996 research outputs found
A unified approach to combinatorial key predistribution schemes for sensor networks
There have been numerous recent proposals for key predistribution schemes for wireless sensor networks based on various types of combinatorial structures such as designs and codes. Many of these schemes have very similar properties and are analysed in a similar manner. We seek to provide a unified framework to study these kinds of schemes. To do so, we define a new, general class of designs, termed “partially balanced t-designs”, that is sufficiently general that it encompasses almost all of the designs that have been proposed for combinatorial key predistribution schemes. However, this new class of designs still has sufficient structure that we are able to derive general formulas for the metrics of the resulting key predistribution schemes. These metrics can be evaluated for a particular scheme simply by substituting appropriate parameters of the underlying combinatorial structure into our general formulas. We also compare various classes of schemes based on different designs, and point out that some existing proposed schemes are in fact identical, even though their descriptions may seem different. We believe that our general framework should facilitate the analysis of proposals for combinatorial key predistribution schemes and their comparison with existing schemes, and also allow researchers to easily evaluate which scheme or schemes present the best combination of performance metrics for a given application scenario
NIHAO project II: Halo shape, phase-space density and velocity distribution of dark matter in galaxy formation simulations
We use the NIHAO (Numerical Investigation of Hundred Astrophysical Objects)
cosmological simulations to study the effects of galaxy formation on key
properties of dark matter (DM) haloes. NIHAO consists of
high-resolution SPH simulations that include (metal-line) cooling, star
formation, and feedback from massive stars and SuperNovae, and cover a wide
stellar and halo mass range: ( ). When compared to DM-only simulations,
the NIHAO haloes have similar shapes at the virial radius, R_{\rm vir}, but are
substantially rounder inside . In NIHAO simulations
increases with halo mass and integrated star formation efficiency,
reaching at the Milky Way mass (compared to 0.5 in DM-only),
providing a plausible solution to the long-standing conflict between
observations and DM-only simulations. The radial profile of the phase-space
parameter () is best fit with a single power law in DM-only
simulations, but shows a flattening within for NIHAO
for total masses . Finally, the global velocity
distribution of DM is similar in both DM-only and NIHAO simulations, but in the
solar neighborhood, NIHAO galaxies deviate substantially from Maxwellian. The
distribution is more symmetric, roughly Gaussian, with a peak that shifts to
higher velocities for Milky Way mass haloes. We provide the distribution
parameters which can be used for predictions for direct DM detection
experiments. Our results underline the ability of the galaxy formation
processes to modify the properties of dark matter haloes.Comment: 19 pages, 17 figures, analysis strongly improved, main conclusions
unchanged, accepted for publication in MNRA
Implementing Shor's algorithm on Josephson Charge Qubits
We investigate the physical implementation of Shor's factorization algorithm
on a Josephson charge qubit register. While we pursue a universal method to
factor a composite integer of any size, the scheme is demonstrated for the
number 21. We consider both the physical and algorithmic requirements for an
optimal implementation when only a small number of qubits is available. These
aspects of quantum computation are usually the topics of separate research
communities; we present a unifying discussion of both of these fundamental
features bridging Shor's algorithm to its physical realization using Josephson
junction qubits. In order to meet the stringent requirements set by a short
decoherence time, we accelerate the algorithm by decomposing the quantum
circuit into tailored two- and three-qubit gates and we find their physical
realizations through numerical optimization.Comment: 12 pages, submitted to Phys. Rev.
