11,446 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
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.
Optical observations of very low ionization HII regions in the large Magellanic cloud
Several very low ionization isolated HII regions were detected on a prism-objective plate of the Large Magellanic Cloud. Most of the objects show a very weak (OIII) lambda 5007 emission line and, on the other hand the (OII) lambda 6584 doublets are very intense. This kind of objects seem to be ideal in order to determine accurate N and O abundance, avoiding the use of large ionization correction factors in the N abundance determination. Spectrophotometric observations of these regions were carried out with the 4 m telescope and the 2-D Frutti spectrograph at Cerro Tololo, and with the 1.52 m and the Image Dissector Scanner (IDS) at La Silla, ESO. The wavelength range lambda lambda 3700 to 7000 A was covered. Calibrated fluxes of the emission lines detected were measured, and from these data preliminary results of physical conditions of the gas as well as some ionic abundances were derived. Comparisons of the observations with ionization structure models show that the effective temperatures of the ionizing stars are less than 35,000 K. Possible abundances gradients across the large megallanic cloud are discussed
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
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
- …