https://www.ge.infn.it/jlab12/IT/files/0612014-C.F..pdf

Nucleon electromagnetic form factors

The ratio of the proton elastic electromagnetic form factors, GEp/GMp, was obtained by measuring Pt and P, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic epepreaction in the four-momentum transfer squared range of 0.5 to 3.5 GeV2. In the single-photon exchange approximation, GEp/GMp is directly proportional to Pt/P. The simultaneous measurement of Pt and P in a polarimeter reduces systematic uncertainties. The results for GEp/GMp show a systematic decrease with increasing Q2, indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously. (Less)

/pdf/proton-elastic-form-factor-ratios-to-q-2-3-5-gev-2-by-2d43qknpy1.pdf

Proton elastic form-factor ratios to Q**2 = 3.5-GeV**2 by polarization transfer

Proton electromagnetic form factors: Basic notions, present achievements and future perspectives

Precise measurements of the proton electromagnetic form factor ratio R = mu(p)G(E)(p)/G(M)(p) using the polarization transfer method at Jefferson Lab have revolutionized the understanding of nucleon structure by revealing the strong decrease of R with momentum transfer Q(2) for Q(2) greater than or similar to 1 GeV2, in strong disagreement with previous extractions of R from cross-section measurements. In particular, the polarization transfer results have exposed the limits of applicability of the one-photon-exchange approximation and highlighted the role of quark orbital angular momentum in the nucleon structure. The GEp-II experiment in Jefferson Lab's Hall A measured R at four Q(2) values in the range 3.5 GeV2 <= Q(2) <= 5.6 GeV2. A possible discrepancy between the originally published GEp-II results and more recent measurements at higher Q(2) motivated a new analysis of the GEp-II data. This article presents the final results of the GEp-II experiment, including details of the new analysis, an expanded description of the apparatus, and an overview of theoretical progress since the original publication. The key result of the final analysis is a systematic increase in the results for R, improving the consistency of the polarization transfer data in the high-Q(2) region. This increase is the result of an improved selection of elastic events which largely removes the systematic effect of the inelastic contamination, underestimated by the original analysis. (Less)

/pdf/final-analysis-of-proton-form-factor-ratio-data-at-q-2-4-0-4-4zu7a6gjxb.pdf

Final analysis of proton form factor ratio data at Q(2)=4.0, 4.8, and 5.6 GeV2

The recent 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 2 fm. 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.

A coherent interpretation of the form factors of the nucleon in terms of a pion cloud and constituent quarks

The proton elastic form factors ${\mathit{G}}_{\mathit{E}\mathit{p}}$(${\mathit{Q}}^{2}$) and ${\mathit{G}}_{\mathit{M}\mathit{p}}$(${\mathit{Q}}^{2}$) have been extracted for ${\mathit{Q}}^{2}$=1.75 to 8.83 (GeV/c${)}^{2}$ via a Rosenbluth separation to ep elastic cross section measurements in the angular range 13\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}\ensuremath{\theta}\ensuremath{\le}90\ifmmode^\circ\else\textdegree\fi{}. The ${\mathit{Q}}^{2}$ range covered more than doubles that of the existing data. For ${\mathit{Q}}^{2}$4 (GeV/c${)}^{2}$, where the data overlap with previous measurements, the total uncertainties have been reduced to 14% in ${\mathit{G}}_{\mathit{E}\mathit{p}}$ and 1.5% in ${\mathit{G}}_{\mathit{M}\mathit{p}}$. Results for ${\mathit{G}}_{\mathit{E}\mathit{p}}$(${\mathit{Q}}^{2}$) are consistent with the dipole fit ${\mathit{G}}_{\mathit{D}}$(${\mathit{Q}}^{2}$)=(1+${\mathit{Q}}^{2}$/0.71${)}^{\mathrm{\ensuremath{-}}2}$, while those for ${\mathit{G}}_{\mathit{M}\mathit{p}}$(${\mathit{Q}}^{2}$)/${\mathrm{\ensuremath{\mu}}}_{\mathit{p}}$${\mathit{G}}_{\mathit{D}}$(${\mathit{Q}}^{2}$) decrease smoothly from 1.05 to 0.92. Deviations from form factor scaling are observed up to 20%. The ratio ${\mathit{Q}}^{2}$${\mathit{F}}_{2}$/${\mathit{F}}_{1}$ is observed to approach a constant value for ${\mathit{Q}}^{2}$g3 (GeV/c${)}^{2}$. Comparisons are made to vector meson dominance, dimensional scaling, QCD sum rule, diquark, and constituent quark models, none of which fully characterize all the new data.

/pdf/measurements-of-the-electric-and-magnetic-form-factors-of-43d4mqoxry.pdf

Measurements of the electric and magnetic form factors of the proton from Q2=1.75 to 8.83 (GeV/c)2.

Measurements of inclusive electron-scattering cross sections using hydrogen and deuterium targets in the region of the $\ensuremath{\Delta}(1232)$ resonance are reported. A global fit to these new data and previous data in the resonance region is also reported for the proton. Transition form factors have been extracted from the proton cross sections for this experiment over the four-momentum transfer squared range $1.64l{Q}^{2}l6.75 (\mathrm{GeV}{/c)}^{2}$ and from previous data over the range $2.41l{Q}^{2}l9.82 (\mathrm{GeV}{/c)}^{2}.$ The results confirm previous reports that the $\ensuremath{\Delta}(1232)$ transition form factor decreases more rapidly with ${Q}^{2}$ than expected from perturbative QCD. The ratio of ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}$ in the $\ensuremath{\Delta}(1232)$ resonance region has been extracted from the deuteron data for this experiment in the range $1.64l{Q}^{2}l3.75 (\mathrm{G}\mathrm{e}\mathrm{V}/{c)}^{2}$ and for a previous experiment in the range $2.4l{Q}^{2}l7.9 (\mathrm{GeV}{/c)}^{2}.$ A study has been made of the model dependence of these results. This ratio ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}$ for $\ensuremath{\Delta}(1232)$ production is slightly less than unity, while ${\ensuremath{\sigma}}_{n}/{\ensuremath{\sigma}}_{p}$ for the nonresonant cross sections is approximately 0.5, which is consistent with deep inelastic scattering results.

/pdf/measurements-of-the-delta-1232-transition-form-factor-and-9i17in7o9g.pdf

Measurements of the Delta(1232) transition form factor and the ratio sigma(n)/sigma(p) from inelastic electron-proton and electron-deuteron scattering

https://digital.library.unt.edu/ark:/67531/metadc891591/m2/1/high_res_d/883166.pdf

W. R. Dodge

Papers

Measurements of the electric and magnetic form factors of the proton from Q2=1.75 to 8.83 (GeV/c)2.

Measurements of the Delta(1232) transition form factor and the ratio sigma(n)/sigma(p) from inelastic electron-proton and electron-deuteron scattering

Measurements of the nucleon form factors at large momentum transfers