12 research outputs found
Time-like and space-like electromagnetic form factors of nucleons, a unified description
The extended Lomon-Gari-Kr\"umpelmann model of nucleon electromagnetic form
factors, which embodies \rho, \rho', \omega, \omega' and \phi vector meson
contributions and the perturbative QCD high momentum transfer behavior has been
extended to the time-like region. Breit-Wigner formulae with momentum-dependent
widths have been considered for broad resonances in order to have a
parametrization for the electromagnetic form factors that fulfills, in the
time-like region, constraints from causality, analyticity, and unitarity. This
analytic extension of the Lomon-Gari-Kr\"umpelmann model has been used to
perform a unified fit to all the nucleon electromagnetic form factor data, in
the space-like and time-like region (where form factor values are extracted
from e+e- nucleon-antinucleon cross sections data). The knowledge of the
complete analytic structure of form factors enables predictions at extended
momentum transfer, and also of time-like observables such as the ratio between
electric and magnetic form factors and their relative phase.Comment: 21 pages, 17 figures. Accepted for publication in PR
Analytic pion form factor
The pion electromagnetic form factor and two-pion production in electron-positron collisions are simultaneously fitted by a vector dominance model evolving to perturbative QCD at large momentum transfer. This model was previously successful in simultaneously fitting the nucleon electromagnetic form factors (spacelike region) and the electromagnetic production of nucleon-antinucleon pairs (timelike region). For this pion case dispersion relations are used to produce the analytic connection of the spacelike and timelike regions. The fit to all the data is good, especially for the newer sets of timelike data. The description of high-q[superscript 2] data, in the timelike region, requires one more meson with Ï quantum numbers than listed in the 2014 Particle Data Group review.United States. Dept. of Energy (Award DE-SC0011090
Effect of recent R_p and R_n measurements on extended Gari-Krumpelmann model fits to nucleon electromagnetic form factors
The Gari-Krumpelmann (GK) models of nucleon electromagnetic form factors, in
which the rho, omega, and phi vector meson pole contributions evolve at high
momentum transfer to conform to the predictions of perturbative QCD (pQCD), was
recently extended to include the width of the rho meson by substituting the
result of dispersion relations for the pole and the addition of rho' (1450)
isovector vector meson pole. This extended model was shown to produce a good
overall fit to all the available nucleon electromagnetic form factor (emff)
data. Since then new polarization data shows that the electric to magnetic
ratios R_p and R_n obtained are not consistent with the older G_{Ep} and G_{En}
data in their range of momentum transfer. The model is further extended to
include the omega' (1419) isoscalar vector meson pole. It is found that while
this GKex cannot simultaneously fit the new R_p and the old G_{En} data, it can
fit the new R_p and R_n well simultaneously. An excellent fit to all the
remaining data is obtained when the inconsistent G_{Ep} and G_{En} is omitted.
The model predictions are shown up to momentum transfer squared, Q^2, of 8
GeV^2/c^2.Comment: 14 pages, 8 figures, using RevTeX4; email correspondence to
[email protected] ; minor typos corrected, figures added, conclusions
extende
New empirical fits to the proton electromagnetic form factors
Recent measurements of the ratio of the elastic electromagnetic form factors
of the proton, G_Ep/G_Mp, using the polarization transfer technique at
Jefferson Lab show that this ratio decreases dramatically with increasing Q^2,
in contradiction to previous measurements using the Rosenbluth separation
technique. Using this new high quality data as a constraint, we have reanalyzed
most of the world e-p elastic cross section data. In this paper, we present a
new empirical fit to the reanalyzed data for the proton elastic magnetic form
factor in the region 0 < Q^2 < 30 GeV^2. As well, we present an empirical fit
to the proton electromagnetic form factor ratio, G_Ep/G_Mp, which is valid in
the region 0.1 < Q^2 < 6 GeV^2
Extended Gari-Krumpelmann model fits to nucleon electromagnetic form factors
Nucleon electromagnetic form factor data (including recent data) is fitted
with models that respect the confinement and asymptotic freedom properties of
QCD. Gari-Krumpelmann (GK) type models, which include the major vector meson
pole contributions and at high momentum transfer conform to the predictions of
perturbative QCD, are combined with Hohler-Pietarinen (HP) models, which also
include the width of the rho meson and the addition of higher mass vector meson
exchanges, but do not evolve into the explicit form of PQCD at high momentum
transfer. Different parameterizations of the GK model's hadronic form factors,
the effect of including the width of the rho meson and the addition of the next
(in mass) isospin 1 vector meson are considered. The quality of fit and the
consistency of the parameters select three of the combined HP/GK type models.
Projections are made to the higher momentum transfers which are relevant to
electron-deuteron experiments. The projections vary little for the preferred
models, removing much of the ambiguity in electron-nucleus scattering
predictions.Comment: 18pp, 7 figures, using RevTeX with BoxedEPS macros; 1 new figure,
minor textual changes; email correspondence to [email protected]
Isobar width effects in the coupling of nucleon to isobar channels
The investigation of the effects of isobar coupling to two-nucleon channels has been extended to include additional physical features. A new code discretizes the mass distribution of the isobar widths and treats each mass as a separate channel. This allows the treatment of width in the presence of coupling by transition potentials, in addition to the previously permitted boundary coupling. It also produces the S-matrix components required to describe the many-body final-state distributions. When indicated by the one-pion-exchange coupling strength new isobar channels are included. The new results for nucleon-nucleon scattering fit the data better, starting from more realistic models. The observed 1D2 and 3F3 structures are well understood as coupled-channel effects, without exotic-quark contributions. The existence of a structure in the 3P0 channel depends on the amount of inelasticity, which differs among the phase shift solutions. The energy dependence seen in recent analyses, of the 3P2 phase shift near TL=800 MeV, is shown to be a consequence of the isobar channel coupling. Improved models obtained for the 1S0 and 3S1-3D1 channels are being developed. They are important to accurately extrapolate those phases to higher energies where six-quark effects are expected
Analytic pion form factor
The pion electromagnetic form factor and two-pion production in
electron-positron collisions are simultaneously fitted by a vector dominance
model evolving to perturbative QCD at large momentum transfer. This model was
previously successful in simultaneously fitting the nucleon electromagnetic
form factors (spacelike region) and the electromagnetic production of
nucleon-antinucleon pairs (timelike region). For this pion case dispersion
relations are used to produce the analytic connection of the spacelike and
timelike regions. The fit to all the data is good, especially for the newer
sets of time-like data. The description of high- data, in the time-like
region, requires one more meson with quantum numbers than listed in the
2014 Particle Data Group review.Comment: 10 pages, 7 figure