333 research outputs found
On the equilibria of finely discretized curves and surfaces
Our goal is to identify the type and number of static equilibrium points of
solids arising from fine, equidistant -discretrizations of smooth, convex
surfaces. We assume uniform gravity and a frictionless, horizontal, planar
support. We show that as approaches infinity these numbers fluctuate around
specific values which we call the imaginary equilibrium indices associated with
the approximated smooth surface. We derive simple formulae for these numbers in
terms of the principal curvatures and the radial distances of the equilibrium
points of the solid from its center of gravity. Our results are illustrated on
a discretized ellipsoid and match well the observations on natural pebble
surfaces.Comment: 21 pages, 2 figure
Cosmological distance indicators
We review three distance measurement techniques beyond the local universe:
(1) gravitational lens time delays, (2) baryon acoustic oscillation (BAO), and
(3) HI intensity mapping. We describe the principles and theory behind each
method, the ingredients needed for measuring such distances, the current
observational results, and future prospects. Time delays from strongly lensed
quasars currently provide constraints on with < 4% uncertainty, and with
1% within reach from ongoing surveys and efforts. Recent exciting discoveries
of strongly lensed supernovae hold great promise for time-delay cosmography.
BAO features have been detected in redshift surveys up to z <~ 0.8 with
galaxies and z ~ 2 with Ly- forest, providing precise distance
measurements and with < 2% uncertainty in flat CDM. Future BAO
surveys will probe the distance scale with percent-level precision. HI
intensity mapping has great potential to map BAO distances at z ~ 0.8 and
beyond with precisions of a few percent. The next years ahead will be exciting
as various cosmological probes reach 1% uncertainty in determining , to
assess the current tension in measurements that could indicate new
physics.Comment: Review article accepted for publication in Space Science Reviews
(Springer), 45 pages, 10 figures. Chapter of a special collection resulting
from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in
the Space Ag
KL to pi0 e+ e- and KL to pi0 mu+ mu- : A binary star on the stage of flavor physics
A systematic analysis of New Physics impacts on the rare decays KL to pi0 l+
l- is performed. Thanks to their different sensitivities to flavor-changing
local effective interactions, these two modes could provide valuable
information on the nature of the possible New Physics at play. In particular, a
combined measurement of both modes could disentangle scalar/pseudoscalar from
vector or axial-vector contributions. For the latter, model-independent bounds
are derived. Finally, the KL to pi0 mu+ mu- forward-backward CP-asymmetry is
considered, and shown to give interesting complementary information.Comment: 20 pages, 5 figures. Comments and references added, minor numerical
changes, conclusion unchanged, to appear in JHE
"Razne vijesti"
The present work shows the synthesis of nano-sized hybrid zeolitic imidazolate frameworks (ZIFs) with the rho topology based on a mixture of the linkers benzimidazole (bIm) and 4-methyl-5-imidazolecarboxaldehyde (4-m-5-ica). The hybrid ZIF was obtained by post-synthetic modification of ZIF-93 in a bIm solution. The use of different solvents, MeOH and N, N-dimethylacetamide (DMAc), and reaction times led to differences in the quantity of bIm incorporated to the framework, from 7.4 to 23 % according to solution-state NMR spectroscopy. XPS analysis showed that the mixture of linkers was also present at the surface of the particles. The inclusion of bIm to the ZIF-93 nanoparticles improved the thermal stability of the framework and also increased the hydrophobicity according to water adsorption results. N2 and CO2 adsorption experiments revealed that the hybrid material has an intermediate adsorption capacity, between those of ZIF-93 and ZIF-11. Finally, ZIF-93/11 hybrid materials were applied as fillers in polybenzimidazole (PBI) mixed matrix membranes (MMMs). These MMMs were used for H2/CO2 separation (at 180 °C) reaching values of 207 Barrer of H2 and a H2/CO2 selectivity of 7.7 that clearly surpassed the Robeson upper bound (corrected for this temperature)
First Results from the Arcminute Cosmology Bolometer Array Receiver
We review the first science results from the Arcminute Cosmology Bolometer
Array Receiver (ACBAR); a multifrequency millimeter-wave receiver optimized for
observations of the Cosmic Microwave Background (CMB) and the
Sunyaev-Zel'dovich (SZ) effect in clusters of galaxies. ACBAR was installed on
the 2 m Viper telescope at the South Pole in January 2001 and the results
presented here incorporate data through July 2002. We present the power
spectrum of the CMB at 150 GHz over the range \ell = 150 - 3000 measured by
ACBAR as well as estimates for the values of the cosmological parameters within
the context of Lambda-CDM models. We find that the inclusion of Omega_Lambda
greatly improves the fit to the power spectrum. We also observe a slight excess
of small-scale anisotropy at 150 GHz; if interpreted as power from the SZ
effect of unresolved clusters, the measured signal is consistent with CBI and
BIMA within the context of the SZ power spectrum models tested.Comment: To be published in the proceedings of "The Cosmic Microwave
