27,144 research outputs found
Dynamical Coupled-Channels Effects on Pion Photoproduction
The electromagnetic pion production reactions are investigated within the
dynamical coupled-channels model developed in {\bf Physics Reports, 439, 193
(2007)}. The meson-baryon channels included in this study are , , , and the , and resonant components
of the channel. With the hadronic parameters of the model determined
in a recent study of scattering, we show that the pion photoproduction
data up to the second resonance region can be described to a very large extent
by only adjusting the bare helicity amplitudes, while the
non-resonant electromagnetic couplings are taken from previous works. It is
found that the coupled-channels effects can contribute about 10 - 20 % of the
production cross sections in the (1232) resonance region, and can
drastically change the magnitude and shape of the cross sections in the second
resonance region. The importance of the off-shell effects in a dynamical
approach is also demonstrated. The meson cloud effects as well as the
coupled-channels contributions to the form factors are found
to be mainly in the low region. For the magnetic M1
(1232) form factor, the results are close to that of the Sato-Lee Model.
Necessary improvements to the model and future developments are discussed.Comment: Corrected version. 14 pages, 10 figure
Search for missing baryon resonances via associated strangeness photoproduction
Differential cross-section and single polarization observables in the process
gamma p --> K^+ Lambda are investigated within a constituent quark model and a
dynamical coupled-channel formalism. The effects of two new nucleon resonances
and of the K*(892)- and K1(1270)-exchanges are briefly presented.Comment: Contributed paper to the IVth International Conference on Quarks and
Nuclear Physics, Madrid June 5-10, 200
Quark-Exchange Mechanism of Reaction At 2-6 GeV
Within the constituent quark model, we examine the extent to which the
deuteron photo-disintegration at 2-6 GeV can be described by the quark-exchange
mechanism. With the parameters constrained by the scattering, the
calculated differential cross sections disagree with the data in both magnitude
and energy-dependence. The results can be improved if we use a smaller size
parameter for quark wavefunctions. We also find that the on-shell approximation
used in a previous investigation is not accurateComment: To be published in the Proceeeding of Second Asia Pacific Conference
on Few-Body Problems in Physics, Shanghai, China, August 27-30, 200
Shearing Interferometer for Quantifying the Coherence of Hard X-Ray Beams
We report a quantitative measurement of the full transverse coherence function of the 14.4 keV x-ray radiation produced by an undulator at the Swiss Light Source. An x-ray grating interferometer consisting of a beam splitter phase grating and an analyzer amplitude grating has been used to measure the degree of coherence as a function of the beam separation out to 30 m. Importantly, the technique provides a model-free and spatially resolved measurement of the complex coherence function and is not restricted to high resolution detectors and small fields of view. The spatial characterization of the wave front has important applications in discovering localized defects in beam line optics
The Nature and Frequency of Outflows from Stars in the Central Orion Nebula Cluster
Recent Hubble Space Telescope images have allowed the determination with
unprecedented accuracy of motions and changes of shocks within the inner Orion
Nebula. These originate from collimated outflows from very young stars, some
within the ionized portion of the nebula and others within the host molecular
cloud. We have doubled the number of Herbig-Haro objects known within the inner
Orion Nebula. We find that the best-known Herbig-Haro shocks originate from a
relatively few stars, with the optically visible X-ray source COUP 666 driving
many of them.
While some isolated shocks are driven by single collimated outflows, many
groups of shocks are the result of a single stellar source having jets oriented
in multiple directions at similar times. This explains the feature that shocks
aligned in opposite directions in the plane of the sky are usually blue shifted
because the redshifted outflows pass into the optically thick Photon Dominated
Region behind the nebula. There are two regions from which optical outflows
originate for which there are no candidate sources in the SIMBAD data base.Comment: 152 pages, 46 figures, 7 tables. Accepted by A
Crystal and magnetic structure of the oxypnictide superconductor LaO(1-x)FxFeAs: evidence for magnetoelastic coupling
High-resolution and high-flux neutron as well as X-ray powder-diffraction
experiments were performed on the oxypnictide series LaO(1-x)FxFeAs with
0<x<0.15 in order to study the crystal and magnetic structure. The magnetic
symmetry of the undoped compound corresponds to those reported for ReOFeAs
(with Re a rare earth) and for AFe2As2 (A=Ba, Sr) materials. We find an ordered
magnetic moment of 0.63(1)muB at 2 K in LaOFeAs, which is significantly larger
than the values previously reported for this compound. A sizable ordered
magnetic moment is observed up to a F-doping of 4.5% whereas there is no
magnetic order for a sample with a F concentration of x=0.06. In the undoped
sample, several interatomic distances and FeAs4 tetrahedra angles exhibit
pronounced anomalies connected with the broad structural transition and with
the onset of magnetism supporting the idea of strong magneto-elastic coupling
in this material.Comment: 8 pages, 7 figures, regular articl
Effect of Tuned Parameters on a LSA MCQ Answering Model
This paper presents the current state of a work in progress, whose objective
is to better understand the effects of factors that significantly influence the
performance of Latent Semantic Analysis (LSA). A difficult task, which consists
in answering (French) biology Multiple Choice Questions, is used to test the
semantic properties of the truncated singular space and to study the relative
influence of main parameters. A dedicated software has been designed to fine
tune the LSA semantic space for the Multiple Choice Questions task. With
optimal parameters, the performances of our simple model are quite surprisingly
equal or superior to those of 7th and 8th grades students. This indicates that
semantic spaces were quite good despite their low dimensions and the small
sizes of training data sets. Besides, we present an original entropy global
weighting of answers' terms of each question of the Multiple Choice Questions
which was necessary to achieve the model's success.Comment: 9 page
Determining the Chirality of Yukawa Couplings via Single Charged Higgs Boson Production in Polarized Photon Collision
When the charged Higgs boson is too heavy to be produced in pairs, the
predominant production mechanism at Linear Colliders is via the single charged
Higgs boson production processes, such as and . We show that
the yield of a heavy charged Higgs boson at a collider is
typically one or two orders of magnitude larger than that at an
collider. Furthermore, a polarized collider can determine the
chirality of the Yukawa couplings of fermions with charged Higgs boson via
single charged Higgs boson production, and thus discriminate models of new
physics.Comment: Version accepted by Physical Review Letters (references added, minor
rewording, RevTex4
Holographic formula for the determinant of the scattering operator in thermal AdS
A 'holographic formula' expressing the functional determinant of the
scattering operator in an asymptotically locally anti-de Sitter(ALAdS) space
has been proposed in terms of a relative functional determinant of the scalar
Laplacian in the bulk. It stems from considerations in AdS/CFT correspondence
of a quantum correction to the partition function in the bulk and the
corresponding subleading correction at large N on the boundary. In this paper
we probe this prediction for a class of quotients of hyperbolic space by a
discrete subgroup of isometries. We restrict to the simplest situation of an
abelian group where the quotient geometry describes thermal AdS and also the
non-spinning BTZ instanton. The bulk computation is explicitly done using the
method of images and the answer can be encoded in a (Patterson-)Selberg
zeta-function.Comment: 11 pages, published JPA versio
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