339 research outputs found
Extraction of proton form factors in the timelike region from unpolarized e+e- --> p pbar events
We have performed numerical simulations of the unpolarized e+e- --> p pbar
process in kinematic conditions under discussion for a possible upgrade of the
existing DAFNE facility. By fitting the cross section angular distribution with
a typical Born expression, we can extract information on the ratio |G_E/G_M| of
the proton electromagnetic form factors in the timelike region within a 5-10%
uncertainty. We have explored also non-Born contributions to the cross section
by introducing a further component in the angular fit, which is related to
two-photon exchange diagrams. We show that these corrections can be identified
if larger than 5% of the Born contribution, and if relative phases of the
complex form factors do not produce severe cancellations.Comment: 10 pages in RevTeX style; 8 figures in eps forma
Li - O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas
Elemental correlations and anti-correlations are known to be present in
globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its
fragility Li is destroyed at the temperature at which the CNO cycling occurs,
and this makes Li a crucial study for the nature of the contaminating stars. We
observed 112 un-evolved stars at the Turnoff of the NGC6752 cluster with FLAMES
at the VLT to investigate the presence and the extent of a Li-O correlation.
This correlation is expected if there is a simple pollution scenario. Li (670.8
nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to
a very narrow region of the color-magnitude diagram, so they have similar
stellar parameters (Teff, log g). We find that O and Li correlate, with a high
statistical significance that confirms the early results for this cluster. At
first glance this is what is expected if a simple pollution of pristine gas
with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship,
however, is about 0.4, and differs from unity by over 7 Sigma. A slope of one
is the value predicted for a pure contamination model. We confirm an extended
Li-O correlation in non evolved stars of NGC 6752. At the same time the
characteristic of the correlation shows that a simple pollution scenario is not
sufficient to explain the observations. Within this scenario the contaminant
gas must have been enriched in Li. This would rule out massive stars as main
polluters, and favor the hypothesis that the polluting gas was enriched by
intermediate or high-mass AGB stars, unless the former can be shown to be able
to produce Li. According to our observations, the fraction of polluting gas
contained in the stars observed is a considerable fraction of the stellar mass
of the cluster.Comment: 8pages, 2 figures, accepted by A&A Lette
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
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
Unified framework for generalized and transverse-momentum dependent parton distributions within a 3Q light-cone picture of the nucleon
We present a systematic study of generalized transverse-momentum dependent
parton distributions (GTMDs). By taking specific limits or projections, these
GTMDs yield various transverse-momentum dependent and generalized parton
distributions, thus providing a unified framework to simultaneously model
different observables. We present such simultaneous modeling by considering a
light-cone wave function overlap representation of the GTMDs. We construct the
different quark-quark correlation functions from the 3-quark Fock components
within both the light-front constituent quark model as well as within the
chiral quark-soliton model. We provide a comparison with available data and
make predictions for different observables.Comment: version to appear in JHE
Azimuthal asymmetries in lepton-pair production at a fixed-target experiment using the LHC beams (AFTER)
A multi-purpose fixed-target experiment using the proton and lead-ion beams
of the LHC was recently proposed by Brodsky, Fleuret, Hadjidakis and Lansberg,
and here we concentrate our study on some issues related to the spin physics
part of this project (referred to as AFTER). We study the nucleon spin
structure through and processes with a fixed-target experiment using
the LHC proton beams, for the kinematical region with 7 TeV proton beams at the
energy in center-of-mass frame of two nucleons GeV. We calculate
and estimate the azimuthal asymmetries of unpolarized and
dilepton production processes in the Drell--Yan continuum region and at the
-pole. We also calculate the , and
azimuthal asymmetries of and dilepton production
processes with the target proton and deuteron longitudinally or transversally
polarized in the Drell--Yan continuum region and around resonances region.
