1,336 research outputs found
Evaluation of the Primary Energy of UHE Photon-induced Atmospheric Showers from Ground Array Measurements
A photon induced shower at eV exhibits very specific
features and is different from a hadronic one. At such energies, the LPM effect
delays in average the first interactions of the photon in the atmosphere and
hence slows down the whole shower development. They also have a smaller muonic
content than hadronic ones. The response of a surface detector such as that of
the Auger Observatory to these specific showers is thus different and has to be
accounted for in order to enable potential photon candidates reconstruction
correctly. The energy reconstruction in particular has to be adapted to the
late development of photon showers. We propose in this article a method for the
reconstruction of the energy of photon showers with a surface detector. The key
feature of this method is to rely explicitly on the development stage of the
shower. This approach leads to very satisfactory results (). At
even higher energies ( eV and above) the probability for the photon
to convert into a pair of ee in the geomagnetic field becomes non
negligible and requires a different function to evaluate the energy with the
proposed method. We propose several approaches to deal with this issue in the
scope of the establishment of an upper bound on the photon fraction in UHECR.Comment: 10 page
What have we learned from observational cosmology ?
We review the observational foundations of the CDM model, considered
by most cosmologists as the standard model of cosmology. The Cosmological
Principle, a key assumption of the model is shown to be verified with
increasing accuracy. The fact that the Universe seems to have expanded from and
hot and dense past is supported by many independent probes (galaxy redshifts,
Cosmic Microwave Background, Big-Bang Nucleosynthesis and reionization). The
explosion of detailed observations in the last few decades has allowed for
precise measurements of the cosmological parameters within
Friedman-Lema\^itre-Robertson-Walker cosmologies leading to the CDM
model: an apparently flat Universe, dominated by a cosmological constant, whose
matter component is dominantly dark. We describe and discuss the various
observational probes that led to this conclusion and conclude that the
CDM model, although leaving a number of open questions concerning the
deep nature of the constituents of the Universe, provides the best theoretical
framework to explain the observations.Comment: 23 pages, no figure. Proceedings of "Philosophical Aspects of Modern
Cosmology" held in Granada, Spain, 22-23 Sept. 2011. Studies in History and
Philosophy of Modern Physics (2013
Preliminary results and perspectives in the Archeops experiment
Observations of the Cosmic Microwave Background (CMB) temperature
fluctuations are a powerful tool for testing theories of the early Universe and
for measuring cosmological parameters. We present basics of CMB physics, review
some of the most recent results and discuss their implications for cosmology.
The Archeops balloon-borne experiment is designed to map the CMB with an
angular resolution of 10 arcminutes and a precision of 30 per
pixel. This will allow the measurement of the CMB fluctuation power spectrum
from large to small angular scales. We describe preliminary results from the
test flight which took place in July 1999 and present perspectives for upcoming
scientific flights (January 2001).Comment: 10 pages, 6 figures, proceedings to "Les rencontres de Physique de la
Vallee d'Aoste", la Thuile, Italy, March 2000. Bibliography update
A demonstrator for bolometric interferometry
Bolometric Interferometry (BI) is one of the most promising techniques for
precise measurements of the Cosmic Microwave Background polarization. In this
paper, we present the results of DIBO (Demonstrateur d'Interferometrie
Bolometrique), a single-baseline demonstrator operating at 90 GHz, built to
proof the validity of the BI concept applied to a millimeter-wave
interferometer. This instrument has been characterized in the laboratory with a
detector at room temperature and with a 4 K bolometer. This allowed us to
measure interference patterns in a clean way, both (1) rotating the source and
(2) varying with time the phase shift among the two interferometer's arms.
Detailed modelisation has also been performed and validated with measurements.Comment: 15 pages, 14 figure
Effects of potassium fertilization and throughfall exclusion on the hydraulic redistribution of soil water in Eucalyptus grandis plantations
The transport of water from moist soil layers to dry through the roots of some species is an important process for plant survival during long dry periods. The objective of the present study was to evaluate if Eucalyptus grandis roots growing in a tropical region characterized by long dry periods passively move water from deep to shallow soil layers, which is known as “hydraulic redistribution”. The experiment was carried out at the Itatinga experimental station (SP, Brazil) that included four contrasting experimental plots resulting from the combination of two set of treatments: with/without potassium fertilization (+K/-K, respectively) and with/without throughfall exclusion (+W/-W, respectively). Sap flow was measured in superficial Eucalyptus coarse roots from the end of the 2014 dry season to the end of the 2015 rainy. We detected reverse sap flow (water in superficial roots going to the soil surface far from the trunks) all of the months, even during the rainy season, and in all the treatments, except in -K-W, where reverse flow started two months after the beginning of the rains (January). The lowest flow densities in superficial roots were observed in -K and/or -W, but reverse flow occurred in more roots or during more days per month than in treatments +K and +W
Baryon Acoustic Oscillations in the Ly{\alpha} forest of BOSS DR11 quasars
We report a detection of the baryon acoustic oscillation (BAO) feature in the
flux-correlation function of the Ly{\alpha} forest of high-redshift quasars
with a statistical significance of five standard deviations. The study uses
137,562 quasars in the redshift range from the Data Release
11 (DR11) of the Baryon Oscillation Spectroscopic Survey (BOSS) of SDSS-III.
