4,225 research outputs found
Walking on Eggshells: The Effect of the United States Supreme Court\u27s Ruling in Burlington Northern & Santa Fe Railway Co. v. White
Coupled channel study of photoproduction
A coupled channel model with , and channels has been
used to analyze the recent data of . The non-resonant
interactions within the subspace are derived from effective
Lagrangians using a unitary transformation method. The direct photoproduction
reaction is obtained from a chiral constituent quark model with breaking. Missing baryon resonances issues are briefly discussed.Comment: Part of the proceedings of the International Workshop on the Physics
of Excited Baryons NSTAR05, 12-15 October 2005, Tallahassee, Florida, US
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
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
Disentangling the dynamical origin of P11 Nucleon Resonances
We show that two almost degenerate poles near the threshold and
the next higher mass pole in the partial wave of scattering
evolve from a single bare state through its coupling with , and
reaction channels. This finding provides new information on
understanding the dynamical origins of the Roper and
resonances listed by Particle Data Group. Our results for the resonance poles
in other partial waves are also presented.Comment: Improved version, accepted Phys. Rev. Let
Chandra X-ray spectroscopy of the focused wind in the Cygnus X-1 system. I. The non-dip spectrum in the low/hard state
We present analyses of a 50 ks observation of the supergiant X-ray binary
system Cygnus X-1/HDE 226868 taken with the Chandra High Energy Transmission
Grating Spectrometer (HETGS). Cyg X-1 was in its spectrally hard state and the
observation was performed during superior conjunction of the black hole,
allowing for the spectroscopic analysis of the accreted stellar wind along the
line of sight. A significant part of the observation covers X-ray dips as
commonly observed for Cyg X-1 at this orbital phase, however, here we only
analyze the high count rate non-dip spectrum. The full 0.5-10 keV continuum can
be described by a single model consisting of a disk, a narrow and a
relativistically broadened Fe Kalpha line, and a power law component, which is
consistent with simultaneous RXTE broad band data. We detect absorption edges
from overabundant neutral O, Ne and Fe, and absorption line series from highly
ionized ions and infer column densities and Doppler shifts. With emission lines
of He-like Mg XI, we detect two plasma components with velocities and densities
consistent with the base of the spherical wind and a focused wind. A simple
simulation of the photoionization zone suggests that large parts of the
spherical wind outside of the focused stream are completely ionized, which is
consistent with the low velocities (<200 km/s) observed in the absorption
lines, as the position of absorbers in a spherical wind at low projected
velocity is well constrained. Our observations provide input for models that
couple the wind activity of HDE 226868 to the properties of the accretion flow
onto the black hole.Comment: 16 pages, 15 figures, uses emulateapj, published as ApJ 690:330-346,
2009 January
Multi-Satellite Observations of Cygnus X-1 to Study the Focused Wind and Absorption Dips
High-mass X-ray binary systems are powered by the stellar wind of their donor
stars. The X-ray state of Cygnus X-1 is correlated with the properties of the
wind which defines the environment of mass accretion. Chandra-HETGS
observations close to orbital phase 0 allow for an analysis of the photoionzed
stellar wind at high resolution, but because of the strong variability due to
soft X-ray absorption dips, simultaneous multi-satellite observations are
required to track and understand the continuum, too. Besides an earlier joint
Chandra and RXTE observation, we present first results from a recent campaign
which represents the best broad-band spectrum of Cyg X-1 ever achieved: On 2008
April 18/19 we observed this source with XMM-Newton, Chandra, Suzaku, RXTE,
INTEGRAL, Swift, and AGILE in X- and gamma-rays, as well as with VLA in the
radio. After superior conjunction of the black hole, we detect soft X-ray
absorption dips likely due to clumps in the focused wind covering >95 % of the
X-ray source, with column densities likely to be of several 10^23 cm^-2, which
also affect photon energies above 20 keV via Compton scattering.Comment: 10 pages, contributed talk at the 7th Microquasar Workshop, Foca,
Turkey, Sept. 1-5, 200
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