1,908 research outputs found
Sivers and Boer-Mulders functions in Light-Cone Quark Models
Results for the naive-time-reversal-odd quark distributions in a light-cone
quark model are presented. The final-state interaction effects are generated
via single-gluon exchange mechanism. The formalism of light-cone wave functions
is used to derive general expressions in terms of overlap of wave-function
amplitudes describing the different orbital angular momentum components of the
nucleon. In particular, the model predictions show a dominant contribution from
S- and P-wave interference in the Sivers function and a significant
contribution also from the interference of P and D waves in the Boer-Mulders
function. The favourable comparison with existing phenomenological
parametrizations motivates further applications to describe azimuthal
asymmetries in hadronic reactions.Comment: references and explanations added; version to appear in Phys. Rev.
The Nuclear Shell Model Toward the Drip Lines
We describe the "islands of inversion" that occur when approaching the
neutron drip line around the magic numbers N=20, N=28 and N=40 in the framework
of the Interacting Shell Model in very large valence spaces. We explain these
configuration inversions (and the associated shape transitions) as the result
of the competition between the spherical mean field (monopole) which favors
magicity and the correlations (multipole) which favor deformed intruder states.
We also show that the N=20 and N=28 islands are in reallity a single one, which
for the Magnesium isotopes is limited by N=18 and N=32.Comment: Proceedings of the Nordic Conference in Nuclear Physics 2011,
Stockholm, to appear in Physica Script
Cornering the revamped BMV model with neutrino oscillation data
Using the latest global determination of neutrino oscillation parameters
from~\cite{deSalas:2017kay} we examine the status of the simplest revamped
version of the BMV (Babu-Ma-Valle) model, proposed in~\cite{Morisi:2013qna}.
The model predicts a striking correlation between the "poorly determined"
atmospheric angle and CP phase , leading to either
maximal CP violation or none, depending on the preferred octants.
We determine the allowed BMV parameter regions and compare with the general
three-neutrino oscillation scenario. We show that in the BMV model the higher
octant is possible only at C.L., a stronger rejection than found in the
general case. By performing quantitative simulations of forthcoming DUNE and
T2HK experiments, using only the four "well-measured" oscillation parameters
and the indication for normal mass ordering, we also map out the potential of
these experiments to corner the model. The resulting global sensitivities are
given in a robust form, that holds irrespective of the true values of the
oscillation parameters.Comment: 5 pages, 2 figure
Resonant Destruction as a Possible Solution to the Cosmological Lithium Problem
We explore a nuclear physics resolution to the discrepancy between the
predicted standard big-bang nucleosynthesis (BBN) abundance of 7Li and its
observational determination in metal-poor stars. The theoretical 7Li abundance
is 3-4 times greater than the observational values, assuming the
baryon-to-photon ratio, eta_wmap, determined by WMAP. The 7Li problem could be
resolved within the standard BBN picture if additional destruction of A=7
isotopes occurs due to new nuclear reaction channels or upward corrections to
existing channels. This could be achieved via missed resonant nuclear
reactions, which is the possibility we consider here. We find some potential
candidate resonances which can solve the lithium problem and specify their
required resonant energies and widths. For example, a 1^- or 2^- excited state
of 10C sitting at approximately 15.0 MeV above its ground state with an
effective width of order 10 keV could resolve the 7Li problem; the existence of
this excited state needs experimental verification. Other examples using known
states include 7Be+t \rightarrow 10B(18.80 MeV), and 7Be+d \rightarrow 9B(16.71
MeV). For all of these states, a large channel radius (a > 10 fm) is needed to
give sufficiently large widths. Experimental determination of these reaction
strengths is needed to rule out or confirm these nuclear physics solutions to
the lithium problem.Comment: 37 pages, 9 figures. Additional discussion of channel widths and
radii. Matches published versio
Daily variability of Ceres' Albedo detected by means of radial velocities changes of the reflected sunlight
Bright features have been recently discovered by Dawn on Ceres, which extend
previous photometric and Space Telescope observations. These features should
produce distortions of the line profiles of the reflected solar spectrum and
therefore an apparent radial velocity variation modulated by the rotation of
the dwarf planet. Here we report on two sequences of observations of Ceres
performed in the nights of 31 July, 26-27 August 2015 by means of the
high-precision HARPS spectrograph at the 3.6-m La Silla ESO telescope. The
observations revealed a quite complex behaviour which likely combines a radial
velocity modulation due to the rotation with an amplitude of approx +/- 6 m/s
and an unexpected diurnal effect. The latter changes imply changes in the
albedo of Occator's bright features due to the blaze produced by the exposure
to solar radiation. The short-term variability of Ceres' albedo is on
timescales ranging from hours to months and can both be confirmed and followed
by means of dedicated radial velocity observations.Comment: 5 pag, 1fig, two tables, MNRAS Letters 201
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
Consistent metallicity scale for cool dwarfs and giants. A benchmark test using the Hyades
In several instances chemical abundances of dwarf and giant stars are used
simultaneously under the assumption that they share the same abundance scale.
