1,913 research outputs found
A view of the Galactic halo using beryllium as a time scale
Beryllium stellar abundances were suggested to be a good tracer of time in
the early Galaxy. In an investigation of its use as a cosmochronometer, using a
large sample of local halo and thick-disk dwarfs, evidence was found that in a
log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components.
One is consistent with predictions of evolutionary models while the other is
chemically indistinguishable from the thick-disk stars. This is interpreted as
a difference in the star formation history of the two components and suggests
that the local halo is not a single uniform population where a clear
age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU
Symposium, Volume 265, Chemical abundances in the Universe: connecting first
stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure
Beryllium abundances and the formation of the halo and the thick disk
The single stable isotope of beryllium is a pure product of cosmic-ray
spallation in the ISM. Assuming that the cosmic-rays are globally transported
across the Galaxy, the beryllium production should be a widespread process and
its abundance should be roughly homogeneous in the early-Galaxy at a given
time. Thus, it could be useful as a tracer of time. In an investigation of the
use of Be as a cosmochronometer and of its evolution in the Galaxy, we found
evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate
into two components. One is consistent with predictions of evolutionary models
while the other is chemically indistinguishable from the thick-disk stars. This
is interpreted as a difference in the star formation history of the two
components and suggests that the local halo is not a single uniform population
where a clear age-metallicity relation can be defined. We also found evidence
that the star formation rate was lower in the outer regions of the thick disk,
pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 -
Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C.
Chiappini, eds
The spin structure of the nucleon in light-cone quark models
The quark spin densities related to generalized parton distributions in
impact-parameter space and to transverse-momentum dependent parton
distributions are reviewed within a light-cone quark model, with focus on the
role of the different spin-spin and spin-orbit correlations of quarks.Results
for azimuthal spin asymmetries in semi-inclusive deep-inelastic scattering due
to T-even transverse-momentum dependent parton distributions are also
discussed.Comment: proceedings of the workshop "Recent Advances in Perturbative QCD and
Hadronic Physics", 20-25 July 2009, ECT*, Trento (Italy), in Honor of Prof.
Anatoly Efremov's 75th Birthday Celebration; also based on the talk given at
the workshop "Transverse Partonic Structure of Hadrons", 21-26 June 2009,
Yerevan (Armenia); to appear in Mod. Phys. Lett.
Proton scalar dipole polarizabilities from real Compton scattering data, using fixed-t subtracted dispersion relations and the bootstrap method
We perform a fit of the real Compton scattering (RCS) data below
pion-production threshold to extract the electric () and magnetic
() static scalar dipole polarizabilities of the proton, using
fixed- subtracted dispersion relations and a bootstrap-based fitting
technique. The bootstrap method provides a convenient tool to include the
effects of the systematic errors on the best values of and
and to propagate the statistical errors of the model parameters
fixed by other measurements. We also implement various statistical tests to
investigate the consistency of the available RCS data sets below
pion-production threshold and we conclude that there are not strong motivations
to exclude any data point from the global set. Our analysis yields and , with p-value .Comment: 19 pages, 11 figures, 4 tables; final version accepted for
publication in J. Phys.
First extraction of the scalar proton dynamical polarizabilities from real Compton scattering data
We present the first attempt to extract the scalar dipole dynamical
polarizabilities from proton real Compton scattering data below pion-production
threshold. The theoretical framework combines dispersion relations technique,
low-energy expansion and multipole decomposition of the scattering amplitudes.
The results are obtained with statistical tools that have never been applied so
far to Compton scattering data and are crucial to overcome problems inherent to
the analysis of the available data set.Comment: 8 pages, 4 figures, 2 tables; extended version to appear in Phys.
Rev.
Transverse pion structure beyond leading twist in constituent models
The understanding of the pion structure as described in terms of
transverse-momentum dependent parton distribution functions (TMDs) is of
importance for the interpretation of currently ongoing Drell-Yan experiments
with pion beams. In this work we discuss the description of pion TMDs beyond
leading twist in a pion model formulated in the light-front constituent
framework. For comparison, we also review and derive new results for pion TMDs
in the bag and spectator models.Comment: 17 pages, 7 figures; v2: modified presentation, updated references;
matches the journal versio
Naive time-reversal odd phenomena in semi-inclusive deep-inelastic scattering from light-cone constituent quark models
We present results for leading-twist azimuthal asymmetries in semi-inclusive
lepton-nucleon deep-inelastic scattering due to naively time-reversal odd
transverse-momentum dependent parton distribution functions from the light-cone
constituent quark model. We carefully discuss the range of applicability of the
model, especially with regard to positivity constraints and evolution effects.
We find good agreement with available experimental data from COMPASS and
HERMES, and present predictions to be tested in forthcoming experiments at
Jefferson Lab.Comment: 10 pages, 7 figures, discussion of evolution effects extended, to
appear in Phys.Rev.
Shell-model phenomenology of low-momentum interactions
The first detailed comparison of the low-momentum interaction V_{low k} with
G matrices is presented. We use overlaps to measure quantitatively the
similarity of shell-model matrix elements for different cutoffs and oscillator
frequencies. Over a wide range, all sets of V_{low k} matrix elements can be
approximately obtained from a universal set by a simple scaling. In an
oscillator mean-field approach, V_{low k} reproduces satisfactorily many
features of the single-particle and single-hole spectra on closed-shell nuclei,
in particular through remarkably good splittings between spin-orbit partners on
top of harmonic oscillator closures. The main deficiencies of pure two-nucleon
interactions are associated with binding energies and with the failure to
ensure magicity for the extruder-intruder closures. Here, calculations
including three-nucleon interactions are most needed. V_{low k} makes it
possible to define directly a meaningful unperturbed monopole Hamiltonian, for
which the inclusion of three-nucleon forces is tractable.Comment: 5 pages, 4 figures, minor additions, to appear as Rapid Comm. in
Phys. Rev.
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