15,953 research outputs found
Enhancing signal detectability in learning-based CT reconstruction with a model observer inspired loss function
Deep neural networks used for reconstructing sparse-view CT data are
typically trained by minimizing a pixel-wise mean-squared error or similar loss
function over a set of training images. However, networks trained with such
pixel-wise losses are prone to wipe out small, low-contrast features that are
critical for screening and diagnosis. To remedy this issue, we introduce a
novel training loss inspired by the model observer framework to enhance the
detectability of weak signals in the reconstructions. We evaluate our approach
on the reconstruction of synthetic sparse-view breast CT data, and demonstrate
an improvement in signal detectability with the proposed loss
Detecting a gravitational-wave background with next-generation space interferometers
Future missions of gravitational-wave astronomy will be operated by
space-based interferometers, covering very wide range of frequency. Search for
stochastic gravitational-wave backgrounds (GWBs) is one of the main targets for
such missions, and we here discuss the prospects for direct measurement of
isotropic and anisotropic components of (primordial) GWBs around the frequency
0.1-10 Hz. After extending the theoretical basis for correlation analysis, we
evaluate the sensitivity and the signal-to-noise ratio for the proposed future
space interferometer missions, like Big-Bang Observer (BBO), Deci-Hertz
Interferometer Gravitational-wave Observer (DECIGO) and recently proposed
Fabry-Perot type DECIGO. The astrophysical foregrounds which are expected at
low frequency may be a big obstacle and significantly reduce the
signal-to-noise ratio of GWBs. As a result, minimum detectable amplitude may
reach h^2 \ogw = 10^{-15} \sim 10^{-16}, as long as foreground point sources
are properly subtracted. Based on correlation analysis, we also discuss
measurement of anisotropies of GWBs. As an example, the sensitivity level
required for detecting the dipole moment of GWB induced by the proper motion of
our local system is closely examined.Comment: 19 pages, 6 figures, references added, typos correcte
On the time delay in binary systems
The aim of this paper is to study the time delay on electromagnetic signals
propagating across a binary stellar system. We focus on the antisymmetric
gravitomagnetic contribution due to the angular momentum of one of the stars of
the pair. Considering a pulsar as the source of the signals, the effect would
be manifest both in the arrival times of the pulses and in the frequency shift
of their Fourier spectra. We derive the appropriate formulas and we discuss the
influence of different configurations on the observability of gravitomagnetic
effects. We argue that the recently discovered PSR J0737-3039 binary system
does not permit the detection of the effects because of the large size of the
eclipsed region.Comment: 7 pages, 2 eps figures, RevTex, to appear in Physical Review
Output-input stability and minimum-phase nonlinear systems
This paper introduces and studies the notion of output-input stability, which
represents a variant of the minimum-phase property for general smooth nonlinear
control systems. The definition of output-input stability does not rely on a
particular choice of coordinates in which the system takes a normal form or on
the computation of zero dynamics. In the spirit of the ``input-to-state
stability'' philosophy, it requires the state and the input of the system to be
bounded by a suitable function of the output and derivatives of the output,
modulo a decaying term depending on initial conditions. The class of
output-input stable systems thus defined includes all affine systems in global
normal form whose internal dynamics are input-to-state stable and also all
left-invertible linear systems whose transmission zeros have negative real
parts. As an application, we explain how the new concept enables one to develop
a natural extension to nonlinear systems of a basic result from linear adaptive
control.Comment: Revised version, to appear in IEEE Transactions on Automatic Control.
See related work in http://www.math.rutgers.edu/~sontag and
http://black.csl.uiuc.edu/~liberzo
Gravitational Waves from Phase Transitions at the Electroweak Scale and Beyond
If there was a first order phase transition in the early universe, there
should be an associated stochastic background of gravitational waves. In this
paper, we point out that the characteristic frequency of the spectrum due to
phase transitions which took place in the temperature range 100 GeV - 10^7 GeV
is precisely in the window that will be probed by the second generation of
space-based interferometers such as the Big Bang Observer (BBO). Taking into
account the astrophysical foreground, we determine the type of phase
transitions which could be detected either at LISA, LIGO or BBO, in terms of
the amount of supercooling and the duration of the phase transition that are
needed. Those two quantities can be calculated for any given effective scalar
potential describing the phase transition. In particular, the new models of
electroweak symmetry breaking which have been proposed in the last few years
typically have a different Higgs potential from the Standard Model. They could
lead to a gravitational wave signature in the milli-Hertz frequency, which is
precisely the peak sensitivity of LISA. We also show that the signal coming
from phase transitions taking place at T ~ 1-100 TeV could entirely screen the
relic gravitational wave signal expected from standard inflationary models.Comment: 18 pages, 24 figure
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