462 research outputs found
Disseminated Effects in Agent Based Models: A Potential Outcomes Framework and Application to Inform Pre-Exposure Prophylaxis Coverage Levels for HIV Prevention
Pre-exposure prophylaxis (PrEP) for HIV prevention may not only benefit the individual who uses it, but also their uninfected sexual risk contacts. We developed an agent-based model using a novel trial emulation approach to quantify disseminated effects of PrEP use among men who have sex with men in Atlanta, USA from 2015 to 2017. Components (subsets of agents connected through partnerships in a sexual network, but not sharing partnerships with any other agents) were first randomized to an intervention coverage level or control, then within intervention components, eligible agents were randomized to PrEP. We estimated direct and disseminated (indirect) effects using randomization-based estimators and reported corresponding 95% simulation intervals across scenarios ranging from 10% to 90% coverage in the intervention components. A population of 11,245 agents was simulated with an average of 1,551 components identified. Comparing agents randomized to PrEP in 70% coverage components to control agents, there was a 15% disseminated risk reduction in HIV incidence (95% simulation intervals = 0.65, 1.05). Individuals not on PrEP may receive a protective benefit by being in a sexual network with higher PrEP coverage. Agent-based models are useful to evaluate possible direct and disseminated effects of HIV prevention modalities in sexual networks
Spacetime as a membrane in higher dimensions
By means of a simple model we investigate the possibility that spacetime is a
membrane embedded in higher dimensions. We present cosmological solutions of
d-dimensional Einstein-Maxwell theory which compactify to two dimensions. These
solutions are analytically continued to obtain dual solutions in which a
(d-2)-dimensional Einstein spacetime "membrane" is embedded in d-dimensions.
The membrane solutions generalise Melvin's 4-dimensional flux tube solution.
The flat membrane is shown to be classically stable. It is shown that there are
zero mode solutions of the d-dimensional Dirac equation which are confined to a
neighbourhood of the membrane and move within it like massless chiral
(d-2)-dimensional fermions. An investigation of the spectrum of scalar
perturbations shows that a well-defined mass gap between the zero modes and
massive modes can be obtained if there is a positive cosmological term in (d-2)
dimensions or a negative cosmological term in d dimensions.Comment: 30 pages, 4 figures in 10 files, epsf. This early brane world paper
is being placed on the archive to make it more easily accessible, as its
results are used in a new brane world construction in an accompanying
submissio
A multi-layer extension of the stochastic heat equation
Motivated by recent developments on solvable directed polymer models, we
define a 'multi-layer' extension of the stochastic heat equation involving
non-intersecting Brownian motions.Comment: v4: substantially extended and revised versio
Templates for Convex Cone Problems with Applications to Sparse Signal Recovery
This paper develops a general framework for solving a variety of convex cone
problems that frequently arise in signal processing, machine learning,
statistics, and other fields. The approach works as follows: first, determine a
conic formulation of the problem; second, determine its dual; third, apply
smoothing; and fourth, solve using an optimal first-order method. A merit of
this approach is its flexibility: for example, all compressed sensing problems
can be solved via this approach. These include models with objective
functionals such as the total-variation norm, ||Wx||_1 where W is arbitrary, or
a combination thereof. In addition, the paper also introduces a number of
technical contributions such as a novel continuation scheme, a novel approach
for controlling the step size, and some new results showing that the smooth and
unsmoothed problems are sometimes formally equivalent. Combined with our
framework, these lead to novel, stable and computationally efficient
algorithms. For instance, our general implementation is competitive with
state-of-the-art methods for solving intensively studied problems such as the
LASSO. Further, numerical experiments show that one can solve the Dantzig
selector problem, for which no efficient large-scale solvers exist, in a few
hundred iterations. Finally, the paper is accompanied with a software release.
