241 research outputs found
The couplings derived from QCD sum rules
The light cone QCD sum rules are derived for vector and tensor
couplings simultaneously. The vacuum gluon field contribution is taken into
account. Our results are .Comment: To appear in Phys. Rev. C (Brief Report
Dispersion-theoretical analysis of the nucleon electromagnetic formfactors
Dispersion relations allow for a coherent description of the nucleon electromagnetic form factors measured over a large range of momentum transfer, Q^2 \simeq 0 \ldots 35 GeV^2. Including constraints from unitarity and perturbative QCD, we present a novel parametrisation of the absorptive parts of the various isoscalar and isovector nucleon form factors. Using the current world data, we obtain results for the electromagnetic form factors, nucleon radii and meson couplings. We stress the importance of measurements at large momentum transfer to test the predictions of perturbative QCD
Updated dispersion-theoretical analysis of the nucleon electromagnetic form factors
In the light of the new data on the various neutron and proton
electromagnetic form factors taken in recent years, we update the
dispersion-theoretical analysis of the nucleon electromagnetic form factors
from the mid-nineties. The parametrization of the spectral functions includes
constraints from unitarity, perturbative QCD, and recent measurements of the
neutron charge radius. We obtain a good description of most modern form factor
data, with the exception of the Jefferson Lab data on G_E^p/G_M^p in the
four-momentum transfer range Q^2=3...6 GeV^2. For the magnetic radii of the
proton and the neutron we find r_M^p = 0.857 fm and r_M^n = 0.879 fm, which is
consistent with the recent determinations using continued fraction expansions.Comment: 5 pages, 3 ps figures, final version, exp. errors in Figs. 1 and 3
correcte
Fluctuating semiflexible polymer ribbon constrained to a ring
Twist stiffness and an asymmetric bending stiffness of a polymer or a polymer
bundle is captured by the elastic ribbon model. We investigate the effects a
ring geometry induces to a thermally fluctuating ribbon, finding bend-bend
coupling in addition to twist-bend coupling. Furthermore, due to the geometric
constraint the polymer's effective bending stiffness increases. A new parameter
for experimental investigations of polymer bundles is proposed: the mean square
diameter of a ribbonlike ring, which is determined analytically in the
semiflexible limit. Monte Carlo simulations are performed which affirm the
model's prediction up to high flexibility.Comment: 6 pages, 3 figures, Version as published in Eur. Phys. J.
Proton charge and magnetic rms radii from the elastic scattering data
The elastic electron-proton scattering data are analysed in order to
determine proton charge and magnetic rms radii, r_E and r_M. Along with the
usual statistical error, we try to estimate a systematic error in the radii,
caused by the inadequacy of particular form factor parameterization employed.
The range of data to use in the analysis is chosen so as to minimize the total
(statistical + systematic) error. We obtain r_E = 0.912 +- 0.009 (stat) +-
0.007 (syst) fm, and r_M = 0.876 +- 0.010 (stat) +- 0.016 (syst) fm. The
cross-section data were corrected for two-photon exchange. We found that
without such corrections obtained r_E and r_M are somewhat smaller while the
quality of fit is worse.Comment: 6 pages, 4 figures. Numbers slightly changed due to discovered error
in minimization program. Sec.III revised, discussion of G_E behaviour added
The Strangeness Radius and Magnetic Moment of the Nucleon Revisited
We update Jaffe's estimate of the strange isoscalar radius and magnetic
moment of the nucleon. We make use of a recent dispersion--theoretical fit to
the nucleon electromagnetic form factors and an improved description of
symmetry breaking in the vector nonet. We find ~n.m.
and ~fm. The strange formfactor follows
a dipole with a cut--off mass of 1.46~GeV, . These numbers should be considered as upper limits on the
strange vector current matrix--elements in the nucleon.Comment: 8 pp, LaTeX, uses epsf, 1 figure in separate fil
reaction near threshold
We analyze the total cross section data for near threshold
measured recently at SATURNE. Using an effective range approximation for the
on-shell S-wave final state interaction we extract from these data the
modulus fm of the threshold transition amplitude
. We present a calculation of various (tree-level) meson exchange
diagrams contributing to . It is essential that -emission from
the anomalous -vertex interferes destructively with
-emission from the proton lines. The contribution of scalar
-meson exchange to turns out to be negligibly small. Without
introducing off-shell meson-nucleon form factors the experimental value
fm can be reproduced with an -coupling constant
of . The results of the present approach agree qualitatively
with the J\"ulich model. We also perform a combined analysis of the reactions
and near threshold.Comment: Latex-file 6 pages, 2 Figure
The Role of 5-quark Components on the Nucleon Form Factors
The covariant quark model is shown to allow a phenomenological description of
the neutron electric form factor, G_E^n(Q^2), in the impulse approximation,
provided that the wave function contains minor (~ 3 %) admixtures of the lowest
sea-quark configurations. While that form factor is not very sensitive to
whether the \bar q in the qqqq\bar q component is in the P-state or in the
S-state, the calculated nucleon magnetic form factors are much closer to the
empirical values in the case of the former configuration. In the case of the
electric form factor of the proton, G_E^p(Q^2), a zero appears in the impulse
approximation close to 9 GeV^2, when the \bar q is in the P-state. That
configuration, which may be interpreted as a pion loop ("cloud") fluctuation,
also leads to a clearly better description of the nucleon magnetic moments.
When the amplitude of the sea-quark admixtures are set so as to describe the
electric form factor of the neutron, the qqqq\bar q admixtures have the
phenomenologically desirable feature, that the electric form factor of the
proton falls at a more rapid rate with momentum transfer than the magnetic form
factor.Comment: To appear in Nuclear Physics
On the rms-radius of the proton
We study the world data on elastic electron-proton scattering in order to
determine the proton charge rms-radius. After accounting for the Coulomb
distortion and using a parameterization that allows to deal properly with the
higher moments we find a radius of 0.895+-0.018 fm, which is significantly
larger than the radii used in the past.Comment: 9 pages, 2 figures, submitted to Phys.Lett.
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
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