NIHAO IV: Core creation and destruction in dark matter density profiles across cosmic time
We use the NIHAO simulations to investigate the effects of baryonic physics
on the time evolution of Dark Matter central density profiles. The sample is
made of independent high resolution hydrodynamical simulations of
galaxy formation and covers a wide mass range: 1e10< Mhalo <1e12, i.e., from
dwarfs to L* . We confirm previous results on the dependence of the inner dark
matter density slope, , on the ratio between stellar-to-halo mass. We
show that this relation holds approximately at all redshifts (with an intrinsic
scatter of ~0.18 in ). This implies that in practically all haloes the
shape of their inner density profile changes quite substantially over cosmic
time, as they grow in stellar and total mass. Thus, depending on their final
stellar-to-halo mass ratio, haloes can either form and keep a substantial
density core (size~1 kpc), or form and then destroy the core and re-contract
the halo, going back to a cuspy profile, which is even steeper than CDM
predictions for massive galaxies (~1e12 Msun). We show that results from the
NIHAO suite are in good agreement with recent observational measurements of
in dwarf galaxies. Overall our results suggest that the notion of a
universal density profile for dark matter haloes is no longer valid in the
presence of galaxy formation.Comment: 11 pages, 13 figures. Corrected typo in table 2 (middle row) with
respect to the version published in MNRA
Evolution of interfaces and expansion in width
Interfaces in a model with a single, real nonconserved order parameter and
purely dissipative evolution equation are considered. We show that a systematic
perturbative approach, called the expansion in width and developed for curved
domain walls, can be generalized to the interfaces. Procedure for calculating
curvature corrections is described. We also derive formulas for local velocity
and local surface tension of the interface. As an example, evolution of
spherical interfaces is discussed, including an estimate of critical size of
small droplets.Comment: Discussion of stability of the interface is added, and the numerical
estimates of width and velocity of the interface in the liquid crystal
example are corrected. 25 pages, Latex2
Developing a Model for Evidence-based Clinical Forensic Interviewing
Much of the work undertaken in forensic settings, such as diagnosis, formulation and judgements about treatment and placement are based on information gathered through clinical forensic interviewing. Despite this, the evidence base on which clinical forensic interviewing is founded is extremely limited. This paper is divided into two sections; the first examines the nature of interviewing and provides an introduction to this area of practice. Drawing on some of the research undertaken with specific forms of interview such as those for diagnosis and investigative purposes allows factors such as the evidence concerning interview quality, interview effectiveness, underlying competencies and methods for skills training to be outlined. The second part of the paper, which provides the main focus, describes a forensic clinical interview framework which seeks to draw together a broad range of considerations and areas for research in relation to the clinical forensic interview. This framework is explicitly intended to provoke and guide practitioners and researchers in the pursuit of evidence-based interviewing
Dynamics of Weak First Order Phase Transitions
The dynamics of weak vs. strong first order phase transitions is investigated
numerically for 2+1 dimensional scalar field models. It is argued that the
change from a weak to a strong transition is itself a (second order) phase
transition, with the order parameter being the equilibrium fractional
population difference between the two phases at the critical temperature, and
the control parameter being the coefficient of the cubic coupling in the
free-energy density. The critical point is identified, and a power law
controlling the relaxation dynamics at this point is obtained. Possible
applications are briefly discussed.Comment: 11 pages, 4 figures in uuencoded compressed file (see instructions in
main text), RevTeX, DART-HEP-94/0
Is there Evidence for Flat Cores in the Halos of Dwarf Galaxies?: The Case of NGC 3109 and NGC 6822
Two well studied dwarf galaxies -- NGC 3109 and NGC 6822 -- present some of
the strongest observational support for a flat core at the center of galactic
dark matter (DM) halos. We use detailed cosmologically motivated numerical
models to investigate the systematics and the accuracy of recovering parameters
of the galaxies. Some of our models match the observed structure of the two
galaxies remarkably well. Our analysis shows that the rotation curves of these
two galaxies are instead quite compatible with their DM halos having steep
cuspy density profiles. The rotation curves in our models are measured using
standard observational techniques. The models reproduce the rotation curves of
both galaxies, the disk surface brightness profiles as well as the profile of
isophotal ellipticity and position angle. The models are centrally dominated by
baryons; however, the dark matter component is globally dominant. The simulated
disk mass is marginally consistent with a stellar mass-to-light ratio in
agreement with the observed colors. We show that non-circular motions combined
with gas pressure support and projection effects results in a large
underestimation of the circular velocity in the central kpc region,
creating the illusion of a constant density core. Although the systematic
effects mentioned above are stronger in barred systems, they are also present
in axisymetric disks. Our results strongly suggest that there is no
contradiction between the observed rotation curves in dwarf galaxies and the
cuspy central dark matter density profiles predicted by Cold Dark Matter
models.Comment: Accepted for publication in the ApJ. New discussion, figures and one
appendix. High resolution version
at:http://www.astro.washington.edu/octavio/N3109_paper.ps.g
Thermal Phase Mixing During First Order Phase Transitions
The dynamics of first order phase transitions are studied in the context of
(3+1)-dimensional scalar field theories. Particular attention is paid to the
question of quantifying the strength of the transition, and how `weak' and
`strong' transitions have different dynamics. We propose a model with two
available low temperature phases separated by an energy barrier so that one of
them becomes metastable below the critical temperature . The system is
initially prepared in this phase and is coupled to a thermal bath.
Investigating the system at its critical temperature, we find that `strong'
transitions are characterized by the system remaining localized within its
initial phase, while `weak' transitions are characterized by considerable phase
mixing. Always at , we argue that the two regimes are themselves separated
by a (second order) phase transition, with an order parameter given by the
fractional population difference between the two phases and a control parameter
given by the strength of the scalar field's quartic self-coupling constant. We
obtain a Ginzburg-like criterion to distinguish between `weak' and `strong'
transitions, in agreement with previous results in (2+1)-dimensions.Comment: 28 pages RevTeX, 9 postscript figures, IMPERIAL/TP/93-94/58,
DART-HEP-94/0
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