Background and its Polarization", New Astronomy Reviews, (eds. S. Hanany and
K.A. Olive). 10 pages, 2 figure
The Fluorescence Detector of the Pierre Auger Observatory
The Pierre Auger Observatory is a hybrid detector for ultra-high energy
cosmic rays. It combines a surface array to measure secondary particles at
ground level together with a fluorescence detector to measure the development
of air showers in the atmosphere above the array. The fluorescence detector
comprises 24 large telescopes specialized for measuring the nitrogen
fluorescence caused by charged particles of cosmic ray air showers. In this
paper we describe the components of the fluorescence detector including its
optical system, the design of the camera, the electronics, and the systems for
relative and absolute calibration. We also discuss the operation and the
monitoring of the detector. Finally, we evaluate the detector performance and
precision of shower reconstructions.Comment: 53 pages. Submitted to Nuclear Instruments and Methods in Physics
Research Section
Anisotropy studies around the galactic centre at EeV energies with the Auger Observatory
Data from the Pierre Auger Observatory are analyzed to search for
anisotropies near the direction of the Galactic Centre at EeV energies. The
exposure of the surface array in this part of the sky is already significantly
larger than that of the fore-runner experiments. Our results do not support
previous findings of localized excesses in the AGASA and SUGAR data. We set an
upper bound on a point-like flux of cosmic rays arriving from the Galactic
Centre which excludes several scenarios predicting sources of EeV neutrons from
Sagittarius . Also the events detected simultaneously by the surface and
fluorescence detectors (the `hybrid' data set), which have better pointing
accuracy but are less numerous than those of the surface array alone, do not
show any significant localized excess from this direction.Comment: Matches published versio
Atmospheric effects on extensive air showers observed with the Surface Detector of the Pierre Auger Observatory
Atmospheric parameters, such as pressure (P), temperature (T) and density,
affect the development of extensive air showers initiated by energetic cosmic
rays. We have studied the impact of atmospheric variations on extensive air
showers by means of the surface detector of the Pierre Auger Observatory. The
rate of events shows a ~10% seasonal modulation and ~2% diurnal one. We find
that the observed behaviour is explained by a model including the effects
associated with the variations of pressure and density. The former affects the
longitudinal development of air showers while the latter influences the Moliere
radius and hence the lateral distribution of the shower particles. The model is
validated with full simulations of extensive air showers using atmospheric
profiles measured at the site of the Pierre Auger Observatory.Comment: 24 pages, 9 figures, accepted for publication in Astroparticle
Physic
The exposure of the hybrid detector of the Pierre Auger Observatory
The Pierre Auger Observatory is a detector for ultra-high energy cosmic rays.
It consists of a surface array to measure secondary particles at ground level
and a fluorescence detector to measure the development of air showers in the
atmosphere above the array. The "hybrid" detection mode combines the
information from the two subsystems. We describe the determination of the
hybrid exposure for events observed by the fluorescence telescopes in
coincidence with at least one water-Cherenkov detector of the surface array. A
detailed knowledge of the time dependence of the detection operations is
crucial for an accurate evaluation of the exposure. We discuss the relevance of
monitoring data collected during operations, such as the status of the
fluorescence detector, background light and atmospheric conditions, that are
used in both simulation and reconstruction.Comment: Paper accepted by Astroparticle Physic
Update on the correlation of the highest energy cosmic rays with nearby extragalactic matter
Data collected by the Pierre Auger Observatory through 31 August 2007 showed
evidence for anisotropy in the arrival directions of cosmic rays above the
Greisen-Zatsepin-Kuz'min energy threshold, \nobreak{eV}. The
anisotropy was measured by the fraction of arrival directions that are less
than from the position of an active galactic nucleus within 75 Mpc
(using the V\'eron-Cetty and V\'eron catalog). An updated
measurement of this fraction is reported here using the arrival directions of
cosmic rays recorded above the same energy threshold through 31 December 2009.
The number of arrival directions has increased from 27 to 69, allowing a more
precise measurement. The correlating fraction is , compared
with expected for isotropic cosmic rays. This is down from the early
estimate of . The enlarged set of arrival directions is
examined also in relation to other populations of nearby extragalactic objects:
galaxies in the 2 Microns All Sky Survey and active galactic nuclei detected in
hard X-rays by the Swift Burst Alert Telescope. A celestial region around the
position of the radiogalaxy Cen A has the largest excess of arrival directions
relative to isotropic expectations. The 2-point autocorrelation function is
shown for the enlarged set of arrival directions and compared to the isotropic
expectation.Comment: Accepted for publication in Astroparticle Physics on 31 August 201
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