28,545 research outputs found
Quark-Hadron Duality in Structure Functions
Quark-hadron duality is studied in a systematic way for both the unpolarized
and polarized structure functions, by taking into account all the available
data in the resonance region.In both cases, a detailed perturbative QCD based
analysis of the structure functions integrals in the resonance region is
performed: non perturbative contributions are disentangled, and higher twist
terms are evaluated. A different behavior between the unpolarized and polarized
structure functions at low Q^2 is found.Comment: 5 pages, 4 figure
A Perturbative QCD Based Study of Polarized Nucleon Structure in the Transition Region and Beyond: "Quarks, Color Neutral Clusters, and Hadrons"
A large fraction of the world data on both polarized and unpolarized
inclusive scattering at large Bjorken lies in the resonance region
where a correspondence with the deep inelastic regime, known as Bloom and
Gilman's duality, was observed. Recent analyses of the dependence of the
data show that parton-hadron duality is inconsistent with the twist expansion
at low values of the final state invariant mass. We investigate the nature of
this disagreement, and we interpret its occurrence in terms of contributions
from non partonic degrees of freedom in a preconfinement model.Comment: 5 pages, 1 figure, to be published in the Proceedings of the "3rd
International Symposium on the Gerasimov-Drell-Hearn Sum Rule and its
Extensions", Editors, J.P. Chen and S. Kuh
From Passive to Active: A New Model for Library Orientation
Objectives This poster shares the experience of academic librarians turning a traditional, passive library orientation at a Graduate Medical Education Fair for new residents into an active learning activity.
Methods Every summer, new residents enter postgraduate medical training programs at the medical center. In an effort to introduce the library early in their clinical careers, liaison librarians have participated at a Graduate Medical Education fair since 2012. In the past, the library’s orientation activity consisted of a table full of paper handouts, staffed by overzealous librarians. Feedback from the residents, however, revealed that they politely collected the paperwork but frequently used or understood little of its content. In 2016, the library orientation for new residents was completely redesigned and updated. The traditional library handouts were replaced by an active learning exercise centered around a poster that highlighted the essential resources and services provided by the library. Residents were asked to spend just 1 minute reading through the library poster, and then invited to complete a brief poster survey.
Results Results of this new orientation format were quite revealing. Completed poster surveys were returned by more than 85% of the residents, showing that they liked the poster format and its effectiveness in introducing the library. The poster also prompted many questions and interesting conversations among residents and librarians right on the spot. The simple design of the survey questions encouraged residents to identify what they liked and wanted to learn more about the library. In addition, the survey fostered a perfect opportunity to ask questions about anything important that was included or missing from the poster.
Conclusions Librarians plan to continue to use this poster driven learning activity, to study its results, to modify its content when appropriate, and to build on its success in other library presentations
A perturbative re-analysis of N=4 supersymmetric Yang--Mills theory
The finiteness properties of the N=4 supersymmetric Yang-Mills theory are
reanalyzed both in the component formulation and using N=1 superfields, in
order to discuss some subtleties that emerge in the computation of gauge
dependent quantities. The one-loop corrections to various Green functions of
elementary fields are calculated. In the component formulation it is shown that
the choice of the Wess-Zumino gauge, that is standard in supersymmetric gauge
theories, introduces ultraviolet divergences in the propagators at the one-loop
level. Such divergences are exactly cancelled when the contributions of the
fields that are put to zero in the Wess-Zumino gauge are taken into account. In
the description in terms of N=1 superfields infrared divergences are found for
every choice of gauge different from the supersymmetric generalization of the
Fermi-Feynman gauge. Two-, three- and four-point functions of N=1 superfields
are computed and some general features of the infrared problem are discussed.
