167 research outputs found
Pion photo- and electroproduction and the partially-conserved axial current
The relevance of the axial current for pion production processes off the
nucleon with real or virtual photons is revisited. Employing the hypothesis of
a partially conserved axial current (PCAC), it is shown that, when all of the
relevant contributions are taken into account, PCAC does not provide any
additional constraint for threshold production processes that goes beyond the
Goldberger-Treiman relation. In particular, it is shown that pion
electroproduction processes at threshold cannot be used to extract any
information regarding the weak axial form factor. The relationships found in
previous investigations are seen to be an accident of the approximations
usually made in this context.Comment: 4 pages, 3 figures; typos corrected; references updated; some
rewording; conclusions unchange
Comment about pion electro-production and the axial form factors
The claim by Haberzettl (Phys.Rev.Lett.85 (2000) 3576) that the axial form
factor of the nucleon cannot be accessed through threshold pion
electroproduction is unfounded
Image analysis in sedimentology as application for high-resolution grain size analysis: a case study of lake sediments from Tangra Yumco (Tibetan Plateau)
Abstract HKT-ISTP 2013
B
Low surface brightness galaxies around the HDF-S - I. Object extraction and photometric results
This study reports on photometric results of a search for LSB galaxies in a
0.76deg^2 field centered on the HDF-S. We present results from photometric
analysis of the derived sample galaxies and compare number densities to results
of former surveys. We used public data from the NOAO Deep Wide-Field survey and
the multi-wavelength Goddard Space Flight Center survey. The former reaches a
limiting surface brightness of mu_BW~29 magarcsec^-2 and is therefore one of
the most sensitive ground based data sets systematically analyzed for LSB
galaxies. To reduce the contamination by High Surface Brightness (HSB) galaxies
at higher redshift, mimicking LSBs due to the ''Tolman Dimming'' effect, we
placed a lower diameter limit of 10.8 arcsec and compared the colors of our
candidate galaxies with the redshift tracks of 5 ''standard'' HSB galaxy types.
We report the detection of 37 galaxies with low apparent central surface
brightness (mu_BW>=22 magarcsec^-2). Using color-color diagrams we were able to
derive a subsample of 9 LSB galaxy candidates with intrinsic central surface
brightnesses below mu_(0,BW)=22.5 magarcsec^-2 and diameters larger than the
preselected size limit of 10.8 arcsec. We selected three additional LSB
candidates due to there extreme low blue central surface brightness (mu_BW>=25
magarcsec^-2). These galaxies were only found in the larger and more sensitive
NOAO data. So finally we derived a sample of 12 LSB galaxy candidates and
therfore this survey results in a four times higher surface density than other
CCD based surveys for field galaxies before.Comment: 22 pages, 23 figures, A&A in pres
GaBoDS: The Garching-Bonn Deep Survey -- II. Confirmation of EIS cluster candidates by weak gravitational lensing
We report the first confirmation of colour-selected galaxy cluster candidates
by means of weak gravitational lensing. Significant lensing signals were
identified in the course of the shear-selection programme of dark matter haloes
in the Garching-Bonn Deep Survey, which currently covers 20 square degrees of
deep, high-quality imaging data on the southern sky. The detection was made in
a field that was previously covered by the ESO Imaging Survey (EIS) in 1997. A
highly significant shear-selected mass-concentration perfectly coincides with
the richest EIS cluster candidate at z~0.2, thus confirming its cluster nature.
Several other shear patterns in the field can also be identified with cluster
candidates, one of which could possibly be part of a filament at z~0.45.Comment: 4 pages, 4 figures, submitted to A&A Letter
Gauge-invariant theory of pion photoproduction with dressed hadrons
Based on an effective field theory of hadrons in which quantum chromodynamics
is assumed to provide the necessary bare cutoff functions, a gauge-invariant
theory of pion photoproduction with fully dressed nucleons is developed. The
formalism provides consistent dynamical descriptions of pi-N --> pi-N
scattering and Gamma-N --> pi-N production mechanisms in terms of nonlinear
integral equations for fully dressed hadrons. Defining electromagnetic currents
via the gauging of hadronic n-point Green's functions, dynamically detailed
currents for dressed nucleons are introduced. The dressed hadron currents and
the pion photoproduction current are explicitly shown to satisfy gauge
invariance in a self-consistent manner. Approximations are discussed that make
the nonlinear formalism manageable in practice and yet preserve gauge
invariance. This is achieved by recasting the gauge conditions for all
contributing interaction currents as continuity equations with ``surface''
terms for the individual particle legs coming into or going out of the hadronic
interaction region. General procedures are given that approximate any type of
(global) interaction current in a gauge-invariance preserving manner as a sum
of single-particle ``surface'' currents. It is argued that these prescriptions
carry over to other reactions, irrespective of the number or type of
contributing hadrons or hadronic systems.Comment: 33 pages, RevTeX; includes 8 postscript figures (requires psfig.sty).
This version corrects some minor errors, etc.; contains updated references.
Accepted for publication in Phys. Rev. C56 (Oct. 97
Low Surface Brightness Galaxies around the HDF-S: II. Distances and volume densities
With this study we aim at the spectroscopic verification of a photometrically
selected sample of Low Surface Brightness (LSB) galaxy candidates in a field
around the Hubble Deep Field-South (HDF-S). The sample helps to extend the
parameter space for LSB galaxies to lower central surface brightnesses and to
provide better estimates on the volume densities of these objects. To derive
redshifts for the LSB candidates, long-slit spectra were obtained covering a
spectral range from 3400{\AA} to 7500{\AA}. The observations have been obtained
using the ESO 3.6m telescope, equipped with the EFOSC2 spectrograph. From the
measured radial velocities, distances could be estimated. With this distance
information, it is possible to differentiate between true LSB galaxies and
higher redshift High Surface Brightness (HSB) galaxies which may contaminate
the sample. A correction for the surface brightnesses can then be applied,
accounting for the cosmological dimming effect (``Tolman Dimming''). We show
that ~70% of the LSB candidates, selected based on their location in the
color-color space, are real LSB galaxies. Their position in the color-color
diagrams, therefore, indicate that the LSB galaxies have a different stellar
population mix resulting from a different star formation history compared to
HSBs. Our LSB galaxy sample consists only of large disk galaxies with
scale-length between 2.5kpc and 7.3kpc. We confirm the flat central surface
brightness distribution of previous surveys and extend this distribution down
to central surface brightnesses of 27 B mag arcsec^-2.Comment: 12 pages, 20 figures, accepted by A&
GALEX selected Lyman Break Galaxies at z~2: Comparison with other Populations
We present results of a search for bright Lyman break galaxies at 1.5<=z<=2.5
in the GOODS-S field using a NUV-dropout technique in combination with
color-selection. We derived a sample of 73 LBG candidates. We compare our
selection efficiencies to BM/BX- and BzK methods (techniques solely based on
ground-based data sets), and find the NUV data to provide greater efficiency
for selecting star-forming galaxies. We estimate LBG candidate ages, masses,
star formation rates, and extinction from fitting PEGASE synthesis evolution
models. We find about 20% of our LBG candidates are comparable to infrared
luminous LBGs or sub-millimeter galaxies which are thought to be precursors of
massive elliptical galaxies today. Overall, we can show that although BM/BX and
BzK methods do identify star-forming galaxies at z~2, the sample they provide
biases against those star-forming galaxies which are more massive and contain
sizeable red stellar populations. A true Lyman break criterion at z~2 is
therefore more directly comparable to the populations found at z~3, which does
contain a red fraction.Comment: 14 pages, 11 figures, accepted for publication in Ap
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