2,244 research outputs found
Random acyclic networks
Directed acyclic graphs are a fundamental class of networks that includes
citation networks, food webs, and family trees, among others. Here we define a
random graph model for directed acyclic graphs and give solutions for a number
of the model's properties, including connection probabilities and component
sizes, as well as a fast algorithm for simulating the model on a computer. We
compare the predictions of the model to a real-world network of citations
between physics papers and find surprisingly good agreement, suggesting that
the structure of the real network may be quite well described by the random
graph.Comment: 4 pages, 2 figure
The optical depth of the Universe to ultrahigh energy cosmic ray scattering in the magnetized large scale structure
This paper provides an analytical description of the transport of ultrahigh
energy cosmic rays in an inhomogeneously magnetized intergalactic medium. This
latter is modeled as a collection of magnetized scattering centers such as
radio cocoons, magnetized galactic winds, clusters or magnetized filaments of
large scale structure, with negligible magnetic fields in between. Magnetic
deflection is no longer a continuous process, it is rather dominated by
scattering events. We study the interaction between high energy cosmic rays and
the scattering agents. We then compute the optical depth of the Universe to
cosmic ray scattering and discuss the phenomological consequences for various
source scenarios. For typical parameters of the scattering centers, the optical
depth is greater than unity at 5x10^{19}eV, but the total angular deflection is
smaller than unity. One important consequence of this scenario is the
possibility that the last scattering center encountered by a cosmic ray be
mistaken with the source of this cosmic ray. In particular, we suggest that
part of the correlation recently reported by the Pierre Auger Observatory may
be affected by such delusion: this experiment may be observing in part the last
scattering surface of ultrahigh energy cosmic rays rather than their source
population. Since the optical depth falls rapidly with increasing energy, one
should probe the arrival directions of the highest energy events beyond
10^{20}eV on an event by event basis to circumvent this effect.Comment: version to appear in PRD; substantial improvements: extended
introduction, sections added on angular images and on direction dependent
effects with sky maps of optical depth, enlarged discussion of Auger results
(conclusions unchanged); 27 pages, 9 figure
Distortion of Ultra-high-energy sky by Galactic Magnetic Field
We investigate the deflections of UHE protons by Galactic magnetic field(GMF)
using four conventional GMF models in order to discuss the positional
correlation between the arrival distribution of UHECRs and their sources. UHE
protons coming from the direction around the Galactic center are highly
deflected above by the dipole magnetic field during their
propagation in Galactic space. However, in bisymmetric spiral field models,
there are directions with the deflection angle below . One of these
directions is toward Centaurus A, the nearest radio-loud active galactic nuclei
that is one of possible candidates of UHECR sources. On the other hand, UHE
protons arriving from the direction of the anti-Galactic center are less
deflected, especially in bisymmetric spiral field models. Thus, the northern
hemisphere, not including the Galactic center, is suitable for the studies of
correlation with sources. The dependence on model parameters is also
investigated. The deflection angles of UHE protons are dependent on the pitch
angle of the spiral field. We also investigate distortion of the supergalactic
plane by GMF. Since the distortion in the direction around Galactic center
strongly depends on the GMF model, we can obtain information on GMF around
Galactic center if Pierre Auger Observatory finds the significant positional
correlation around the supergalactic plane.Comment: 9 pages, 6 figures, submitted to Ap
Resonant inelastic x-ray scattering study of charge excitations in La2CuO4
We report a resonant inelastic x-ray scattering study of the dispersion
relations of charge transfer excitations in insulating LaCuO. These
data reveal two peaks, both of which show two-dimensional characteristics. The
lowest energy excitation has a gap energy of eV at the zone center,
and a dispersion of eV. The spectral weight of this mode becomes
dramatically smaller around (, ). The second peak shows a smaller
dispersion ( eV) with a zone-center energy of eV. We argue
that these are both highly dispersive exciton modes damped by the presence of
the electron-hole continuum.Comment: 5 pages, 3 figure
Stock mechanics: predicting recession in S&P500, DJIA, and NASDAQ
An original method, assuming potential and kinetic energy for prices and
conservation of their sum is developed for forecasting exchanges. Connections
with power law are shown. Semiempirical applications on S&P500, DJIA, and
NASDAQ predict a coming recession in them. An emerging market, Istanbul Stock
Exchange index ISE-100 is found involving a potential to continue to rise.Comment: 14 pages, 4 figure
Damped finite-time-singularity driven by noise
We consider the combined influence of linear damping and noise on a dynamical
finite-time-singularity model for a single degree of freedom. We find that the
noise effectively resolves the finite-time-singularity and replaces it by a
first-passage-time or absorbing state distribution with a peak at the
singularity and a long time tail. The damping introduces a characteristic
cross-over time. In the early time regime the probability distribution and
first-passage-time distribution show a power law behavior with scaling exponent
depending on the ratio of the non linear coupling strength to the noise
strength. In the late time regime the behavior is controlled by the damping.
The study might be of relevance in the context of hydrodynamics on a nanometer
scale, in material physics, and in biophysics.Comment: 9 pages, 4 eps-figures, revtex4 fil
Monitoring Ion Channel Function In Real Time Through Quantum Decoherence
In drug discovery research there is a clear and urgent need for non-invasive
detection of cell membrane ion channel operation with wide-field capability.
Existing techniques are generally invasive, require specialized nano
structures, or are only applicable to certain ion channel species. We show that
quantum nanotechnology has enormous potential to provide a novel solution to
this problem. The nitrogen-vacancy (NV) centre in nano-diamond is currently of
great interest as a novel single atom quantum probe for nanoscale processes.
However, until now, beyond the use of diamond nanocrystals as fluorescence
markers, nothing was known about the quantum behaviour of a NV probe in the
complex room temperature extra-cellular environment. For the first time we
explore in detail the quantum dynamics of a NV probe in proximity to the ion
channel, lipid bilayer and surrounding aqueous environment. Our theoretical
results indicate that real-time detection of ion channel operation at
millisecond resolution is possible by directly monitoring the quantum
decoherence of the NV probe. With the potential to scan and scale-up to an
array-based system this conclusion may have wide ranging implications for
nanoscale biology and drug discovery.Comment: 7 pages, 6 figure
Variational assimilation of Lagrangian data in oceanography
We consider the assimilation of Lagrangian data into a primitive equations
circulation model of the ocean at basin scale. The Lagrangian data are
positions of floats drifting at fixed depth. We aim at reconstructing the
four-dimensional space-time circulation of the ocean. This problem is solved
using the four-dimensional variational technique and the adjoint method. In
this problem the control vector is chosen as being the initial state of the
dynamical system. The observed variables, namely the positions of the floats,
are expressed as a function of the control vector via a nonlinear observation
operator. This method has been implemented and has the ability to reconstruct
the main patterns of the oceanic circulation. Moreover it is very robust with
respect to increase of time-sampling period of observations. We have run many
twin experiments in order to analyze the sensitivity of our method to the
number of floats, the time-sampling period and the vertical drift level. We
compare also the performances of the Lagrangian method to that of the classical
Eulerian one. Finally we study the impact of errors on observations.Comment: 31 page
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