3,473 research outputs found
A model of ballistic aggregation and fragmentation
A simple model of ballistic aggregation and fragmentation is proposed. The
model is characterized by two energy thresholds, Eagg and Efrag, which
demarcate different types of impacts: If the kinetic energy of the relative
motion of a colliding pair is smaller than Eagg or larger than Efrag, particles
respectively merge or break; otherwise they rebound. We assume that particles
are formed from monomers which cannot split any further and that in a
collision-induced fragmentation the larger particle splits into two fragments.
We start from the Boltzmann equation for the mass-velocity distribution
function and derive Smoluchowski-like equations for concentrations of particles
of different mass. We analyze these equations analytically, solve them
numerically and perform Monte Carlo simulations. When aggregation and
fragmentation energy thresholds do not depend on the masses of the colliding
particles, the model becomes analytically tractable. In this case we show the
emergence of the two types of behavior: the regime of unlimited cluster growth
arises when fragmentation is (relatively) weak and the relaxation towards a
steady state occurs when fragmentation prevails. In a model with mass-dependent
Eagg and Efrag the evolution with a cross-over from one of the regimes to
another has been detected
Sum Rule Description of Color Transparency
The assumption that a small point-like configuration does not interact with
nucleons leads to a new set of sum rules that are interpreted as models of the
baryon-nucleon interaction. These models are rendered semi-realistic by
requiring consistency with data for cross section fluctuations in proton-proton
diffractive collisions.Comment: 22 pages + 3 postscript figures attache
Clinical Practice: Direct-to-consumer genetic testing: To test or not to test, that is the question
In direct-to-consumer (DTC) genetic testing, laboratory-based genetic services are offered directly to the public without an independent healthcare professional being involved. The committee of the Southern African Society for Human Genetics (SASHG) appeals to the public and clinicians to be cautious when considering and interpreting such testing. It is important to stress that currently, the clinical validity and utility of genetic tests for complex multifactorial disorders such as type 2 diabetes mellitus and cardiovascular diseases is questionable. The majority of such tests are not scientifically validated and are based on a few preliminary studies. Potential consumers should be aware of the implications of genetic testing that could lead to stigmatisation and discrimination by insurance companies or potential employers of themselves and their family members. Guidelines and recommendations for DTC genetic testing in South Africa (SA) are currently lacking. We provide recommendations that seek to protect consumers and healthcare providers in SA from possible exploitation
Multiple-Scattering Series For Color Transparency
Color transparency CT depends on the formation of a wavepacket of small
spatial extent. It is useful to interpret experimental searches for CT with a
multiple scattering scattering series based on wavepacket-nucleon scattering
instead of the standard one using nucleon-nucleon scattering. We develop
several new techniques which are valid for differing ranges of energy. These
techniques are applied to verify some early approximations; study new forms of
the wave-packet-nucleon interaction; examine effects of treating wave packets
of non-zero size; and predict the production of 's in electron scattering
experiments.Comment: 26 pages, U.Wa. preprint 40427-23-N9
Synchronization Landscapes in Small-World-Connected Computer Networks
Motivated by a synchronization problem in distributed computing we studied a
simple growth model on regular and small-world networks, embedded in one and
two-dimensions. We find that the synchronization landscape (corresponding to
the progress of the individual processors) exhibits Kardar-Parisi-Zhang-like
kinetic roughening on regular networks with short-range communication links.
