24,654 research outputs found
Nuclear Medium Modifications of Hadrons from Generalized Parton Distributions
We study the structure of generalized parton distributions in spin 0 nuclei
within a microscopic approach for nuclear dynamics. GPDs can be used on one
side as tools to unravel the deep inelastic transverse structure of nuclei in
terms of both transverse spatial and transverse momentum degrees of freedom. On
the other, one can obtain information on GPDs themselves by observing how they
become modified in the nuclear environment. We derive the structure of the
nuclear deeply virtual Compton scattering tensor and generalized parton
distributions at leading order in in a field-theoretical framework. The
nuclear generalized parton distributions are calculated using a two step
process -- the convolution approach -- where the scattering process happens
from a quark inside a nucleon, itself inside a nucleus, disregarding final
state interactions with both the nuclear and nucleon debris. We point out that
details of the nuclear long range interactions such as two-body currents, can
be disregarded compared to the deep inelastic induced modifications of the
bound GPDs. We show how the pattern of nuclear modifications predicted, and in
particular the deviations of off-shell effects from the longitudinal
convolution provide clear signals to be sought in experimental measurements.
Finally, we find interesting relationships by studying Mellin moments in
nuclei: in particular we predict the -dependence for the -term of GPDs
within a microscopic approach, and the behavior with of the total momentum
carried by quarks in a nucleus. The latter provides an important element for
the evaluation of nuclear hadronization phenomena which are vital for
interpreting current and future data at RHIC, HERMES and Jefferson Lab.Comment: 29 pages, 10 figure
High intensity radiation heat source is capable of sustained operation
Water cooled, high intensity radiation source rated at 125 kw, with an efficiency of 31 to 34 percent is used in the evaluation of ablative materials under simulated conditions of high velocity entry into planetary atmospheres. The source operates repeatedly at maximum power for periods of 10 to 20 minutes
Temperature dependence of trapped magnetic field in MgB2 bulk superconductor
Based on DC magnetization measurements, the temperature dependencies of the
trapped magnetic field have been calculated for two MgB2 samples prepared by
two different techniques: the high-pressure sintering and the hot pressing.
Experimentally measured trapped field values for the first sample coincide
remarkably well with calculated ones in the whole temperature range. This
proves, from one side, the validity of the introduced calculation approach, and
demonstrates, from another side, the great prospects of the hot pressing
technology for large scale superconducting applications of the MgB2.Comment: 3 pages, 3 figures, submitted to AP
Mortality Model for a Perennial Grass in Australian Semi-Arid Wooded Grasslands Grazed by Sheep
Grazing of sheep in marginal semi-arid environments is risky because grazing appears to predispose grass plants, especially palatable species, to sudden death (Hodgkinson, 1994; 1995). These early observations were based on a preliminary analysis of perennial grass survival in a single drought and supported the concept of tactical grazing proposed by Westoby et al. (1989) as a preferred management. Later this idea was developed by suggesting the existence of critical thresholds for perennial grass survival, which when crossed, collapses grass populations (Hodgkinson, 1994). Here we examine the relationship between mortality of a palatable perennial grass, Thyridolepis mitchelliana, and a number of variables measured during a 10-year period
Two-Dimensional Hydrodynamics of Pre-Core Collapse: Oxygen Shell Burning
By direct hydrodynamic simulation, using the Piecewise Parabolic Method (PPM)
code PROMETHEUS, we study the properties of a convective oxygen burning shell
in a SN 1987A progenitor star prior to collapse. The convection is too
heterogeneous and dynamic to be well approximated by one-dimensional
diffusion-like algorithms which have previously been used for this epoch.
Qualitatively new phenomena are seen.
The simulations are two-dimensional, with good resolution in radius and
angle, and use a large (90-degree) slice centered at the equator. The
microphysics and the initial model were carefully treated. Many of the
qualitative features of previous multi-dimensional simulations of convection
are seen, including large kinetic and acoustic energy fluxes, which are not
accounted for by mixing length theory. Small but significant amounts of
carbon-12 are mixed non-uniformly into the oxygen burning convection zone,
resulting in hot spots of nuclear energy production which are more than an
order of magnitude more energetic than the oxygen flame itself. Density
perturbations (up to 8%) occur at the `edges' of the convective zone and are
the result of gravity waves generated by interaction of penetrating flows into
the stable region. Perturbations of temperature and electron fraction at the
base of the convective zone are of sufficient magnitude to create angular
inhomogeneities in explosive nucleosynthesis products, and need to be included
in quantitative estimates of yields. Combined with the plume-like velocity
structure arising from convection, the perturbations will contribute to the
mixing of nickel-56 throughout supernovae envelopes. Runs of different
resolution, and angular extent, were performed to test the robustness of theseComment: For mpeg movies of these simulations, see
http://www.astrophysics.arizona.edu/movies.html Submitted to the
Astrophysical Journa
A Low-cost Multi-channel Analogue Signal Generator
A scalable multi-channel analogue signal generator is presented. It uses a commercial low-cost graphics card with multiple outputs in a standard PC as signal source. Each color signal serves as independent channel to generate an analogue signal. A custom-built external PCB was developed to adjust the graphics card output voltage levels for a specific task, which needed differential signals. The system furthermore comprises a software package to program the signal shape. The implementation of the signal generator is presented as well as an application where it was successfully utilized
Levels of genetic polymorphism: marker loci versus quantitative traits
Species are the units used to measure ecological diversity and alleles are the units of genetic diversity. Genetic variation within and among species has been documented most extensively using allozyme electrophoresis. This reveals wide differences in genetic variability within, and genetic distances among, species, demonstrating that species are not equivalent units of diversity. The extent to which the pattern observed for allozymes can be used to infer patterns of genetic variation in quantitative traits depends on the forces generating and maintaining variability. Allozyme variation is probably not strictly neutral but, nevertheless, heterozygosity is expected to be influenced by population size and genetic distance will be affected by time since divergence. The same is true for quantitative traits influenced by many genes and under weak stabilizing selection. However, the limited data available suggest that allozyme variability is a poor predictor of genetic variation in quantitative traits within populations. It is a better predictor of general phenotypic divergence and of postzygotic isolation between populations or species, but is only weakly correlated with prezygotic isolation. Studies of grasshopper and planthopper mating signal variation and assortative mating illustrate how these characters evolve independently of general genetic and morphological variation. The role of such traits in prezygotic isolation, and hence speciation, means that they will contribute significantly to the diversity of levels of genetic variation within and among species
Impacts of Economic and Psychological Factors on Women’s Obesity and Food Assistance Program Participation: Evidence from the NLSY Panel
Over the past thirty-five years, the U.S. adult obesity rate has more than doubled from roughly 15% to 35%, reflecting a general diffusion of obesity across all segments of the adult population (USDHHS). Obesity is a concern because it increases the risk for cardiovascular diseases, diabetes, and most forms of cancer, except for lung. Earlier studies of obesity of U.S. adults have largely focused on data in a single cross-section or one round of a panel survey. Chen and Huffman (2010) show that food and drink prices significantly affect U.S. women’s probability of being obese but not for men. However, the impact of individual food and drink prices are not always as expected
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