3,856 research outputs found
Impact of Interatomic Electronic Decay Processes on Xe 4d Hole Decay in the Xenon Fluorides
A hole in a 4d orbital of atomic xenon relaxes through Auger decay after a
lifetime of 3 fs. Adding electronegative fluorine ligands to form xenon
fluoride molecules, results in withdrawal of valence-electron density from Xe.
Thus, within the one-center picture of Auger decay, a lowered Xe 4d Auger width
would be expected, in contradiction, however, with experiment. Employing
extensive ab initio calculations within the framework of many-body Green's
functions, we determine all available decay channels in XeFn and characterize
these channels by means of a two-hole population analysis. We derive a relation
between two-hole population numbers and partial Auger widths. On this basis,
interatomic electronic decay processes are demonstrated to be so strong in the
xenon fluorides that they overcompensate the reduction in intra-atomic Auger
width and lead to the experimentally observed trend. The nature of the relevant
processes is discussed. These processes presumably underlie Auger decay in a
variety of systems.Comment: 11 pages, 5 figures, 3 tables, RevTeX4, extensively revised, the
discussion of single ionization of XeFn was published separately: J. Chem.
Phys. 119, 7763--7771 (2003), preprint arXiv: physics/030612
Recommendation Subgraphs for Web Discovery
Recommendations are central to the utility of many websites including
YouTube, Quora as well as popular e-commerce stores. Such sites typically
contain a set of recommendations on every product page that enables visitors to
easily navigate the website. Choosing an appropriate set of recommendations at
each page is one of the key features of backend engines that have been deployed
at several e-commerce sites.
Specifically at BloomReach, an engine consisting of several independent
components analyzes and optimizes its clients' websites. This paper focuses on
the structure optimizer component which improves the website navigation
experience that enables the discovery of novel content.
We begin by formalizing the concept of recommendations used for discovery. We
formulate this as a natural graph optimization problem which in its simplest
case, reduces to a bipartite matching problem. In practice, solving these
matching problems requires superlinear time and is not scalable. Also,
implementing simple algorithms is critical in practice because they are
significantly easier to maintain in production. This motivated us to analyze
three methods for solving the problem in increasing order of sophistication: a
sampling algorithm, a greedy algorithm and a more involved partitioning based
algorithm.
We first theoretically analyze the performance of these three methods on
random graph models characterizing when each method will yield a solution of
sufficient quality and the parameter ranges when more sophistication is needed.
We complement this by providing an empirical analysis of these algorithms on
simulated and real-world production data. Our results confirm that it is not
always necessary to implement complicated algorithms in the real-world and that
very good practical results can be obtained by using heuristics that are backed
by the confidence of concrete theoretical guarantees
Resolving on 100 pc scales the UV-continuum in Lyman- emitters between redshift 2 to 3 with gravitational lensing
We present a study of seventeen LAEs at redshift 23 gravitationally
lensed by massive early-type galaxies (ETGs) at a mean redshift of
approximately 0.5. Using a fully Bayesian grid-based technique, we model the
gravitational lens mass distributions with elliptical power-law profiles and
reconstruct the UV-continuum surface brightness distributions of the background
sources using pixellated source models. We find that the deflectors are close
to, but not consistent with isothermal models in almost all cases, at the
-level. We take advantage of the lensing magnification (typically
20) to characterise the physical and morphological properties of
these LAE galaxies. From reconstructing the ultra-violet continuum emission, we
find that the star-formation rates range from 0.3 to 8.5 M yr
and that the galaxies are typically composed of several compact and diffuse
components, separated by 0.4 to 4 kpc. Moreover, they have peak star-formation
rate intensities that range from 2.1 to 54.1 M yr kpc.
These galaxies tend to be extended with major axis ranging from 0.2 to 1.8 kpc
(median 561 pc), and with a median ellipticity of 0.49. This morphology is
consistent with disk-like structures of star-formation for more than half of
the sample. However, for at least two sources, we also find off-axis components
that may be associated with mergers. Resolved kinematical information will be
needed to confirm the disk-like nature and possible merger scenario for the
LAEs in the sample.Comment: 19 pages, 7 figures, accepted for publication on MNRA
Dynamical Behavior of a stochastic SIRS epidemic model
In this paper we study the Kernack - MacKendrick model under telegraph noise.
