42,634 research outputs found
Nucleon electromagnetic form factors
Elastic electromagnetic nucleon form factors have long provided vital
information about the structure and composition of these most basic elements of
nuclear physics. The form factors are a measurable and physical manifestation
of the nature of the nucleons' constituents and the dynamics that binds them
together. Accurate form factor data obtained in recent years using modern
experimental facilities has spurred a significant reevaluation of the nucleon
and pictures of its structure; e.g., the role of quark orbital angular
momentum, the scale at which perturbative QCD effects should become evident,
the strangeness content, and meson-cloud effects. We provide a succinct survey
of the experimental studies and theoretical interpretation of nucleon
electromagnetic form factors.Comment: Topical review invited by Journal of Physics G: Nuclear and Particle
Physics; 34 pages (contents listed on page 34), 11 figure
Nucleon Electromagnetic Form Factors
A review of data on the nucleon electromagnetic form factors in the
space-like region is presented. Recent results from experiments using polarized
beams and polarized targets or nucleon recoil polarimeters have yielded a
significant improvement on the precision of the data obtained with the
traditional Rosenbluth separation. Future plans for extended measurements are
outlined.Comment: 12 pages, 6 figures, Talk presented at the Bates25 Symposiu
K* nucleon hyperon form factors and nucleon strangeness
A crucial input for recent meson hyperon cloud model estimates of the nucleon
matrix element of the strangeness current are the nucleon-hyperon-K* (NYK*)
form factors which regularize some of the arising loops. Prompted by new and
forthcoming information on these form factors from hyperon-nucleon potential
models, we analyze the dependence of the loop model results for the
strange-quark observables on the NYK* form factors and couplings. We find, in
particular, that the now generally favored soft N-Lambda-K* form factors can
reduce the magnitude of the K* contributions in such models by more than an
order of magnitude, compared to previous results with hard form factors. We
also discuss some general implications of our results for hadronic loop models.Comment: 9 pages, 8 figures, new co-author, discussion extended to the
momentum dependence of the strange vector form factor
On Nucleon Electromagnetic Form Factors
A Poincare' covariant Faddeev equation, which describes baryons as composites
of confined-quarks and -nonpointlike-diquarks, is solved to obtain masses and
Faddeev amplitudes for the nucleon and Delta. The amplitudes are a component of
a nucleon-photon vertex that automatically fulfills the Ward-Takahashi identity
for on-shell nucleons. These elements are sufficient for the calculation of a
quark core contribution to the nucleons' electromagnetic form factors. An
accurate description of the static properties is not possible with the core
alone but the error is uniformly reduced by the incorporation of meson-loop
contributions. Such contributions to form factors are noticeable for Q^2 < ~2
GeV^2 but vanish with increasing momentum transfer. Hence, larger Q^2
experiments probe the quark core. The calculated behaviour of
G_E^p(Q^2)/G_M^p(Q^2) on Q^2 \in [2,6] GeV^2 agrees with that inferred from
polarisation transfer data. Moreover, \sqrt{Q^2} F_2(Q^2)/F_1(Q^2) is
approximately constant on this domain. These outcomes result from correlations
in the proton's amplitude.Comment: 31 pages, 7 figures, 5 table
Strange chiral nucleon form factors
We investigate the strange electric and magnetic form factors of the nucleon
in the framework of heavy baryon chiral perturbation theory to third order in
the chiral expansion. All counterterms can be fixed from data. In particular,
the two unknown singlet couplings can be deduced from the parity-violating
electron scattering experiments performed by the SAMPLE and the HAPPEX
collaborations. Within the given uncertainties, our analysis leads to a small
and positive electric strangeness radius, .
We also deduce the consequences for the upcoming MAMI A4 experiment.Comment: 7 pp, REVTeX, uses epsf, minor correction
The Nucleon Elastic Form Factors
The nucleon elastic form factors are of fundamental interest, provide a
unique testing ground for QCD motivated models of nucleon structure and are of
critical importance to our understanding of the electromagnetic properties of
nuclei. Even after an experimental effort spanning nearly 50 years the nucleon
form factors are still the subject of active investigation. Advances in
polarized beams, polarized targets and recoil polarimetry have been exploited
over the last decade to produce an important and precise set of data. I review
the status of the experimental efforts to measure the nucleon elastic form
factors.Comment: Contribution to the proceedings of the Baryons04 conference. To be
published in the journal Nuclear Physics
Survey of nucleon electromagnetic form factors
A dressed-quark core contribution to nucleon electromagnetic form factors is
calculated. It is defined by the solution of a Poincare' covariant Faddeev
equation in which dressed-quarks provide the elementary degree of freedom and
correlations between them are expressed via diquarks. The nucleon-photon vertex
involves a single parameter; i.e., a diquark charge radius. It is argued to be
commensurate with the pion's charge radius. A comprehensive analysis and
explanation of the form factors is built upon this foundation. A particular
feature of the study is a separation of form factor contributions into those
from different diagram types and correlation sectors, and subsequently a
flavour separation for each of these. Amongst the extensive body of results
that one could highlight are: r_1^{n,u}>r_1^{n,d}, owing to the presence of
axial-vector quark-quark correlations; and for both the neutron and proton the
ratio of Sachs electric and magnetic form factors possesses a zero.Comment: 43 pages, 17 figures, 12 tables, 5 appendice
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