15,047 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
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
The Temporal and Spectral Characteristics of "Fast Rise and Exponential Decay" Gamma-Ray Burst Pulses
In this paper we have analyzed the temporal and spectral behavior of 52 Fast
Rise and Exponential Decay (FRED) pulses in 48 long-duration gamma-ray bursts
(GRBs) observed by the CGRO/BATSE, using a pulse model with two shape
parameters and the Band model with three shape parameters, respectively. It is
found that these FRED pulses are distinguished both temporally and spectrally
from those in long-lag pulses. Different from these long-lag pulses only one
parameter pair indicates an evident correlation among the five parameters,
which suggests that at least 4 parameters are needed to model burst
temporal and spectral behavior. In addition, our studies reveal that these FRED
pulses have correlated properties: (i) long-duration pulses have harder spectra
and are less luminous than short-duration pulses; (ii) the more asymmetric the
pulses are the steeper the evolutionary curves of the peak energy () in
the spectrum within pulse decay phase are. Our statistical
results give some constrains on the current GRB models.Comment: 18 pages, 7 figures, accepted for publication in the Astrophysical
Journa
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
Temperature dependence of interlayer coupling in perpendicular magnetic tunnel junctions with GdOx barriers
Perpendicular magnetic tunnel junctions with GdOX tunneling barriers have
shown a unique voltage controllable interlayer magnetic coupling effect. Here
we investigate the quality of the GdOX barrier and the coupling mechanism in
these junctions by examining the temperature dependence of the tunneling
magnetoresistance and the interlayer coupling from room temperature down to 11
K. The barrier is shown to be of good quality with the spin independent
conductance only contributing a small portion, 14%, to the total room
temperature conductance, similar to AlOX and MgO barriers. The interlayer
coupling, however, shows an anomalously strong temperature dependence including
sign changes below 80 K. This non-trivial temperature dependence is not
described by previous models of interlayer coupling and may be due to the large
induced magnetic moment of the Gd ions in the barrier.Comment: 14 pages, 4 figure
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