22,769 research outputs found
The Low Column Density Lyman-alpha Forest
We develop an analytical method based on the lognormal approximation to
compute the column density distribution of the Lyman-alpha forest in the low
column density limit. We compute the column density distributions for six
different cosmological models and found that the standard, COBE-normalized CDM
model cannot fit the observations of the Lyman-alpha forest at z=3. The
amplitude of the fluctuations in that model has to be lowered by a factor of
almost 3 to match observations. However, the currently viable cosmological
models like the lightly tilted COBE-normalized CDM+Lambda model, the CHDM model
with 20% neutrinos, and the low-amplitude Standard CDM model are all in
agreement with observations, to within the accuracy of our approximation, for
the value of the cosmological baryon density at or higher than the old Standard
Bing Bang Nucleosynthesis value of 0.0125 for the currently favored value of
the ionizing radiation intensity. With the low value for the baryon density
inferred by Hogan & Rugers (1996), the models can only marginally match
observations.Comment: three postscript figures included, submitted to ApJ
Bubble statistics and positioning in superhelically stressed DNA
We present a general framework to study the thermodynamic denaturation of
double-stranded DNA under superhelical stress. We report calculations of
position- and size-dependent opening probabilities for bubbles along the
sequence. Our results are obtained from transfer-matrix solutions of the
Zimm-Bragg model for unconstrained DNA and of a self-consistent linearization
of the Benham model for superhelical DNA. The numerical efficiency of our
method allows for the analysis of entire genomes and of random sequences of
corresponding length ( base pairs). We show that, at physiological
conditions, opening in superhelical DNA is strongly cooperative with average
bubble sizes of base pairs (bp), and orders of magnitude higher
than in unconstrained DNA. In heterogeneous sequences, the average degree of
base-pair opening is self-averaging, while bubble localization and statistics
are dominated by sequence disorder. Compared to random sequences with identical
GC-content, genomic DNA has a significantly increased probability to open large
bubbles under superhelical stress. These bubbles are frequently located
directly upstream of transcription start sites.Comment: to be appeared in Physical Review
Muon anomalous magnetic moment and lepton flavor violation in MSSM
We give a thorough analysis of the correlation between the muon anomalous
magnetic moment and the radiative lepton flavor violating (LFV) processes
within the minimal supersymmetric standard model. We find that in the case when
the slepton mass eigenstates are nearly degenerate, , coming from
SUSY contributions, hardly depends on the lepton flavor mixing and, thus, there
is no direct relation between and the LFV processes. On the
contrary, if the first two generations' sleptons are much heavier than the 3rd
one, i.e., in the effective SUSY scenario, the two quantities are closely
related. In the latter scenario, the SUSY parameter space to account for the
experimental is quite different from the case of no lepton
flavor mixing. Especially, the Higgsino mass parameter can be either
positive or negative.Comment: 22 pages, 9 figures; Some discussions are modifie
Polaronic effects in strongly coupled electron-phonon systems: Exact diagonalization results for the 2D Holstein t-J model
Ground-state and dynamical properties of the 2D Holstein t-J model are
examined by means of direct Lanczos diagonalization, using a truncation method
of the phononic Hilbert space. The single-hole spectral function shows the
formation of a narrow hole-polaron band as the electron-phonon coupling
increases, where the polaronic band collapse is favoured by strong Coulomb
correlations. In the two-hole sector, the hole-hole correlations unambiguously
indicate the existence of inter-site bipolaronic states. At quarter-filling, a
polaronic superlattice is formed in the adiabatic strong-coupling regime.Comment: 3 pages, LaTeX, 6 Postscript figures, Proc. Int. Conf. on Strongly
Correlated Electron Systems, Zuerich, August 1996, accepted for publication
in Physica
Difference of optical conductivity between one- and two-dimensional doped nickelates
We study the optical conductivity in doped nickelates, and find the dramatic
difference of the spectrum in the gap (\alt4 eV) between one- (1D)
and two-dimensional (2D) nickelates. The difference is shown to be caused by
the dependence of hopping integral on dimensionality. The theoretical results
explain consistently the experimental data in 1D and
2D nickelates, YCaBaNiO and LaSrNiO,
respectively. The relation between the spectrum in the X-ray aborption
experiments and the optical conductivity in LaSrNiO is
discussed.Comment: RevTeX, 4 pages, 4 figure
VHE gamma ray absorption by galactic interstellar radiation field
Adopting a recent calculation of the Galactic interstellar radiation field,
we calculate the attenuation of the very high energy gamma rays from the
Galactic sources. The infra-red radiation background near the Galactic Center
is very intense due to the new calculation and our result shows that a cutoff
of high energy gamma ray spectrum begins at about 20 TeV and reaches about 10%
for 50 TeV gamma rays.Comment: 6 pages, 1 figure, figure is changed, conclusion not change
Synthesis of Positron Emission Tomography (PET) Images via Multi-channel Generative Adversarial Networks (GANs)
Positron emission tomography (PET) image synthesis plays an important role,
which can be used to boost the training data for computer aided diagnosis
systems. However, existing image synthesis methods have problems in
synthesizing the low resolution PET images. To address these limitations, we
propose multi-channel generative adversarial networks (M-GAN) based PET image
synthesis method. Different to the existing methods which rely on using
low-level features, the proposed M-GAN is capable to represent the features in
a high-level of semantic based on the adversarial learning concept. In
addition, M-GAN enables to take the input from the annotation (label) to
synthesize the high uptake regions e.g., tumors and from the computed
tomography (CT) images to constrain the appearance consistency and output the
synthetic PET images directly. Our results on 50 lung cancer PET-CT studies
indicate that our method was much closer to the real PET images when compared
with the existing methods.Comment: 9 pages, 2 figure
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