7,527 research outputs found
Dynamics of Surface Roughening with Quenched Disorder
We study the dynamical exponent for the directed percolation depinning
(DPD) class of models for surface roughening in the presence of quenched
disorder. We argue that for dimensions is equal to the exponent
characterizing the shortest path between two sites in an
isotropic percolation cluster in dimensions. To test the argument, we
perform simulations and calculate for DPD, and for
percolation, from to .Comment: RevTex manuscript 3 pages + 6 figures (obtained upon request via
email [email protected]
Prospective Risk of Intrauterine Death of Monochorionic-Diamniotic Twins
OBJECTIVE: The purpose of this study was to calculate the prospective risk of fetal death in monochorionic-diamniotic twins.
STUDY DESIGN: We evaluated 193 monochorionic diamniotic twin pregnancies that were followed and delivered after 24 weeks. Surveillance included cardiotocography and sonography performed at least once weekly. The prospective risk of fetal death was calculated as the total number of deaths at the beginning of the gestational period divided by the number of continuing pregnancies at or beyond that period.
RESULTS: The fetal death rate was 5 of 193 pregnancies (2.6%; 95% CI, 1.1, 5.9); the prospective risk of stillbirth per pregnancy after 32 weeks of gestation was 1.2% (95% CI, 0.3% - 4.2%).
CONCLUSION: Under intensive surveillance, the prospective risk of fetal death in monochorionic-diamniotic pregnancies after 32 weeks of gestation is much lower than reported and does not support a policy of elective preterm delivery
Effects of gluon number fluctuations on photon - photon collisions at high energies
We investigate the effects of gluon number fluctuations on the total
, cross sections and the photon structure
function . Considering a model which relates the
dipole-dipole and dipole-hadron scattering amplitudes, we estimate these
observables by using event-by-event and physical amplitudes. We demonstrate
that both analyses are able to describe the LEP data, but predict different
behaviours for the observables at high energies, with the gluon fluctuations
effects decreasing the cross sections. We conclude that the study of interactions can be useful to constrain the QCD dynamics.Comment: 9 pages, 6 figures. Improved version with two new figures. Version to
be published in Physical Review
Pipe network model for scaling of dynamic interfaces in porous media
We present a numerical study on the dynamics of imbibition fronts in porous
media using a pipe network model. This model quantitatively reproduces the
anomalous scaling behavior found in imbibition experiments [Phys. Rev. E {\bf
52}, 5166 (1995)]. Using simple scaling arguments, we derive a new identity
among the scaling exponents in agreement with the experimental results.Comment: 13 pages, 3 figures, REVTeX, to appear in Phys. Rev. Let
cross section from the dipole model in momentum space
We reproduce the DIS measurements of the proton structure function at high
energy from the dipole model in momentum space. To model the dipole-proton
forward scattering amplitude, we use the knowledge of asymptotic solutions of
the Balitsky-Kovchegov equation, describing high-energy QCD in the presence of
saturation effects. We compare our results with the previous analysis in
coordinate space and discuss possible extensions of our approach.Comment: 9 pages, 3 figure
Scaling of the distribution of price fluctuations of individual companies
We present a phenomenological study of stock price fluctuations of individual
companies. We systematically analyze two different databases covering
securities from the three major US stock markets: (a) the New York Stock
Exchange, (b) the American Stock Exchange, and (c) the National Association of
Securities Dealers Automated Quotation stock market. Specifically, we consider
(i) the trades and quotes database, for which we analyze 40 million records for
1000 US companies for the 2-year period 1994--95, and (ii) the Center for
Research and Security Prices database, for which we analyze 35 million daily
records for approximately 16,000 companies in the 35-year period 1962--96. We
study the probability distribution of returns over varying time scales , where varies by a factor of ---from 5 min up to
4 years. For time scales from 5~min up to approximately 16~days, we
find that the tails of the distributions can be well described by a power-law
decay, characterized by an exponent ---well outside the
stable L\'evy regime . For time scales days, we observe results consistent with a slow
convergence to Gaussian behavior. We also analyze the role of cross
correlations between the returns of different companies and relate these
correlations to the distribution of returns for market indices.Comment: 10pages 2 column format with 11 eps figures. LaTeX file requiring
epsf, multicol,revtex. Submitted to PR
Depinning transition and thermal fluctuations in the random-field Ising model
We analyze the depinning transition of a driven interface in the 3d
random-field Ising model (RFIM) with quenched disorder by means of Monte Carlo
simulations. The interface initially built into the system is perpendicular to
the [111]-direction of a simple cubic lattice. We introduce an algorithm which
is capable of simulating such an interface independent of the considered
dimension and time scale. This algorithm is applied to the 3d-RFIM to study
both the depinning transition and the influence of thermal fluctuations on this
transition. It turns out that in the RFIM characteristics of the depinning
transition depend crucially on the existence of overhangs. Our analysis yields
critical exponents of the interface velocity, the correlation length, and the
thermal rounding of the transition. We find numerical evidence for a scaling
relation for these exponents and the dimension d of the system.Comment: 6 pages, including 9 figures, submitted for publicatio
Analytical solution of a model for complex food webs
We investigate numerically and analytically a recently proposed model for
food webs [Nature {\bf 404}, 180 (2000)] in the limit of large web sizes and
sparse interaction matrices. We obtain analytical expressions for several
quantities with ecological interest, in particular the probability
distributions for the number of prey and the number of predators. We find that
these distributions have fast-decaying exponential and Gaussian tails,
respectively. We also find that our analytical expressions are robust to
changes in the details of the model.Comment: 4 pages (RevTeX). Final versio
Universal and non-universal properties of cross-correlations in financial time series
We use methods of random matrix theory to analyze the cross-correlation
matrix C of price changes of the largest 1000 US stocks for the 2-year period
1994-95. We find that the statistics of most of the eigenvalues in the spectrum
of C agree with the predictions of random matrix theory, but there are
deviations for a few of the largest eigenvalues. We find that C has the
universal properties of the Gaussian orthogonal ensemble of random matrices.
Furthermore, we analyze the eigenvectors of C through their inverse
participation ratio and find eigenvectors with large inverse participation
ratios at both edges of the eigenvalue spectrum--a situation reminiscent of
results in localization theory.Comment: 14 pages, 3 figures, Revte
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