This assumption might have implications in different astrophysical contexts. We
aim to ascertain a methodology capable of producing a consistent metallicity
scale for giants and dwarfs. To achieve that, we analyzed giants and dwarfs in
the Hyades open cluster. All these stars have archival high-resolution
spectroscopic data obtained with HARPS and UVES. In addition, the giants have
interferometric measurements of the angular diameters. We analyzed the sample
with two methods. The first method constrains the atmospheric parameters
independently from spectroscopy. For that we present a novel calibration of
microturbulence based on 3D model atmospheres. The second method is the
classical spectroscopic based on Fe lines. We also tested two line lists in an
attempt to minimize possible non-LTE effects and to optimize the treatment of
the giants. We show that it is possible to obtain a consistent metallicity
scale between dwarfs and giants. The preferred method should constrain the
three parameters , , and independent of
spectroscopy. In particular, the lines should be chosen to be free of blends in
the spectra of giants. When attention is paid to the line list, the classical
spectroscopic method can also produce consistent results. The metallicities
derived with the well-constrained set of stellar parameters are consistent
independent of the line list used. Therefore, for this cluster we favor the
metallicity of +0.180.03 dex obtained with this method. The classical
spectroscopic analysis, using the line list optimized for the giants, provides
a metallicity of +0.140.03 dex, in agreement with previous works.Comment: 19 pages, 6 figures, 10 tables. Accepted for publication in A&
The nucleon Drell-Hearn-Gerasimov sum rule within a relativistic constituent quark model
The Drell-Hearn-Gerasimov sum rule for the nucleon is investigated within a relativistic constituent quark model formulated on the light-front. The contribution of the N - Delta(1232) transition is explicitly evaluated using different forms for the baryon wave functions and adopting a one-body relativistic current for the constituent quarks. It is shown that the N - Delta(1232) contribution to the sum rule is sharply sensitive to the introduction of anomalous magnetic moments for the constituent quarks, at variance with the findings of non-relativistic and relativized quark models. The experimental value of the isovector-isovector part of the sum rule is almost totally reproduced by the N - Delta(1232) contribution, when the values of the quark anomalous magnetic moments are fixed by fitting the experimental nucleon magnetic moments. Our results are almost independent of the adopted form of the baryon wave functions and only slightly sensitive to the violation of the angular condition caused by the use of a one-body current. The calculated average slope of the generalized sum rule around the photon point results to be only slightly negative at variance with recent predictions of relativized quark models
Palomar 13: a velocity dispersion inflated by binaries ?
Recently, combining radial velocities from Keck/HIRES echelle spectra with
published proper motion membership probabilities, Cote et al (2002) observed a
sample of 21 stars, probable members of Palomar 13, a globular cluster in the
Galactic halo. Their projected velocity dispersion sigma_p = 2.2 +/-0.4 km/s
gives a mass-to-light ratio M/L_V = 40 +24/-17, about one order of magnitude
larger than the usual estimate for globular clusters. We present here radial
velocities measured from three different CCD frames of commissioning
observations obtained with the new ESO/VLT instrument FLAMES (Fibre Large Array
Multi Element Spectrograph). From these data, now publicly available, we
measure the homogeneous radial velocities of eight probable members of this
globular cluster. A new projected velocity dispersion sigma_p = 0.6-0.9 +/-0.3
km/s implies Palomar 13 mass-to-light ratio M/L_V = 3-7, similar to the usual
value for globular clusters. We discuss briefly the two most obvious reasons
for the previous unusual mass-to-light ratio finding: binaries, now clearly
detected, and more homogeneous data from the multi-fibre FLAMES spectrograph.Comment: 9 pages, 2 Postscript figure
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