We also examine the effect of the introduction of mass terms for the (anti)
chiral superfields in the theory, which break supersymmetry from N=4 to N=1. It
is shown that in the mass deformed model no ultraviolet divergences appear in
two-point functions. It argued that this result can be generalized to n-point
functions, supporting the proposal of a possible of use of this modified model
as a supersymmetry-preserving regularization scheme for N=1 theories.Comment: 41 pages, LaTeX2e, uses feynMP package to draw Feynman diagram
Polar features in the flagellar propulsion of E. coli bacteria
E. coli bacteria swim following a run and tumble pattern. In the run state
all flagella join in a single helical bundle that propels the cell body along
approximately straight paths. When one or more flagellar motors reverse
direction the bundle unwinds and the cell randomizes its orientation. This
basic picture represents an idealization of a much more complex dynamical
problem. Although it has been shown that bundle formation can occur at either
pole of the cell, it is still unclear whether this two run states correspond to
asymmetric propulsion features. Using holographic microscopy we record the 3D
motions of individual bacteria swimming in optical traps. We find that most
cells possess two run states characterised by different propulsion forces,
total torque and bundle conformations. We analyse the statistical properties of
bundle reversal and compare the hydrodynamic features of forward and backward
running states. Our method is naturally multi-particle and opens up the way
towards controlled hydrodynamic studies of interacting swimming cells
The metal and dust yields of the first massive stars
We quantify the role of Population (Pop) III core-collapse supernovae (SNe)
as the first cosmic dust polluters. Starting from a homogeneous set of stellar
progenitors with masses in the range [13 - 80] Msun, we find that the mass and
composition of newly formed dust depend on the mixing efficiency of the ejecta
and the degree of fallback experienced during the explosion. For standard Pop
III SNe, whose explosions are calibrated to reproduce the average elemental
abundances of Galactic halo stars with [Fe/H] < -2.5, between 0.18 and 3.1 Msun
(0.39 - 1.76 Msun) of dust can form in uniformly mixed (unmixed) ejecta, and
the dominant grain species are silicates. We also investigate dust formation in
the ejecta of faint Pop III SN, where the ejecta experience a strong fallback.
By examining a set of models, tailored to minimize the scatter with the
abundances of carbon-enhanced Galactic halo stars with [Fe/H ] < -4, we find
that amorphous carbon is the only grain species that forms, with masses in the
range 2.7 10^{-3} - 0.27 Msun (7.5 10^{-4} - 0.11 Msun) for uniformly mixed
(unmixed) ejecta models. Finally, for all the models we estimate the amount and
composition of dust that survives the passage of the reverse shock, and find
that, depending on circumstellar medium densities, between 3 and 50% (10 - 80%)
of dust produced by standard (faint) Pop III SNe can contribute to early dust
enrichment.Comment: Accepted by MNRAS, 22 pages, 12 figures, 12 table
A Chandra view of the clumpy reflector at the heart of the Circinus galaxy
We present a spectral and imaging analysis of the X-ray reflecting structure
at the heart of the Circinus galaxy, investigating the innermost regions
surrounding the central black hole. By studying an archival 200 ks Chandra
ACIS-S observation, we are able to image the extended clumpy structure
responsible for both cold reflection of the primary radiation and neutral iron
Ka line emission. We measure an excess of the equivalent width of the iron Ka
line which follows an axisymmetric geometry around the nucleus on a hundred pc
scale. Spectra extracted from different regions confirm a scenario in which the
dominant mechanism is the reflection of the nuclear radiation from
Compton-thick gas. Significant differences in the equivalent width of the iron
Ka emission line (up to a factor of 2) are found. It is argued that these
differences are due to different scattering angles with respect to the line of
sight rather than to different iron abundances.Comment: 6 pages, 4 figures, accepted for publication on MNRA
On the reconstruction of planar lattice-convex sets from the covariogram
A finite subset of is said to be lattice-convex if is
the intersection of with a convex set. The covariogram of
is the function associating to each u \in
\integer^d the cardinality of . Daurat, G\'erard, and Nivat and
independently Gardner, Gronchi, and Zong raised the problem on the
reconstruction of lattice-convex sets from . We provide a partial
positive answer to this problem by showing that for and under mild extra
assumptions, determines up to translations and reflections. As a
complement to the theorem on reconstruction we also extend the known
counterexamples (i.e., planar lattice-convex sets which are not
reconstructible, up to translations and reflections) to an infinite family of
counterexamples.Comment: accepted in Discrete and Computational Geometr
Black Holes in the Presence of Cosmological Constant and Large N Brane World
Analytic form has been obtained for four-dimensional black holes with a
minimal Hawking temperature in a theory with cosmological constant, dilaton and
gauge fields. In general dimensions, black hole solutions are shown to exist
and their asymptotic behaviors are obtained. In theories of ten dimension, N
coincident D3-branes as the boundary of an space are constructed by
embedding black D3-branes, with a five-dimensional compactified space of
negligible size if N is large, which provide natural realizations of the
Randall-Sundrum scenario. For this background, the cosmological
constant is a higher order perturbation and its effect on the spectra of
standard model fields on the branes can be calculated.Comment: 12 pages, no figure
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