4,174 research outputs found
A covariant model for the reaction
We apply the covariant spectator quark model to the study of the
electromagnetic structure of the state (),
an important resonance from the second resonance region in both spacelike and
timelike regimes. The contributions from the valence quark effects are
calculated for the helicity amplitudes. The
results are used to parametrize the meson cloud dominant at low .Comment: 3 figure, 3 pages. Contribution to the 13th International Conference
on Meson-Nucleon Physics and the Structure of the Nucleon (MENU 2013), Rome,
September-October 201
Nucleon and gamma N -> Delta lattice form factors in a constituent quark model
A covariant quark model, based both on the spectator formalism and on vector
meson dominance, and previously calibrated by the physical data, is here
extended to the unphysical region of the lattice data by means of one single
extra adjustable parameter - the constituent quark mass in the chiral limit. We
calculated the Nucleon (N) and the gamma N -> Delta form factors in the
universe of values for that parameter described by quenched lattice QCD. A
qualitative description of the Nucleon and gamma N -> Delta form factors
lattice data is achieved for light pions.Comment: To appear in J.Phys.
Valence quark contribution for the gamma N -> Delta quadrupole transition extracted from lattice QCD
Starting with a spectator quark model developed for the nucleon (N) and the
Delta in the physical pion mass region, we extend the predictions of the
reaction gamma N -> Delta to the lattice QCD regime. The quark model includes S
and D waves in the quark-diquark wavefunctions. Within this framework it is the
D-wave part in the Delta wavefunction that generates nonzero valence
contributions for the quadrupole form factors of the transition. Those
contributions are however insufficient to explain the physical data, since the
pion cloud contributions dominate. To separate the two effects we apply the
model to the lattice regime in a region where the pion cloud effects are
negligible, and adjust the D-state parameters directly to the lattice data.
This process allows us to obtain a better determination of the D-state
contributions. Finally, by adding a simple parametrization of the pion cloud we
establish the connection between the experimental data and the lattice data.Comment: To appear in Phys. Rev.
The shape of the baryon in a covariant spectator quark model
Using a covariant spectator quark model that describes the recent lattice QCD
data for the electromagnetic form factors and all available
experimental data on transitions, we analyze the charge
and magnetic dipole distributions of the baryon and discuss its shape.
We conclude that the quadrupole moment of the is a good indicator of
the deformation and that the charge distribution has an oblate
shape. We also calculate transverse moments and find that they do not lead to
unambiguous conclusions about the underlying shape.Comment: Extended introduction, references added, other small modifications.
To appear in Phys. Rev. D. 14 pages, 8 figure
N*(1535) electroproduction at high Q2
A covariant spectator quark model is applied to study the gamma N -> N*(1535)
reaction in the large Q2 region. Starting from the relation between the nucleon
and N*(1535) systems, the N*(1535) valence quark wave function is determined
without the addition of any parameters. The model is then used to calculate the
gamma N -> N*(1535) transition form factors. A very interesting, useful
relation between the A12 and S12 helicity amplitudes for Q2 > GeV^2, is also
derived.Comment: Contribution to the proceedings of the 8th International Workshop on
the Physics of Excited Nucleons (NSTAR2011), Jefferson Lab, Newport News, VA,
USA, May 17-20, 201
Fixed-axis polarization states: covariance and comparisons
Addressing the recent criticisms of Kvinikhidze and Miller, we prove that the
spectator wave functions and currents based on ``fixed-axis'' polarization
states (previously introduced by us) are Lorentz covariant, and find an
explicit connection between them and conventional direction-dependent
polarization states. The discussion shows explicitly how it is possible to
construct pure -wave models of the nucleon.Comment: Changed title and introductory material to match accepted pape
Covariant nucleon wave function with S, D, and P-state components
Expressions for the nucleon wave functions in the covariant spectator theory
(CST) are derived. The nucleon is described as a system with a off-mass-shell
constituent quark, free to interact with an external probe, and two spectator
constituent quarks on their mass shell. Integrating over the internal momentum
of the on-mass-shell quark pair allows us to derive an effective nucleon wave
function that can be written only in terms of the quark and diquark
(quark-pair) variables. The derived nucleon wave function includes
contributions from S, P and D-waves.Comment: 13 pages and 1 figur
Technical Aspects of MRI Signal Change Quantification After Gadolinium-Based Contrast Agents' Administration
Over the last 2years several studies have been published regarding gadolinium deposition in brain structures in patients with normal renal function after repeated administrations of gadolinium-based contrast agents (GBCAs). Most of the publications are magnetic resonance imaging (MRI) based retrospective studies, where gadolinium deposition may be indirectly measured by evaluating changes in T1 signal intensity (SI) in brain tissue, particularly in the dentate nucleus (DN) and/or globus pallidi (GP). The direct correlation between T1 signal changes and gadolinium deposition was validated by human pathology studies. However, the variability of the MR equipment and parameters used across different publications, along with the inherent limitations of MRI to assess gadolinium in human tissues should be acknowledged when interpreting those studies. Nevertheless, MRI studies remain essential regarding gadolinium bio-distribution knowledge. The aim of this paper is to overview current knowledge of technical aspects of T1 signal intensity evaluation by MRI and describe confounding factors, with the intention to achieve higher accuracy and maximize reproducibility.info:eu-repo/semantics/publishedVersio
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