[en] The set of proton elastic form factor ratio data was extended to Q² = 8.5 GeV² with a recent experiment at JLab, GEp(III); the nucleon form factor data base is discussed and several conclusions presented. These data are the result of measurements of the polarization of the recoil proton; they have been known to differ significantly from Rosenbluth separation of cross section data.

[en] A remarkable change of paradigm was precipitated by the results, obtained in the last 12 years, of new measurements of the proton form factor ratio; the two form factors, G_Ep and G_Mp, are not in a constant ratio, as had been concluded from previous cross section experiments, and would be the case if charge and magnetization spatial distributions were the same. Rather, as the series of polarization measurements at JLab shows, the ratio μ_pG_Ep/G_Mp decreases smoothly from 1 at Q² = 0, to about 0.15 at Q² = 8.5 GeV², with Q² the negative of the four-momentum transfer squared. The interesting question is then: how can the results using two methods both related through the Born approximation, be found to lead to a different form factor ratio? The short answer is that cross sections require large radiative corrections, which tend to mask G_Ep for increasing Q², whereas recoil polarization experiments measure the ratio of two components of the recoil polarization, which tends to cancel the effect of radiative corrections. Radiative corrections to the cross section of ep scattering have a long history. They may just not be accurate enough when the ratio G_Ep/G_Mp ∼ 0.05 and τ = Q²/4m_p² ∼ 2.5, as is the case for the largest Q² for which we have double-polarization data, 8.5 GeV². For this Q² and with the Born cross section given by dσ ∼ G_Ep² + (τ/(var_epsilon))G_Mp², with (var_epsilon) the kinematic factor, the contribution to the cross section of G_Ep becomes smaller than 0.1%, i.e. non-measurable in cross section experiments. The other hypothesis, is that the radiative corrections are incomplete, and that the exchange of two hard photons is the source of the discrepancy; the idea has been pursued in numerous works, but to this day there is no direct, experimental evidence that two-hard-photons exchange is the major source of the discrepancy.

[en] The electromagnetic form factors of the nucleon describe the charge and current distribution inside the nucleon. Jefferson Lab experiment 93-027 in Hall A measured P_l and P_t, the longitudinal and transverse recoil proton polarization, respectively, in elastic ep scattering. The ratio P_t/P_l is proportional to the form factor ratio G_Ep/G_Mp; the data show for the first time that the Q² dependence of G_Ep and G_Mp is significantly different

[en] The ratio, μ_pG_Ep/G_Mp, where μ_p is the proton magnetic moment, has been measured extensively over the last decade at the Jefferson Laboratory, using the polarization transfer method. This ratio is extracted directly from the measured ratio of transverse to longitudinal polarizations components of the recoiling proton in elastic electron-proton scattering. The polarization transfer results are of unprecedented high precision and accuracy, due in large part to the small systematic uncertainties associated with the experimental technique. Prior to these measurements, the form factors were empirically observed to exhibit dipole forms, such that μ_pG_Ep/G_Mp ∼ 1 over all regions of momentum transfer studied. With the Hall A results confirming that the ratio μ_pG_Ep/G_Mp shows a steady decrease below unity as a function of Q², beginning around Q² ∼ 1 GeV², discussions revolving around the implication of this deviation from dipole behavior for the structure of the proton have been accompanied by renewed experimental interest in these elastic form factors. Starting in the fall of 2007, two new experiments, GEp-III and GEp-2γ in Hall C at JLab, measured the form factor ratio, G_Ep/G_Mp; the GEp-III experiment pushed the highest Q² limit from 5.6 to 8.49 GeV², with intermediate points at 5.2 and 6.8 GeV², and the GEp-2γ experiment measured the ratio in three different kinematics at the constant value Q² = 2.5 GeV², by changing beam energy and detector angles. Preliminary results from both experiments are reported.

[en] The charge and magnetization distributions of the proton and neutron are encoded in their elastic electromagnetic form factors, which can be measured in elastic electron--nucleon scattering. By measuring the form factors, we probe the spatial distribution of the proton charge and magnetization, providing the most direct connection to the spatial distribution of quarks inside the proton. For decades, the form factors were probed through measurements of unpolarized elastic electron scattering, but by the 1980s, progress slowed dramatically due to the intrinsic limitations of the unpolarized measurements. Early measurements at several laboratories demonstrated the feasibility and power of measurements using polarization degrees of freedom to probe the spatial structure of the nucleon. A program of polarization measurements at Jefferson Lab led to a renaissance in the field of study, and significant new insight into the structure of matter.

[en] The ratio of the proton form factors, G_Ep/G_Mp, has been measured from Q² of 0.5 GeV² to 8.5 GeV², at the Jefferson Laboratory, using the polarization transfer method. This ratio is extracted directly from the measured ratio of the transverse and longitudinal polarization components of the recoiling proton in elastic electron-proton scattering. The discovery that the proton form factor ratio measured in these experiments decreases approximately linearly with four-momentum transfer, Q², for values above ~1 GeV², is one of the most significant results to come out of JLab. These results have had a large impact on progress in hadronic physics; and have required a significant rethinking of nucleon structure. The increasingly common use of the double-polarization technique to measure the nucleon form factors, in the last 15 years, has resulted in a dramatic improvement of the quality of all four nucleon electromagnetic form factors, G_Ep, G_Mp, G_En and G_Mn. There is an approved experiment at JLab, GEP(V), to continue the ratio measurements to 12 GeV². A dedicated experimental setup, the Super Bigbite Spectrometer (SBS), will be built for this purpose. It will be equipped with a focal plane polarimeter to measure the polarization of the recoil protons. The scattered electrons will be detected in an electromagnetic calorimeter. In this presentation, I will review the status of the proton elastic electromagnetic form factors and discuss a number of theoretical approaches to describe nucleon form factors

[en] The charge and magnetization distributions of the proton and neutron are encoded in their elastic electromagnetic form factors, which can be measured in elastic electron-nucleon scattering. By measuring the form factors, we probe the spatial distribution of the proton charge and magnetization, providing the most direct connection to the spatial distribution of quarks inside the proton. For decades, the form factors were probed through measurements of unpolarized elastic electron scattering, but by the 1980s, progress slowed dramatically due to the intrinsic limitations of the unpolarized measurements. Early measurements at several laboratories demonstrated the feasibility and power of measurements using polarization degrees of freedom to probe the spatial structure of the nucleon. A program of polarization measurements at Jefferson Lab led to a renaissance in the field of study, and significant new insight into the structure of matter.

[en] The protons and neutrons in a nucleus can form strongly correlated nucleon pairs. Scattering experiments, in which a proton is knocked out of the nucleus with high-momentum transfer and high missing momentum, show that in carbon-12 the neutron-proton pairs are nearly 20 times as prevalent as proton-proton pairs and, by inference, neutron-neutron pairs. This difference between the types of pairs is due to the nature of the strong force and has implications for understanding cold dense nuclear systems such as neutron stars.

[en] An experimental study of the 16O(e, e'K+)16N_#Lambda# reaction has been performed at Jefferson Lab. A thin film of falling water was used as a target. This permitted a simultaneous measurement of the p(e, e'K+)Λ,Σ₀ exclusive reactions and a precise calibration of the energy scale. A ground-state binding energy of 13.76 ± 0.16 MeV was obtained for 16N_#Lambda# with better precision than previous measurements on the mirror hypernucleus 16O_#Lambda#. Precise energies have been determined for peaks arising from a Lambda in s and p orbits coupled to the p1/2 and p3/2 hole states of the 15N core nucleus.