2,636 research outputs found
Q^2-evolution of nucleon-to-resonance transition form factors in a QCD-inspired vector-meson-dominance model
We adopt the vector-meson-dominance approach to investigate Q^2-evolution of
N-R transition form factors (N denotes nucleon and R an excited resonance) in
the first and second resonance regions. The developed model is based upon
conventional NR\gamma-interaction Lagrangians, introducing three form factors
for spin-3/2 resonances and two form factors for spin-1/2 nucleon excitations.
Lagrangian form factors are expressed as dispersionlike expansions with four or
five poles corresponding to the lowest excitations of the mesons \rho(770) and
\omega(782). Correct high-Q^2 form factor behavior predicted by perturbative
QCD is due to phenomenological logarithmic renormalization of electromagnetic
coupling constants and linear superconvergence relations between the parameters
of the meson spectrum. The model is found to be in good agreement with all the
experimental data on Q^2-dependence of the transitions N-\Delta(1232),
N-N(1440), N-N(1520), N-N(1535). We present fit results and model predictions
for high-energy experiments proposed by JLab. Besides, we make special emphasis
on the transition to perturbative domain of N-\Delta(1232) form factors.Comment: 22 pages, 22 PS figures, REVTeX 4; v2: +3 refs, minor editorial
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A Concept Paper on Networks of Excellence for Research and Education
Research and education ecosystems, foundational components of knowledge-based economies, are relatively underdeveloped in Latin America. The entire ecosystem of a research university â including resources, corporate partnerships, and research â must capitalize on a symbiosis between the research, education and commercialization missions. A university cannot transform unilaterally nor can universities sustain the required transformation without government and industry participation. Initiatives to accelerate the development of research university ecosystems are critical for the realization of knowledge-based economies and resilient civil societies.
To accelerate the development of research and education ecosystems across the Americas, the authors propose to establish âNetworks of Excellenceâ in key focus areas. Each Network of Excellence will be multi-institutional, multi-sector (university, corporate, government, NGO) and multi-national. These multi-faceted networks will allow participants to define and share programs, policies, and content, significantly leverage the resources provided for related programs, and identify opportunities to leapfrog existing programs. Proposed themes for networks include regional grand challenges and cross-cutting capabilities
Origins and early development of human body knowledge
As a knowable object, the human body is highly complex. Evidence from several converging lines of research, including psychological studies, neuroimaging and clinical neuropsychology, indicates that human body knowledge is widely distributed in the adult brain, and is instantiated in at least three partially independent levels of representation. Sensori-motor body knowledge is responsible for on-line control and movement of one's own body and may also contribute to the perception of others' moving bodies; visuo-spatial body knowledge specifies detailed structural descriptions of the spatial attributes of the human body; and lexical-semantic body knowledge contains language-based knowledge about the human body. In the first chapter of this Monograph, we outline the evidence for these three hypothesized levels of human body knowledge, then review relevant literature on infants' and young children's human body knowledge in terms of the three-level framework. In Chapters II and III, we report two complimentary series of studies that specifically investigate the emergence of visuospatial body knowledge in infancy. Our technique is to compare infants' responses to typical and scrambled human bodies, in order to evaluate when and how infants acquire knowledge about the canonical spatial layout of the human body. Data from a series of visual habituation studies indicate that infants first discriminate scrambled from typical human body pictures at 15 to 18 months of age. Data from object examination studies similarly indicate that infants are sensitive to violations of three-dimensional human body stimuli starting at 15-18 months of age. The overall pattern of data supports several conclusions about the early development of human body knowledge: (a) detailed visuo-spatial knowledge about the human body is first evident in the second year of life, (b) visuo-spatial knowledge of human faces and human bodies are at least partially independent in infancy and (c) infants' initial visuo-spatial human body representations appear to be highly schematic, becoming more detailed and specific with development. In the final chapter, we explore these conclusions and discuss how levels of body knowledge may interact in early development
Amorphous Systems in Athermal, Quasistatic Shear
We present results on a series of 2D atomistic computer simulations of
amorphous systems subjected to simple shear in the athermal, quasistatic limit.
