454 research outputs found
Evidence for the fourth P11 resonance predicted by the constituent quark model
It is pointed out that the third of five low-lying P11 states predicted by a
constituent quark model can be identified with the third of four states in a
solution from a three-channel analysis by the Zagreb group. This is one of the
so-called ``missing'' resonances, predicted at 1880 MeV. The fit of the Zagreb
group to the pi N -> eta N data is the crucial element in finding this fourth
resonance in the P11 partial wave.Comment: 8 pages, revtex; expanded acknowledgement
Optimizing an array of antennas for cellular coverage from a high altitude platform
In a wireless communications network served by a high altitude platform (HAP) the cochannel interference is a function of the antenna beamwidth, angular separation and. sidelobe level. At the millimeter wave frequencies proposed for HAPs, an array of aperture type antennas on the platform is a practicable solution for serving the cells. We present a method for predicting cochannel interference based on curve-fit approximations for radiation patterns of elliptic beams which illuminate cell edges with optimum power, and a means of estimating optimum beamwidths for each cell of a regular hexagonal layout. The method is then applied to a 121 cell architecture. Where sidelobes are modeled As a flat floor at 40-dB below peak directivity, a cell cluster size of four yields carrier-to-interference ratios (CIRs), which vary from 15 dB at cell edges to 27 dB at cell centers. On adopting a cluster size of seven, these figures increase, respectively, to 19 and 30 dB. On reducing the sidelobe level, the. improvement in CIR can be quantified. The method also readily allows for regions of overlapping channel coverage to be shown
Constructing Hybrid Baryons with Flux Tubes
Hybrid baryon states are described in quark potential models as having
explicit excitation of the gluon degrees of freedom. Such states are described
in a model motivated by the strong coupling limit of Hamiltonian lattice gauge
theory, where three flux tubes meeting at a junction play the role of the glue.
The adiabatic approximation for the quark motion is used, and the flux tubes
and junction are modeled by beads which are attracted to each other and the
quarks by a linear potential, and vibrate in various string modes. Quantum
numbers and estimates of the energies of the lightest hybrid baryons are
provided.Comment: 4 pages, RevTeX. Submitted to Physical Review Letter
Comment on 's and the H1 and ZEUS High Anomalies
We investigate the effects of extra neutral gauge bosons on the high
region of the cross section at GeV. We found
that the only models with electroweak strength coupling, typical of extended
gauge theories, that give a better fit to the H1 and ZEUS high data than
the standard model, are ruled out by existing data from the Tevatron. From
general scaling arguments, using the allowed contact interactions, the only
allowed models with 's would be those with strong couplings although even
in this case the statistical evidence is not compelling.Comment: Latex file uses revtex version 3, epsfig, 1 postscript figure is
attache
Confinement effects from interacting chromo-magnetic and axion fields
We study a non-Abelian gauge theory with a pseudo scalar coupling \phi
\epsilon ^{\mu \nu \alpha \beta} F_{\mu \nu}^a F_{\alpha \beta}^a in the case
where a constant chromo-electric, or chromo-magnetic, strength expectation
value is present. We compute the interaction potential within the framework of
gauge-invariant, path-dependent, variables formalism. While in the case of a
constant chromo-electric field strength expectation value the static potential
remains Coulombic, in the case of a constant chromo-magnetic field strength the
potential energy is the sum of a Coulombic and a linear potentials, leading to
the confinement of static charges.Comment: 12 pages, no figures, published versio
Violations of Lorentz Covariance in Light Front Quark Models
Electromagnetic form factors of the nucleon from relativistic quark models
are analyzed: results from null-plane projection of the Feynman triangle
diagram are compared with a Bakamjian-Thomas model. The magnetic form factors
of the models differ by about 15% at spacelike momentum transfer 0.5 GeV^2,
while the charge form factors are much closer. Spurious contributions to
electromagnetic form factors due to violations of rotational symmetry are
eliminated from both models. One method changes magnetic form factors by about
10%, whereas the charge form factors stay nearly the same. Another one changes
the charge form factor of the Bakamjian-Thomas model by more than 50%.Comment: 19 pages, 9 figures, Late
Delta isobar masses, large N_c relations, and the quark model
Motivated by recent remarks on the Delta+ mass and comparisons between the
quark model and relations based on large-N_c with perturbative flavor breaking,
two sets of Delta masses consistent with these constraints are constructed.
These two sets, based either on an experimentally determined mass splitting or
a quark model of isospin symmetry breaking, are shown to be inconsistent. The
model dependence of this inconsistency is examined, and suggestions for
improved experiments are made. An explicit quark model calculation and mass
relations based on the large-N_c limit with perturbative flavor breaking are
compared. The expected level of accuracy of such relations is realized in the
quark model, except for mass relations spanning more than one SU(6)
representation. It is shown that the Delta0 and Delta++ pole masses and Delta0
- Delta+ = (Delta- - Delta++)/3 about 1.5 MeV are more consistent with model
expectations than the analogous Breit-Wigner masses and their splittings.Comment: 10 pages, including 1 eps figure, revte
Analysis of Negative Parity Baryon Photoproduction Amplitudes in the Expansion
We study the photoproduction helicity amplitudes of negative parity baryons
in the context of the expansion of QCD. A complete analysis to
next-to-leading order is carried out. The results show sub-leading effects to
be within the magnitude expected from the power counting. They also
show significant deviations from the quark model, in particular the need for
2-body effects.Comment: 7 pages, 5 table
Exotics and all that
This invited contribution summarizes some of the more important aspects of
exotics. We review theoretical expectations for exotic and nonexotic hybrid
mesons, and briefly discuss the leading experimental candidate for an exotic,
the pi_1(1600).Comment: 4 pages. Invited contribution to Electron-Nucleus Scattering VII,
Elba, 24-28 June 200
Excited nucleon spectrum from lattice QCD with maximum entropy method
We study excited states of the nucleon in quenched lattice QCD with the
spectral analysis using the maximum entropy method. Our simulations are
performed on three lattice sizes , and
, at to address the finite volume issue. We find a
significant finite volume effect on the mass of the Roper resonance for light
quark masses. After removing this systematic error, its mass becomes
considerably reduced toward the direction to solve the level order puzzle
between the Roper resonance and the negative-parity nucleon
.Comment: Lattice2003(spectrum), 3 pages, 4 figure
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