This software is not a single, monolithic solver; rather, it is a suite of
programs and routines designed to serve as building blocks for constructing
complete algorithms.Comment: The TFOCS software is available at http://tfocs.stanford.edu This
version has updated reference
Observational Constraints on Cosmological Models with the Updated Long Gamma-Ray Bursts
In the present work, by the help of the newly released Union2 compilation
which consists of 557 Type Ia supernovae (SNIa), we calibrate 109 long
Gamma-Ray Bursts (GRBs) with the well-known Amati relation, using the
cosmology-independent calibration method proposed by Liang {\it et al.}. We
have obtained 59 calibrated high-redshift GRBs which can be used to constrain
cosmological models without the circularity problem (we call them ``Hymnium''
GRBs sample for convenience). Then, we consider the joint constraints on 7
cosmological models from the latest observational data, namely, the combination
of 557 Union2 SNIa dataset, 59 calibrated Hymnium GRBs dataset (obtained in
this work), the shift parameter from the WMAP 7-year data, and the distance
parameter of the measurement of the baryon acoustic oscillation (BAO) peak
in the distribution of SDSS luminous red galaxies. We also briefly consider the
comparison of these 7 cosmological models.Comment: 19 pages, 3 tables, 10 figures, revtex4; v2: accepted for publication
in JCAP; v3: published versio
Magnetization steps in a diluted Heisenberg antiferromagnetic chain: Theory and experiments on TMMC:Cd
A theory for the equilibrium low-temperature magnetization M of a diluted
Heisenberg antiferromagnetic chain is presented. The magnetization curve, M
versus B, is calculated using the exact contributions of finite chains with 1
to 5 spins, and the "rise and ramp approximation" for longer chains. Some
non-equilibrium effects that occur in a rapidly changing B, are also
considered. Specific non-equilibrium models based on earlier treatments of the
phonon bottleneck, and of spin flips associated with cross relaxation and with
level crossings, are discussed. Magnetization data on powders of TMMC diluted
with cadmium [i.e., (CH_3)_4NMn_xCd_(1-x)Cl_3, with 0.16<=x<=0.50 were measured
at 0.55 K in 18 T superconducting magnets. The field B_1 at the first MST from
pairs is used to determine the NN exchange constant, J, which changes from -5.9
K to -6.5 K as x increases from 0.16 to 0.50. The magnetization curves obtained
in the superconducting magnets are compared with simulations based on the
equilibrium theory. Data for the differential susceptibility, dM/dB, were taken
in pulsed magnetic fields (7.4 ms duration) up to 50 T, with the powder samples
in a 1.5 K liquid-helium bath. Non-equilibrium effects, which became more
severe as x decreased, were observed. The non-equilibrium effects are
tentatively interpreted using the "Inadequate Heat Flow Scenario," or to
cross-relaxation, and crossings of energy levels, including those of excited
states.Comment: 16 pages, 14 figure
Measurements of the strong-interaction widths of the kaonic 3He and 4He 2p levels
The kaonic 3He and 4He X-rays emitted in the 3d-2p transitions were measured
in the SIDDHARTA experiment. The widths of the kaonic 3He and 4He 2p states
were determined to be Gamma_2p(3He) = 6 \pm 6 (stat.) \pm 7 (syst.) eV, and
Gamma_2p(4He) = 14 \pm 8 (stat.) \pm 5 (syst.) eV, respectively. Both results
are consistent with the theoretical predictions. The width of kaonic 4He is
much smaller than the value of 55 \pm 34 eV determined by the experiments
performed in the 70's and 80's, while the width of kaonic 3He was determined
for the first time.Comment: Accepted in Phys. Lett.
Young and Intermediate-age Distance Indicators
Distance measurements beyond geometrical and semi-geometrical methods, rely
mainly on standard candles. As the name suggests, these objects have known
luminosities by virtue of their intrinsic proprieties and play a major role in
our understanding of modern cosmology. The main caveats associated with
standard candles are their absolute calibration, contamination of the sample
from other sources and systematic uncertainties. The absolute calibration
mainly depends on their chemical composition and age. To understand the impact
of these effects on the distance scale, it is essential to develop methods
based on different sample of standard candles. Here we review the fundamental
properties of young and intermediate-age distance indicators such as Cepheids,
Mira variables and Red Clump stars and the recent developments in their
application as distance indicators.Comment: Review article, 63 pages (28 figures), Accepted for publication in
Space Science Reviews (Chapter 3 of a special collection resulting from the
May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space
Age
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