7,745 research outputs found
Quark correlations and gluon propagators in elastic vector meson production
We study the behavior of the differential cross section for vector meson
photoproduction at large momentum transfer in the two-gluon exchange model. We
focus on the treatment of two-quark correlation function in the proton and on
gluon propagators with a dynamically generated mass. We find that only the
large t region is sensitive to the particular details of these inputs.Comment: 4 pages, 2 figures, espcrc1.sty included, Proceedings of the Third
International Conference on Perspectives in Hadronic Physic
Real and Virtual Compton Scattering in a Regge Approach
We study Real and Deeply Virtual Compton Scattering in a model based on Regge
trajectories and two-gluon exchange. In the kinematic regime of current
experiments, the hadronic component of the outgoing real photon plays a major
role. We analyze the spin structure of Compton scattering at large momentum
transfer and give predictions for several spin asymmetries. In the DVCS
channel, a fairly good agreement is obtained for the recently measured beam
spin and charge asymmetries.Comment: 12 pages; 6 figures This revised version corrects a mumerical mistake
in the evaluation of the conversion factor between vector mesons and Compton
amplitudes. Also, more details are given on the parameters of the sigma meson
amplitud
Detrimental adsorbate fields in experiments with cold Rydberg gases near surfaces
We observe the shift of Rydberg levels of rubidium close to a copper surface
when atomic clouds are repeatedly deposited on it. We measure transition
frequencies of rubidium to S and D Rydberg states with principal quantum
numbers n between 31 and 48 using the technique of electromagnetically induced
transparency. The spectroscopic measurement shows a strong increase of electric
fields towards the surface that evolves with the deposition of atoms. Starting
with a clean surface, we measure the evolution of electrostatic fields in the
range between 30 and 300 \mum from the surface. We find that after the
deposition of a few hundred atomic clouds, each containing ~10^6 atoms, the
field of adsorbates reaches 1 V/cm for a distance of 30 \mum from the surface.
This evolution of the electrostatic field sets serious limitations on cavity
QED experiments proposed for Rydberg atoms on atom chips.Comment: 4 pages, 3 figures Submitted to Phys. Rev.
Cracking Control in Mezzanine Floor Slabs using Rice Husk Ash and Polypropylene Fibers
The continuous population increase in recent years requires a greater number of households to be built quickly, with good materials and produced under quality standards that guarantee their manufacturing process. The prefabricated concrete, produced and supplied by concrete plants, is poured into the different structural elements, the mezzanine slabs being the most careful surfaces in the appearance of fissures; because being horizontal and having larger dimensions, the dimensional changes in the concrete appear more frequently due to the rapid loss of water from the surface of the concrete before setting; which generates superior stresses to the resistant capacity of the concrete at early ages, which affect the durability and reduce the resistance of the structures, causing greater economic expenses in maintenance and repairs. In the present investigation, 5%, 10% and 15% of rice husk ash was used as a replacement for cement and 900g/m3 of polypropylene fiber; The results indicate that as the percentage of rice husk ash increases, there is a reduction in the slump and the crack fissures, and that the resistance to compression and flexion decreases, with respect to the concrete pattern
OB Stars in the Solar Neighborhood I: Analysis of their Spatial Distribution
We present a newly-developed, three-dimensional spatial classification
method, designed to analyze the spatial distribution of early type stars within
the 1 kpc sphere around the Sun. We propose a distribution model formed by two
intersecting disks -the Gould Belt (GB) and the Local Galactic Disk (LGD)-
defined by their fundamental geometric parameters. Then, using a sample of
about 550 stars of spectral types earlier than B6 and luminosity classes
between III and V, with precise photometric distances of less than 1 kpc, we
estimate for some spectral groups the parameters of our model, as well as
single membership probabilities of GB and LGD stars, thus drawing a picture of
the spatial distribution of young stars in the vicinity of the Sun.Comment: 28 pages including 9 Postscript figures, one of them in color.
Accepted for publication in The Astronomical Journal, 30 January 200
Census of HII regions in NGC 6754 derived with MUSE: Constraints on the metal mixing scale
We present a study of the HII regions in the galaxy NGC 6754 from a two
pointing mosaic comprising 197,637 individual spectra, using Integral Field
Spectrocopy (IFS) recently acquired with the MUSE instrument during its Science
Verification program. The data cover the entire galaxy out to ~2 effective
radii (re ), sampling its morphological structures with unprecedented spatial
resolution for a wide-field IFU. A complete census of the H ii regions limited
by the atmospheric seeing conditions was derived, comprising 396 individual
ionized sources. This is one of the largest and most complete catalogue of H ii
regions with spectroscopic information in a single galaxy. We use this
catalogue to derive the radial abundance gradient in this SBb galaxy, finding a
negative gradient with a slope consistent with the characteristic value for
disk galaxies recently reported. The large number of H ii regions allow us to
estimate the typical mixing scale-length (rmix ~0.4 re ), which sets strong
constraints on the proposed mechanisms for metal mixing in disk galaxies, like
radial movements associated with bars and spiral arms, when comparing with
simulations. We found evidence for an azimuthal variation of the oxygen
abundance, that may be related with the radial migration. These results
illustrate the unique capabilities of MUSE for the study of the enrichment
mechanisms in Local Universe galaxies.Comment: 13 pages, 7 Figurs, accepted for publishing in A&
High Pressure optical nanothermometer based on Er3+ photoluminescence
The optical properties of a sparsely investigated material, LaGdO3 doped with Er3+, are explored regarding its suitability as nanothermometer. Besides its excellent capabilities for dielectric applications, when doping with Er3+, this material provides a highly efficient upconversion photoluminescence (PL) for high temperature thermometry at high pressure due to its structural stability. LaGdO3 belongs to the perovskite-type ABO3 compounds with a B-type monoclinic C2/m space group (a = 14.43 Å; b = 3.69 Å; c = 9.00 Å; and ? = 100.70º) at ambient conditions. It undergoes a structural phase transition to a hexagonal ?3#?1 phase at 3GPa yielding a notable PL enhancement, thus enabling it as a potential high-pressure hightemperature nanothermometer.We acknowledge financial support from MAT2015-69508-P (MINECO/FEDER), MAT2015-71070-REDC; PGC2018-101464-B-I00 (MICIN/FEDER), IDIVAL 18/28, the European Research Council FET-OPEN NCLas H2020 Project (EU829161), and BSH Electrodomésticos España, S.A
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