This sample contains three times the number of quasars used in previous
studies. The measured position of the BAO peak determines the angular distance,
and expansion rate, , both on a scale set by the sound
horizon at the drag epoch, . We find
and
where . The optimal
combination, is determined with a precision of
. For the value , consistent with the CMB power
spectrum measured by Planck, we find
and . Tests with mock
catalogs and variations of our analysis procedure have revealed no systematic
uncertainties comparable to our statistical errors. Our results agree with the
previously reported BAO measurement at the same redshift using the
quasar-Ly{\alpha} forest cross-correlation. The auto-correlation and
cross-correlation approaches are complementary because of the quite different
impact of redshift-space distortion on the two measurements. The combined
constraints from the two correlation functions imply values of and
that are, respectively, 7% low and 7% high compared to the
predictions of a flat CDM cosmological model with the best-fit Planck
parameters. With our estimated statistical errors, the significance of this
discrepancy is .Comment: Accepted for publication in A&A. 17 pages, 18 figure
Exploring cosmic homogeneity with the BOSS DR12 galaxy sample
In this study, we probe the transition to cosmic homogeneity in the Large Scale Structure (LSS) of the Universe using the CMASS galaxy sample of BOSS spectroscopic survey which covers the largest effective volume to date, 3 h-3 Gpc3 at 0.43 ≤ z ≤ 0.7. We study the scaled counts-in-spheres, N(2.97 for r>RH, we find RH = (63.3±0.7) h-1 Mpc, in agreement at the percentage level with the predictions of the ΛCDM model RH=62.0 h-1 Mpc. Thanks to the large cosmic depth of the survey, we investigate the redshift evolution of the transition to homogeneity scale and find agreement with the ΛCDM prediction. Finally, we find that 2 is compatible with 3 at scales larger than 300 h-1 Mpc in all redshift bins. These results consolidate the Cosmological Principle and represent a precise consistency test of the ΛCDM model.PostprintPeer reviewe
The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Analysis of potential systematics
We analyze the density field of galaxies observed by the Sloan Digital Sky
Survey (SDSS)-III Baryon Oscillation Spectroscopic Survey (BOSS) included in
the SDSS Data Release Nine (DR9). DR9 includes spectroscopic redshifts for over
400,000 galaxies spread over a footprint of 3,275 deg^2. We identify,
characterize, and mitigate the impact of sources of systematic uncertainty on
large-scale clustering measurements, both for angular moments of the
redshift-space correlation function and the spherically averaged power
spectrum, P(k), in order to ensure that robust cosmological constraints will be
obtained from these data. A correlation between the projected density of stars
and the higher redshift (0.43 < z < 0.7) galaxy sample (the `CMASS' sample) due
to imaging systematics imparts a systematic error that is larger than the
statistical error of the clustering measurements at scales s > 120h^-1Mpc or k
< 0.01hMpc^-1. We find that these errors can be ameliorated by weighting
galaxies based on their surface brightness and the local stellar density. We
use mock galaxy catalogs that simulate the CMASS selection function to
determine that randomly selecting galaxy redshifts in order to simulate the
radial selection function of a random sample imparts the least systematic error
on correlation function measurements and that this systematic error is
negligible for the spherically averaged correlation function. The methods we
recommend for the calculation of clustering measurements using the CMASS sample
are adopted in companion papers that locate the position of the baryon acoustic
oscillation feature (Anderson et al. 2012), constrain cosmological models using
the full shape of the correlation function (Sanchez et al. 2012), and measure
the rate of structure growth (Reid et al. 2012). (abridged)Comment: Matches version accepted by MNRAS. Clarifications and references have
been added. See companion papers that share the "The clustering of galaxies
in the SDSS-III Baryon Oscillation Spectroscopic Survey:" titl
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