Although the processors, on average, progress at a nonzero rate, their spread
(the width of the synchronization landscape) diverges with the number of nodes
(desynchronized state) hindering efficient data management. When random
communication links are added on top of the one and two-dimensional regular
networks (resulting in a small-world network), large fluctuations in the
synchronization landscape are suppressed and the width approaches a finite
value in the large system-size limit (synchronized state). In the resulting
synchronization scheme, the processors make close-to-uniform progress with a
nonzero rate without global intervention. We obtain our results by ``simulating
the simulations", based on the exact algorithmic rules, supported by
coarse-grained arguments.Comment: 20 pages, 22 figure
Extremal-point Densities of Interface Fluctuations
We introduce and investigate the stochastic dynamics of the density of local
extrema (minima and maxima) of non-equilibrium surface fluctuations. We give a
number of exact, analytic results for interface fluctuations described by
linear Langevin equations, and for on-lattice, solid-on-solid surface growth
models. We show that in spite of the non-universal character of the quantities
studied, their behavior against the variation of the microscopic length scales
can present generic features, characteristic to the macroscopic observables of
the system. The quantities investigated here present us with tools that give an
entirely un-orthodox approach to the dynamics of surface morphologies: a
statistical analysis from the short wavelength end of the Fourier decomposition
spectrum. In addition to surface growth applications, our results can be used
to solve the asymptotic scalability problem of massively parallel algorithms
for discrete event simulations, which are extensively used in Monte-Carlo type
simulations on parallel architectures.Comment: 30 pages, 5 ps figure
Theoretical and experimental evidence of a site-selective Mott transition in Fe2O3 under pressure
We provide experimental and theoretical evidence for a novel type of
pressure-induced insulator-metal transition characterized by site-selective
delocalization of the electrons. M\"ossbauer spectroscopy, X-ray diffraction
and electrical transport measurements on FeO to 100 GPa, along with
dynamical mean-field theory (DFT+DMFT) calculations, reveal this site-selective
Mott transition between 50 and 68 GPa, such that the metallization can be
described by (^\rm{VI}Fe)O [ structure]
(^\rm{VIII}FeFe^\rm{M})O [ structure]
(^\rm{VI}Fe^\rm{M})O [
structure]. Within the crystal structure, characterized by two
distinct coordination sites (VI and VIII), we observe equal abundances of
ferric ions (Fe) and ions having delocalized electrons (Fe^\rm{M}),
and only at higher pressures is a fully metallic structure obtained, all
at room temperature. The transition is characterized by
delocalization/metallization of the electrons on half the Fe sites, with a
site-dependent collapse of local moments. Above 50 GPa, FeO is a
strongly correlated metal with reduced electron mobility (large band
renormalizations) of m*/m4 and 6 near the Fermi level. Upon
decompression, we observe a site-selective (metallic) to conventional Mott
insulator phase transition (^\rm{VIII}FeFe^\rm{M})O
(^\rm{VIII}FeFe)O within the same structure, indicating a decoupling of
the electronic and lattice degrees of freedom, characteristic of a true Mott
transition. Our results show that the interplay of electronic correlations and
lattice may result in rather complex behavior of the electronic structure and
magnetic state.Comment: 18 pages, 5 figure
Safety and pharmacokinetics of rimantadine small-particle aerosol.
The safety and pharmacokinetics of rimantadine administered by small-particle aerosol were assessed in healthy adults and adults with acute influenza virus infection. Aerosolized rimantadine delivered at a concentration of 40 micrograms/liter of air was associated with nasal burning and irritation in normal volunteers. A concentration of 20 micrograms/liter of air was well tolerated for up to 12 h by normal volunteers and was not associated with any changes in pulmonary function, as measured by routine spirometry, plethysmography, or diffusion capacity of carbon monoxide. Mean peak levels of drug in serum were approximately 10-fold lower after 12 h of aerosol administration than they were after oral administration of 200 mg (29.7 versus 255 ng/ml, respectively), while mean nasal wash levels were approximately 100-fold higher (6,650 versus 66.6 ng/ml, respectively). Elimination half-lives were similar after both aerosol and oral administration (24.1 and 25.2 h, respectively), and rimantadine urinary excretion was less than 1% per 24 h in both groups. Twenty micrograms of aerosolized rimantadine per liter of air given 12 h daily for 3 days to nine adults with acute influenza virus infection was well tolerated. Levels in plasma after 12 h of aerosol inhalation were similar to those in normal volunteers, but were higher at the end of the third treatment than they were at the end of the first treatment (88.3 versus 47.9 ng/ml, respectively). Thus, rimantadine delivered via small-particle aerosol at a dose of 20 micrograms/liter of air was well tolerated in normal volunteers and in those with acute influenza and was associated with high local concentrations
Diversity, competition, extinction: the ecophysics of language change
As early indicated by Charles Darwin, languages behave and change very much
like living species. They display high diversity, differentiate in space and
time, emerge and disappear. A large body of literature has explored the role of
information exchanges and communicative constraints in groups of agents under
selective scenarios. These models have been very helpful in providing a
rationale on how complex forms of communication emerge under evolutionary
pressures. However, other patterns of large-scale organization can be described
using mathematical methods ignoring communicative traits. These approaches
consider shorter time scales and have been developed by exploiting both
theoretical ecology and statistical physics methods. The models are reviewed
here and include extinction, invasion, origination, spatial organization,
coexistence and diversity as key concepts and are very simple in their defining
rules. Such simplicity is used in order to catch the most fundamental laws of
organization and those universal ingredients responsible for qualitative
traits. The similarities between observed and predicted patterns indicate that
an ecological theory of language is emerging, supporting (on a quantitative
basis) its ecological nature, although key differences are also present. Here
we critically review some recent advances lying and outline their implications
and limitations as well as open problems for future research.Comment: 17 Pages. A review on current models from statistical Physics and
Theoretical Ecology applied to study language dynamic
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