13,425 research outputs found
Old supernova dust factory revealed at the Galactic center
Dust formation in supernova ejecta is currently the leading candidate to
explain the large quantities of dust observed in the distant, early Universe.
However, it is unclear whether the ejecta-formed dust can survive the hot
interior of the supernova remnant (SNR). We present infrared observations of
~0.02 of warm (~100 K) dust seen near the center of the ~10,000
yr-old Sgr A East SNR at the Galactic center. Our findings signify the
detection of dust within an older SNR that is expanding into a relatively dense
surrounding medium ( ~ 100 ) and has survived the
passage of the reverse shock. The results suggest that supernovae may indeed be
the dominant dust production mechanism in the dense environment of early
Universe galaxies.Comment: 25 pages, 5 figures. Includes supplementary materials. Published
Online March 19 2015 on Science Expres
Recent progress in the development of a solar neutron tracking device (SONTRAC)
We report the results of recent calibration data analysis of a prototype scintillating fiber tracking detector system designed to perform imaging, spectroscopy and particle identification on 20 to 250 MeV neutrons and protons. We present the neutron imaging concept and briefly review the detection principle and the prototype description. The prototype detector system records ionization track data on an event-by-event basis allowing event selection criteria to be used in the off-line analysis. Images of acrylic phantoms from the analysis of recent proton beam calibrations are presented as demonstrations of the particle identification, imaging and energy measurement capabilities. The measured position resolution is \u3c 500 micrometers . The measured energy resolution is 14.2 percent at 35 MeV. The detection techniques employed can be applied to measurements in a variety of disciplines including solar and atmospheric physics, radiation therapy and nuclear materials monitoring. These applications are discussed briefly as are alternative detector configurations and future development plans
Maximin optimal cluster randomized designs for assessing treatment effect heterogeneity
Cluster randomized trials (CRTs) are studies where treatment is randomized at
the cluster level but outcomes are typically collected at the individual level.
When CRTs are employed in pragmatic settings, baseline population
characteristics may moderate treatment effects, leading to what is known as
heterogeneous treatment effects (HTEs). Pre-specified, hypothesis-driven HTE
analyses in CRTs can enable an understanding of how interventions may impact
subpopulation outcomes. While closed-form sample size formulas have recently
been proposed, assuming known intracluster correlation coefficients (ICCs) for
both the covariate and outcome, guidance on optimal cluster randomized designs
to ensure maximum power with pre-specified HTE analyses has not yet been
developed. We derive new design formulas to determine the cluster size and
number of clusters to achieve the locally optimal design (LOD) that minimizes
variance for estimating the HTE parameter given a budget constraint. Given the
LODs are based on covariate and outcome-ICC values that are usually unknown, we
further develop the maximin design for assessing HTE, identifying the
combination of design resources that maximize the relative efficiency of the
HTE analysis in the worst case scenario. In addition, given the analysis of the
average treatment effect is often of primary interest, we also establish
optimal designs to accommodate multiple objectives by combining considerations
for studying both the average and heterogeneous treatment effects. We
illustrate our methods using the context of the Kerala Diabetes Prevention
Program CRT, and provide an R Shiny app to facilitate calculation of optimal
designs under a wide range of design parameters.Comment: 25 pages, 6 figures, 5 tables, 3 appendices; clarified phrasing,
typos correcte
Quantum mutual information of an entangled state propagating through a fast-light medium
Although it is widely accepted that classical information cannot travel
faster than the speed of light in vacuum, the behavior of quantum correlations
and quantum information propagating through actively-pumped fast-light media
has not been studied in detail. To investigate this behavior, we send one half
of an entangled state of light through a gain-assisted fast-light medium and
detect the remaining quantum correlations. We show that the quantum
correlations can be advanced by a small fraction of the correlation time while
the entanglement is preserved even in the presence of noise added by
phase-insensitive gain. Additionally, although we observe an advance of the
peak of the quantum mutual information between the modes, we find that the
degradation of the mutual information due to the added noise appears to prevent
an advancement of the leading edge. In contrast, we show that both the leading
and trailing edges of the mutual information in a slow-light system can be
significantly delayed
Group sequential two-stage preference designs
The two-stage preference design (TSPD) enables the inference for treatment
efficacy while allowing for incorporation of patient preference to treatment.
It can provide unbiased estimates for selection and preference effects, where a
selection effect occurs when patients who prefer one treatment respond
differently than those who prefer another, and a preference effect is the
difference in response caused by an interaction between the patient's
preference and the actual treatment they receive. One potential barrier to
adopting TSPD in practice, however, is the relatively large sample size
required to estimate selection and preference effects with sufficient power. To
address this concern, we propose a group sequential two-stage preference design
(GS-TSPD), which combines TSPD with sequential monitoring for early stopping.
In the GS-TSPD, pre-planned sequential monitoring allows investigators to
conduct repeated hypothesis tests on accumulated data prior to full enrollment
to assess study eligibility for early trial termination without inflating type
I error rates. Thus, the procedure allows investigators to terminate the study
when there is sufficient evidence of treatment, selection, or preference
effects during an interim analysis, thereby reducing the design resource in
expectation. To formalize such a procedure, we verify the independent
increments assumption for testing the selection and preference effects and
apply group sequential stopping boundaries from the approximate sequential
density functions. Simulations are then conducted to investigate the operating
characteristics of our proposed GS-TSPD compared to the traditional TSPD. We
demonstrate the applicability of the design using a study of Hepatitis C
treatment modality.Comment: 27 pages, 7 tables, 5 figures, 4 appendices; under review at
Statistics in Medicin
- …