1,442 research outputs found
The Two-Body Random Ensemble in Nuclei
Combining analytical and numerical methods, we investigate properties of the
two-body random ensemble (TBRE). We compare the TBRE with the Gaussian
orthogonal ensemble of random matrices. Using the geometric properties of the
nuclear shell model, we discuss the information content of nuclear spectra, and
gain insight in the difficulties encountered when fitting the effective
interaction. We exhibit the existence of correlations between spectral widths
pertaining to different quantum numbers. Using these results, we deduce the
preponderance of spin-zero ground states in the TBRE. We demonstrate the
existence of correlations between spectra with different quantum numbers and/or
in different nuclei.Comment: 16 pages, 13 figure
Resonance estimates for single spin asymmetries in elastic electron-nucleon scattering
We discuss the target and beam normal spin asymmetries in elastic
electron-nucleon scattering which depend on the imaginary part of two-photon
exchange processes between electron and nucleon. We express this imaginary part
as a phase space integral over the doubly virtual Compton scattering tensor on
the nucleon. We use unitarity to model the doubly virtual Compton scattering
tensor in the resonance region in terms of
electroabsorption amplitudes. Taking those amplitudes from a phenomenological
analysis of pion electroproduction observables, we present results for beam and
target normal single spin asymmetries for elastic electron-nucleon scattering
for beam energies below 1 GeV and in the 1-3 GeV region, where several
experiments are performed or are in progress.Comment: 36 pages, 16 figure
Using the Ca II Triplet to Trace Abundance Variations in Individual Red Giant Branch stars in Three Nearby Galaxies
Spectroscopic abundance determinations for stars spanning a Hubble time in
age are necessary in order to unambiguously determine the evolutionary
histories of galaxies. Using FORS1 in Multi-Object Spectroscopy mode on ANTU
(UT1) at the ESO-VLT on Paranal we obtained near infrared spectra from which we
measured the equivalent widths of the two strongest Ca II triplet lines to
determine metal abundances for a sample of Red Giant Branch stars, selected
from ESO-NTT optical (I, V-I) photometry of three nearby, Local Group,
galaxies: the Sculptor Dwarf Spheroidal, the Fornax Dwarf Spheroidal and the
Dwarf Irregular NGC 6822. The summed equivalent width of the two strongest
lines in the Ca II triplet absorption line feature, centered at 8500A, can be
readily converted into an [Fe/H] abundance using the previously established
calibrations by Armandroff & Da Costa (1991) and Rutledge, Hesser & Stetson
(1997). We measured metallicities for 37 stars in Sculptor, 32 stars in Fornax,
and 23 stars in NGC 6822, yielding more precise estimates of the metallicity
distribution functions for these galaxies than it is possible to obtain
photometrically. In the case of NGC 6822, this is the first direct measurement
of the abundances of the intermediate-age and old stellar populations. We find
metallicity spreads in each galaxy which are broadly consistent with the
photometric width of the Red Giant Branch, although the abundances of
individual stars do not always appear to correspond to their colour. This is
almost certainly predominantly due to a highly variable star formation rate
with time in these galaxies, which results in a non-uniform,
non-globular-cluster-like, evolution of the Ca/Fe ratio.Comment: Accepted for publication in MNRA
Nucleon electroweak form factors in a meson-cloud model
The meson-cloud model of the nucleon consisting of a system of three valence
quarks surrounded by a meson cloud is applied to study the electroweak
structure of the proton and neutron. The electroweak nucleon form factors are
calculated within a light-front approach, by obtaining an overall good
description of the experimental data. Charge densities as a function of the
transverse distance with respect to the direction of the three-momentum
transfer are also discussed.Comment: Prepared for Proceedings of NSTAR2007, Workshop on the physics of
excited nucleons, Bonn (Germany), 5-8 September 200
Quantum reflection of atoms from a solid surface at normal incidence
We observed quantum reflection of ultracold atoms from the attractive
potential of a solid surface. Extremely dilute Bose-Einstein condensates of
^{23}Na, with peak density 10^{11}-10^{12}atoms/cm^3, confined in a weak
gravito-magnetic trap were normally incident on a silicon surface. Reflection
probabilities of up to 20 % were observed for incident velocities of 1-8 mm/s.
The velocity dependence agrees qualitatively with the prediction for quantum
reflection from the attractive Casimir-Polder potential. Atoms confined in a
harmonic trap divided in half by a solid surface exhibited extended lifetime
due to quantum reflection from the surface, implying a reflection probability
above 50 %.Comment: To appear in Phys. Rev. Lett. (December 2004)5 pages, 4 figure
Virtual meson cloud of the nucleon and generalized parton distributions
We present the general formalism required to derive generalized parton
distributions within a convolution model where the bare nucleon is dressed by
its virtual meson cloud. In the one-meson approximation the Fock states of the
physical nucleon are expanded in a series involving a bare nucleon and
two-particle, meson-baryon, states. The baryon is assumed here to be either a
nucleon or a described within the constituent quark model in terms of
three valence quarks; correspondingly, the meson, assumed to be a pion, is
described as a quark-antiquark pair. Explicit expressions for the unpolarized
generalized parton distributions are obtained and evaluated in different
kinematics.Comment: 37 pages, 9 figures, minor corrections, and figure 3 replaced;
version to appear in Phys. Rev.
Transverse-Momentum Distributions and Spherical Symmetry
Transverse-momentum dependent parton distributions (TMDs) are studied in the
framework of quark models. In particular, quark model relations among TMDs are
reviewed and their physical origin is discussed in terms of rotational-symmetry
properties of the nucleon state in its rest frame.Comment: 8 pages, 2 figures, prepared for the workshop "30 years of strong
interactions", Spa, Belgium, 6-8 April 201
Phase Sensitive Recombination of Two Bose-Einstein Condensates on an Atom Chip
The recombination of two split Bose-Einstein condensates on an atom chip is
shown to result in heating which depends on the relative phase of the two
condensates. This heating reduces the number of condensate atoms between 10 and
40% and provides a robust way to read out the phase of an atom interferometer
without the need for ballistic expansion. The heating may be caused by the
dissipation of dark solitons created during the merging of the condensates.Comment: 5 pages, 4 figure
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