The telegraph noise switches at random between two SIRS models. We give out
conditions for the persistence of the disease and the stability of a disease
free equilibrium. We show that the asymptotic behavior highly depends on the
value of a threshold which is calculated from the intensities of
switching between environmental states, the total size of the population as
well as the parameters of both SIRS systems. According to the value of
, the system can globally tend towards an endemic case or a disease
free case. The aim of this work is also to describe completely the omega-limit
set of all positive solutions to the model. Moreover, the attraction of the
omega-limit set and the stationary distribution of solutions will be pointed
out.Comment: 16 page
A system for online beam emittance measurements and proton beam characterization
A system for online measurement of the transverse beam emittance was
developed. It is named PrOBaM (4-Profiler Online Beam
Emittance Measurement) and was conceived to measure the emittance in a fast and
efficient way using the multiple beam profiler method. The core of the system
is constituted by four consecutive UniBEaM profilers, which are based on silica
fibers passing across the beam. The PrOBaM system was
deployed for characterization studies of the 18~MeV proton beam produced by the
IBA Cyclone 18 MeV cyclotron at Bern University Hospital (Inselspital). The
machine serves daily radioisotope production and multi-disciplinary research,
which is carried out with a specifically conceived Beam Transport Line (BTL).
The transverse RMS beam emittance of the cyclotron was measured as a function
of several machine parameters, such as the magnetic field, RF peak voltage, and
azimuthal angle of the stripper. The beam emittance was also measured using the
method based on the quadrupole strength variation. The results obtained with
both techniques were compared and a good agreement was found. In order to
characterize the longitudinal dynamics, the proton energy distribution was
measured. For this purpose, a method was developed based on aluminum absorbers
of different thicknesses, a UniBEaM detector, and a Faraday cup. The results
were an input for a simulation of the BTL developed in the MAD-X software. This
tool allows machine parameters to be tuned online and the beam characteristics
to be optimized for specific applications.Comment: published in Journal of Instrumentatio
Studying the nuclear mass composition of Ultra-High Energy Cosmic Rays with the Pierre Auger Observatory
The Fluorescence Detector of the Pierre Auger Observatory measures the
atmospheric depth, , where the longitudinal profile of the high energy
air showers reaches its maximum. This is sensitive to the nuclear mass
composition of the cosmic rays. Due to its hybrid design, the Pierre Auger
Observatory also provides independent experimental observables obtained from
the Surface Detector for the study of the nuclear mass composition. We present
-distributions and an update of the average and RMS values in
different energy bins and compare them to the predictions for different nuclear
masses of the primary particles and hadronic interaction models. We also
present the results of the composition-sensitive parameters derived from the
ground level component.Comment: Proceedings of the 12th International Conference on Topics in
Astroparticle and Underground Physics, TAUP 2011, Munich, German
Light Nuclei solving Auger puzzles. The Cen-A imprint
Ultra High Energy Cosmic Rays (UHECR) map at 60 EeV have been found recently
by AUGER group spreading anisotropy signatures in the sky. The result have been
interpreted as a manifestation of AGN sources ejecting protons at GZK edges
mostly from Super-galactic Plane. The result is surprising due to the absence
of much nearer Virgo cluster. Moreover, early GZK cut off in the spectra may be
better reconcile with light nuclei (than with protons). In addition a large
group (nearly a dozen) of events cluster suspiciously along Cen-A. Finally,
proton UHECR composition nature is in sharp disagreement with earlier AUGER
claim of a heavy nuclei dominance at 40 EeV. Therefore we interpret here the
signals as mostly UHECR light nuclei (He, Be, B, C, O), very possibly mostly
the lightest (He,Be) ones, ejected from nearest AGN Cen-A, UHECR smeared by
galactic magnetic fields, whose random vertical bending is overlapping with
super-galactic arm. The eventual AUGER misunderstanding took place because of
such a rare coincidence between the Super Galactic Plane (arm) and the smeared
(randomized) signals from Cen-A, bent orthogonally to the Galactic fields. Our
derivation verify the consistence of the random smearing angles for He, Be and
B, C, O, in reasonable agreement with the AUGER main group events around Cen-A.