We conclude that it is feasible to measure these azimuthal asymmetries,
consequently the three-dimensional or transverse momentum dependent parton
distribution functions (3dPDFs or TMDs), at this new AFTER facility.Comment: 15 pages, 40 figures. Version accepted for publication in EPJ
Pion transverse momentum dependent parton distributions in the Nambu and Jona-Lasinio model
An explicit evaluation of the two pion transverse momentum dependent parton distributions at leading twist is presented, in the framework of the Nambu-Jona Lasinio model with Pauli-Villars regularization. The transverse momentum dependence of the obtained distributions is generated solely by the dynamics of the model. Using these results, the so called generalized Boer-Mulders shift is studied and compared with recent lattice data. The obtained agreement is very encouraging, in particular because no additional parameter has been introduced. A more conclusive comparison would require a precise knowledge of the QCD evolution of the transverse momentum dependent parton distributions under scrutiny
Transverse Spin Structure of the Nucleon through Target Single Spin Asymmetry in Semi-Inclusive Deep-Inelastic Reaction at Jefferson Lab
Jefferson Lab (JLab) 12 GeV energy upgrade provides a golden opportunity to
perform precision studies of the transverse spin and
transverse-momentum-dependent structure in the valence quark region for both
the proton and the neutron. In this paper, we focus our discussion on a
recently approved experiment on the neutron as an example of the precision
studies planned at JLab. The new experiment will perform precision measurements
of target Single Spin Asymmetries (SSA) from semi-inclusive electro-production
of charged pions from a 40-cm long transversely polarized He target in
Deep-Inelastic-Scattering kinematics using 11 and 8.8 GeV electron beams. This
new coincidence experiment in Hall A will employ a newly proposed solenoid
spectrometer (SoLID). The large acceptance spectrometer and the high polarized
luminosity will provide precise 4-D (, , and ) data on the
Collins, Sivers, and pretzelocity asymmetries for the neutron through the
azimuthal angular dependence. The full 2 azimuthal angular coverage in the
lab is essential in controlling the systematic uncertainties. The results from
this experiment, when combined with the proton Collins asymmetry measurement
and the Collins fragmentation function determined from the ee collision
data, will allow for a quark flavor separation in order to achieve a
determination of the tensor charge of the d quark to a 10% accuracy. The
extracted Sivers and pretzelocity asymmetries will provide important
information to understand the correlations between the quark orbital angular
momentum and the nucleon spin and between the quark spin and nucleon spin.Comment: 23 pages, 13 figures, minor corrections, matches published versio
Weighted azimuthal asymmetries in a diquark spectator model
We analytically calculate weighted azimuthal asymmetries in semi-inclusive
lepton-nucleon deep-inelastic scattering and Drell-Yan processes, using
transverse-momentum-dependent partonic densities obtained in a diquark
spectator model. We compare the asymmetries with available preliminary
experimental data, in particular for the Collins and the Sivers effect. We make
predictions for other cases of interest in running and planned experiments.Comment: 21 pages, 13 (multiple) figures in eps format, RevTeX
Study of the cap structure of (3, 3),(4, 4) and (5, 5)-SWCNTs: Application of the sphere-in-contact model
We have applied the sphere-in-contact model supported by hybrid Density Functional Theory (DFT) calculations to elucidate the cap geometry of the sub-nanometer in dimension (3,3), (4,4) and (5,5) single-wall carbon-nanotubes (SWCNTs). Our approach predicts certain cap-geometries that do not comprise of the commonly known for their stability combination of pentagonal and hexagonal carbon rings but also tetragonal, trigonal and all-pentagonal structures. Based on hybrid-DFT calculations carbon atoms in these new cap geometries have similar stability to carbon found in other fullerene-like capped zig-zag and arm-chair nanotubes (i.e., (5,5), (6,6), (9,0) and (10,0)) that are known to be stable and synthetically accessible. We find that the cap structure of the (3,3)-CNTs is a pointy carbon geometry comprised of six pentagonal rings with a single carbon atom at the tip apex. In this tip geometry the carbon atom at the tip apex does not have the usual sp2 or sp3 geometry but an unusual trigonal pyramidal configuration. DFT calculations of the molecular orbitals and density-of-states of the tip show that this tip structure apart from being stable can be used in scanning probe microscopies such as STM for very high resolution imaging
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