The athermal quasistatic trajectories are shown to separate into smooth,
reversible elastic branches which are intermittently broken by discrete
catastrophic plastic events. The onset of a typical plastic event is studied
with precision, and it is shown that the mode of the system which is
responsible for the loss of stability has structure in real space which is
consistent with a quadrupolar source acting on an elastic matrix. The plastic
events themselves are shown to be composed of localized shear transformations
which organize into lines of slip which span the length of the simulation cell,
and a mechanism for the organization is discussed. Although within a single
event there are strong spatial correlations in the deformation, we find little
correlation from one event to the next, and these transient lines of slip are
not to be confounded with the persistent regions of localized shear --
so-called "shear bands" -- found in related studies. The slip lines gives rise
to particular scalings with system length of various measures of event size.
Strikingly, data obtained using three differing interaction potentials can be
brought into quantitative agreement after a simple rescaling, emphasizing the
insensitivity of the emergent plastic behavior in these disordered systems to
the precise details of the underlying interactions. The results should be
relevant to understanding plastic deformation in systems such as metallic
glasses well below their glass temperature, soft glassy systems (such as dense
emulsions), or compressed granular materials.Comment: 21 pages, 18 figure
Beam instrumentation for the Tevatron Collider
The Tevatron in Collider Run II (2001-present) is operating with six times
more bunches and many times higher beam intensities and luminosities than in
Run I (1992-1995). Beam diagnostics were crucial for the machine start-up and
the never-ending luminosity upgrade campaign. We present the overall picture of
the Tevatron diagnostics development for Run II, outline machine needs for new
instrumentation, present several notable examples that led to Tevatron
performance improvements, and discuss the lessons for future colliders
Ordovician and Silurian Strata from Well Core in Schoolcraft County, Michigan
219-229http://deepblue.lib.umich.edu/bitstream/2027.42/48415/2/ID262.pd
Theoretical current-voltage characteristics of ferroelectric tunnel junctions
We present the concept of ferroelectric tunnel junctions (FTJs). These
junctions consist of two metal electrodes separated by a nanometer-thick
ferroelectric barrier. The current-voltage characteristics of FTJs are analyzed
under the assumption that the direct electron tunneling represents the dominant
conduction mechanism. First, the influence of converse piezoelectric effect
inherent in ferroelectric materials on the tunnel current is described. The
calculations show that the lattice strains of piezoelectric origin modify the
current-voltage relationship owing to strain-induced changes of the barrier
thickness, electron effective mass, and position of the conduction-band edge.
Remarkably, the conductance minimum becomes shifted from zero voltage due to
the piezoelectric effect, and a strain-related resistive switching takes place
after the polarization reversal in a ferroelectric barrier. Second, we analyze
the influence of the internal electric field arising due to imperfect screening
of polarization charges by electrons in metal electrodes. It is shown that, for
asymmetric FTJs, this depolarizing-field effect also leads to a considerable
change of the barrier resistance after the polarization reversal. However, the
symmetry of the resulting current-voltage loop is different from that
characteristic of the strain-related resistive switching. The crossover from
one to another type of the hysteretic curve, which accompanies the increase of
FTJ asymmetry, is described taking into account both the strain and
depolarizing-field effects. It is noted that asymmetric FTJs with dissimilar
top and bottom electrodes are preferable for the non-volatile memory
applications because of a larger resistance on/off ratio.Comment: 14 pages, 8 figure
Transforming teacher education, an activity theory analysis
This paper explores the work of teacher education in England and Scotland. It seeks to locate this work within conflicting socio-cultural views of professional practice and academic work. Drawing on an activity theory framework that integrates the analysis of these contradictory discourses with a study of teacher educatorsâ practical activities, including the material artefacts that mediate the work, the paper offers a critical perspective on the social organisation of university-based teacher education. Informed by Engeströmâs activity theory concept of transformation, the paper extends the discussion of contradictions in teacher education to consider the wider socio-cultural relations of the work. The findings raise important questions about the way in which teacher education work within universities is organised and the division of labour between schools and universities
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