Only few other rare events are spread elsewhere. The most collimated from Cen-A
are the lightest. The most spread the heavier. Consequently Cen-A is the best
candidate UHE neutrino tau observable by HEAT and AMIGA as enhanced AUGER array
at tens-hundred PeV energy. This model maybe soon tested by new events
clustering around the Cen-A and by composition imprint study.Comment: 4 pages, 5 figures
Global Production Increased by Spatial Heterogeneity in a Population Dynamics Model
Spatial and temporal heterogeneity are often described as important factors having a strong impact on biodiversity. The effect of heterogeneity is in most cases analyzed by the response of biotic interactions such as competition of predation. It may also modify intrinsic population properties such as growth rate. Most of the studies are theoretic since it is often difficult to manipulate spatial heterogeneity in practice. Despite the large number of studies dealing with this topics, it is still difficult to understand how the heterogeneity affects populations dynamics. On the basis of a very simple model, this paper aims to explicitly provide a simple mechanism which can explain why spatial heterogeneity may be a favorable factor for production.We consider a two patch model and a logistic growth is assumed on each patch. A general condition on the migration rates and the local subpopulation growth rates is provided under which the total carrying capacity is higher than the sum of the local carrying capacities, which is not intuitive. As we illustrate, this result is robust under stochastic perturbations
Results of a self-triggered prototype system for radio-detection of extensive air showers at the Pierre Auger Observatory
We describe the experimental setup and the results of RAuger, a small
radio-antenna array, consisting of three fully autonomous and self-triggered
radio-detection stations, installed close to the center of the Surface Detector
(SD) of the Pierre Auger Observatory in Argentina. The setup has been designed
for the detection of the electric field strength of air showers initiated by
ultra-high energy cosmic rays, without using an auxiliary trigger from another
detection system. Installed in December 2006, RAuger was terminated in May 2010
after 65 registered coincidences with the SD. The sky map in local angular
coordinates (i.e., zenith and azimuth angles) of these events reveals a strong
azimuthal asymmetry which is in agreement with a mechanism dominated by a
geomagnetic emission process. The correlation between the electric field and
the energy of the primary cosmic ray is presented for the first time, in an
energy range covering two orders of magnitude between 0.1 EeV and 10 EeV. It is
demonstrated that this setup is relatively more sensitive to inclined showers,
with respect to the SD. In addition to these results, which underline the
potential of the radio-detection technique, important information about the
general behavior of self-triggering radio-detection systems has been obtained.
In particular, we will discuss radio self-triggering under varying local
electric-field conditions.Comment: accepted for publication in JINS
A Three-Point Cosmic Ray Anisotropy Method
The two-point angular correlation function is a traditional method used to
search for deviations from expectations of isotropy. In this paper we develop
and explore a statistically descriptive three-point method with the intended
application being the search for deviations from isotropy in the highest energy
cosmic rays. We compare the sensitivity of a two-point method and a
"shape-strength" method for a variety of Monte-Carlo simulated anisotropic
signals. Studies are done with anisotropic source signals diluted by an
isotropic background. Type I and II errors for rejecting the hypothesis of
isotropic cosmic ray arrival directions are evaluated for four different event
sample sizes: 27, 40, 60 and 80 events, consistent with near term data
expectations from the Pierre Auger Observatory. In all cases the ability to
reject the isotropic hypothesis improves with event size and with the fraction
of anisotropic signal. While ~40 event data sets should be sufficient for
reliable identification of anisotropy in cases of rather extreme (highly
anisotropic) data, much larger data sets are suggested for reliable
identification of more subtle anisotropies. The shape-strength method
consistently performs better than the two point method and can be easily
adapted to an arbitrary experimental exposure on the celestial sphere.Comment: Fixed PDF erro
- …