268 research outputs found
A Coherent Interpretation of the Form Factors of the Nucleon in Terms of a Pion Cloud and Constituent Quarks
The recent unbiased measurements of the electric form factor of the neutron
suggest that its shape may be interpreted as a smooth broad distribution with a
bump at Q^2 \approx 0.3(GeV/c)^2 superimposed. As a consequence the
corresponding charge distribution in the Breit frame shows a negative charge
extending as far out as 2fm. It is natural to identify this charge with the
pion cloud. This realisation is then used to reanalyse all old and new data of
the electric and magnetic from factors of the proton and the neutron by a
phenomenological fit and by a fit based on the constituent quark model. It is
shown that it is possible to fit all form factors coherently with both
ansaetzen and that they all show the signal of the pion cloud.Comment: 17 pages, 17 figure
Low energy analysis of the nucleon electromagnetic form factors
We analyze the electromagnetic form factors of the nucleon to fourth order in
relativistic baryon chiral perturbation theory. We employ the recently proposed
infrared regularization scheme and show that the convergence of the chiral
expansion is improved as compared to the heavy fermion approach. We also
discuss the inclusion of vector mesons and obtain an accurate description of
all four nucleon form factors for momentum transfer squared up to Q^2 \simeq
0.4 GeV^2.Comment: 44 pp, LaTeX2e, uses epsf and amsbs
Covariant nucleon electromagnetic form factors from the Goldstone-boson-exchange quark model
We present a study of proton and neutron electromagnetic form factors for the
recently proposed Goldstone-boson-exchange constituent quark model. Results for
charge radii, magnetic moments, and electric as well as magnetic form factors
are reported. The calculations are performed in a covariant framework using the
point-form approach to relativistic quantum mechanics. All the predictions by
the Goldstone-boson-exchange constituent quark model are found in remarkably
good agreement with existing experimental data.Comment: LATEX, 10 pages, including 4 ps-figures, slightly modified, one
additional referenc
Precise Neutron Magnetic Form Factors
Precise data on the neutron magnetic form factor G_{mn} have been obtained
with measurements of the ratio of cross sections of D(e,e'n) and D(e,e'p) up to
momentum transfers of Q^2 = 0.9 (GeV/c)^2. Data with typical uncertainties of
1.5% are presented. These data allow for the first time to extract a precise
value of the magnetic radius of the neutron.Comment: 10 pages, 2 figures, submitted to Physics Letters
Dispersion-theoretical analysis of the nucleon electromagnetic form factors: Inclusion of time-like data
We update a recent dispersion--theoretical fit to the nucleon electromagnetic
form factors by including the existing data in the time--like region. We show
that while the time--like data for the proton can be described consistently
with the existing world space--like data, this is not the case for the neutron.
Another measurement of the process is called for. We
furthermore sharpen the previous estimate of the separation between the
perturbative and the non--perturbative regime, which is characterized by a
scale parameter GeV.Comment: 7 pp, LaTeX, uses epsf, 2 figures in separate file, four data points
changed, slight changes in the fits, conclusions unchange
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]
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
Extraction of electromagnetic neutron form factors through inclusive and exclusive polarized electron scattering on polarized 3He target
Inclusive 3He(e,e') and exclusive 3He(e,e'n) processes with polarized
electrons and 3He have been theoretically analyzed and values for the magnetic
and electric neutron form factors have been extracted. In both cases the form
factor values agree well with the ones extracted from processes on the
deuteron. Our results are based on Faddeev solutions, modern NN forces and
partially on the incorporation of mesonic exchange currents.Comment: 28 pages, 29 Postscript figure
Logarithmic corrections and soft photon phenomenology in the multipole model of the nucleon form factors
We analyzed the presently available experimental data on nucleon
electromagnetic form factors within a multipole model based on dispersion
relations. A good fit of the data is achieved by considering the coefficients
of the multipole expansions as logarithmic functions of the momentum transfer
squared. The superconvergence relations, applied to this coefficients, makes
the model agree with unitary constraints and pQCD asymptotics for the Dirac and
Pauli form factors. The soft photon emission is proposed as a mechanism
responsible for the difference between the Rosenbluth, polarization and
beam--target--asymmetry data. It is shown, that the experimentally measured
cross sections depend not only on the Dirac and Pauli form factors, but also on
the average number of the photons emitted. For proton this number is shown to
be different for different types of experimental measurements and then
estimated phenomenologically. For neutron the same mechanism predicts, that the
data form different types of experiments must coincide with high accuracy. A
joint fit of all the experimental data reproduce the dependence with the
accuracy . Predictions of the model, that 1) the ratios of the
proton form factors are different for Rosenbluth, polarization and
beam--target--asymmetry experiments and 2) similar ratios are nearly the same
for neutron, can be used for experimental verification of the model.Comment: 14 pages in 2-column format, 4 figures, references added, typos
corrected, minor changes in the text, accepted in Eur. Phys. Journal
- …