14,123 research outputs found
Time and Ensemble Averages in Bohmian Mechanics
We show that in the framework of one-dimensional Bohmian Quantum
Mechanics[1], for a particle subject to a potential undergoing a weak adiabatic
change, the time averages of the particle's positions typically differ markedly
from the ensemble averages. We Apply this result to the case where the weak
perturbing potential is the back-action of a measuring device (i.e. a
protective measurement). It is shown that under these conditions, most
trajectories never cross the position measured (as already shown for a
particular example in [3]).Comment: 6 page
Dynamics of spin 1/2 quantum plasmas
The fully nonlinear governing equations for spin 1/2 quantum plasmas are
presented. Starting from the Pauli equation, the relevant plasma equations are
derived, and it is shown that nontrivial quantum spin couplings arise, enabling
studies of the combined collective and spin dynamics. The linear response of
the quantum plasma in an electron--ion system is obtained and analyzed.
Applications of the theory to solid state and astrophysical systems as well as
dusty plasmas are pointed out.Comment: 4 pages, 2 figures, to appear in Physical Review Letter
HST/STIS Imaging of the Host Galaxy of GRB980425/SN1998bw
We present HST/STIS observations of ESO 184-G82, the host galaxy of the
gamma-ray burst GRB 980425 associated with the peculiar Type Ic supernova
SN1998bw. ESO 184-G82 is found to be an actively star forming SBc sub-luminous
galaxy. We detect an object consistent with being a point source within the
astrometric uncertainty of 0.018 arcseconds of the position of the supernova.
The object is located inside a star-forming region and is at least one
magnitude brighter than expected for the supernova based on a simple
radioactive decay model. This implies either a significant flattening of the
light curve or a contribution from an underlying star cluster.Comment: 12 pages, 2 figures, AASTeX v5.02 accepted for publication in ApJ
Letter
Discontinuous Percolation Transitions in Epidemic Processes, Surface Depinning in Random Media and Hamiltonian Random Graphs
Discontinuous percolation transitions and the associated tricritical points
are manifest in a wide range of both equilibrium and non-equilibrium
cooperative phenomena. To demonstrate this, we present and relate the
continuous and first order behaviors in two different classes of models: The
first are generalized epidemic processes (GEP) that describe in their spatially
embedded version - either on or off a regular lattice - compact or fractal
cluster growth in random media at zero temperature. A random graph version of
GEP is mapped onto a model previously proposed for complex social contagion. We
compute detailed phase diagrams and compare our numerical results at the
tricritical point in d = 3 with field theory predictions of Janssen et al.
[Phys. Rev. E 70, 026114 (2004)]. The second class consists of exponential
("Hamiltonian", or formally equilibrium) random graph models and includes the
Strauss and the 2-star model, where 'chemical potentials' control the densities
of links, triangles or 2-stars. When the chemical potentials in either graph
model are O(logN), the percolation transition can coincide with a first order
phase transition in the density of links, making the former also discontinuous.
Hysteresis loops can then be of mixed order, with second order behavior for
decreasing link fugacity, and a jump (first order) when it increases
Local dynamics in high-order harmonic generation using Bohmian trajectories
We investigate high-order harmonic generation from a Bohmian-mechanical
perspective, and find that the innermost part of the core, represented by a
single Bohmian trajectory, leads to the main contributions to the high-harmonic
spectra. Using time-frequency analysis, we associate this central Bohmian
trajectory to an ensemble of unbound classical trajectories leaving and
returning to the core, in agreement with the three step model. In the Bohmian
scenario, this physical picture builds up non-locally near the core via the
quantum mechanical phase of the wavefunction. This implies that the flow of the
wavefunction far from the core alters the central Bohmian trajectory. We also
show how this phase degrades in time for the peripheral Bohmian trajectories as
they leave the core region.Comment: 7 pages, 3 figures; the manuscript has been considerably extended and
modified with regard to the previous version
Investigating people: a qualitative analysis of the search behaviours of open-source intelligence analysts
The Internet and the World Wide Web have become integral parts of the lives of many modern individuals, enabling almost instantaneous communication, sharing and broadcasting of thoughts, feelings and opinions. Much of this information is publicly facing, and as such, it can be utilised in a multitude of online investigations, ranging from employee vetting and credit checking to counter-terrorism and fraud prevention/detection. However, the search needs and behaviours of these investigators are not well documented in the literature. In order to address this gap, an in-depth qualitative study was carried out in cooperation with a leading investigation company. The research contribution is an initial identification of Open-Source Intelligence investigator search behaviours, the procedures and practices that they undertake, along with an overview of the difficulties and challenges that they encounter as part of their domain. This lays the foundation for future research in to the varied domain of Open-Source Intelligence gathering
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Search for Q: single grains Xe isotope analysis of carbonaceous residue from Yilmia
We analyse Xe in single grains in HF-HCl residue from Yilmia using RELAX mass spectrometer
Hierarchical Models for Independence Structures of Networks
We introduce a new family of network models, called hierarchical network
models, that allow us to represent in an explicit manner the stochastic
dependence among the dyads (random ties) of the network. In particular, each
member of this family can be associated with a graphical model defining
conditional independence clauses among the dyads of the network, called the
dependency graph. Every network model with dyadic independence assumption can
be generalized to construct members of this new family. Using this new
framework, we generalize the Erd\"os-R\'enyi and beta-models to create
hierarchical Erd\"os-R\'enyi and beta-models. We describe various methods for
parameter estimation as well as simulation studies for models with sparse
dependency graphs.Comment: 19 pages, 7 figure
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