2008 financial crisis, severe droughts in the US corn belt, and the release of a popular documentary film, have spawned a resurgence in public interest in the Dust Bowl. Events of the Dust Bowl era have also proven in recent years to be of considerable interest to scholars researching phenomena related to global environmental change, including atmospheric circulation, drought modeling, land management, institutional behavior, adaptation processes, and human migration. In this review, we draw out common themes in terms of not only what natural and social scientists have learned about the Dust Bowl era itself, but also how insights gained from the study of that period are helping to enhance our understanding of climate–human relations more generally
Quantum interest in two dimensions
The quantum interest conjecture of Ford and Roman asserts that any
negative-energy pulse must necessarily be followed by an over-compensating
positive-energy one within a certain maximum time delay. Furthermore, the
minimum amount of over-compensation increases with the separation between the
pulses. In this paper, we first study the case of a negative-energy square
pulse followed by a positive-energy one for a minimally coupled, massless
scalar field in two-dimensional Minkowski space. We obtain explicit expressions
for the maximum time delay and the amount of over-compensation needed, using a
previously developed eigenvalue approach. These results are then used to give a
proof of the quantum interest conjecture for massless scalar fields in two
dimensions, valid for general energy distributions.Comment: 17 pages, 4 figures; final version to appear in PR
The Energy Density in the Casimir Effect
We compute the expectations of the squares of the electric and magnetic
fields in the vacuum region outside a half-space filled with a uniform
dispersive dielectric. We find a positive energy density of the electromagnetic
field which diverges at the interface despite the inclusion of dispersion in
the calculation. We also investigate the mean squared fields and the energy
density in the vacuum region between two parallel half-spaces. Of particular
interest is the sign of the energy density. We find that the energy density is
described by two terms: a negative position independent (Casimir) term, and a
positive position dependent term with a minimum value at the center of the
vacuum region. We argue that in some cases, including physically realizable
ones, the negative term can dominate in a given region between the two
half-spaces, so the overall energy density can be negative in this region.Comment: 16 pages, 4 figures; 3 references and some new material in Sect. 4.4
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