Low Q^2 measurements of the proton form factor ratio $mu_p G_E / G_M$
We present an updated extraction of the proton electromagnetic form factor ratio, {mu}{sub p}G{sub E}/G{sub M}, at low Q{sup 2}. The form factors are sensitive to the spatial distribution of the proton, and precise measurements can be used to constrain models of the proton. An improved selection of the elastic events and reduced background contributions yielded a small systematic reduction in the ratio {mu}{sub p}G{sub E}/G{sub M} compared to the original analysis.
The structure of the nucleon: Elastic electromagnetic form factors
Precise proton and neutron form factor measurements at Jefferson Lab, using spin observables, have recently made a significant contribution to the unraveling of the internal structure of the nucleon. Accurate experimental measurements of the nucleon form factors are a test-bed for understanding how the nucleon’s static properties and dynamical behavior emerge from QCD, the theory of the strong interactions between quarks. There has been enormous theoretical progress, since the publication of the Jefferson Lab proton form factor ratio data, aiming at reevaluating the picture of the nucleon. We will review the experimental and theoretical developments in this field and discuss the outlook for the future.
Complete experiment in e∓N-elastic scattering, in presence of two-photon exchange
Abstract Using a general parametrization of the spin structure of the matrix element for elastic eN-scattering, in terms of three independent complex amplitudes, we found the expressions for all polarization observables, in electron–proton elastic scattering. This allows to suggest possible methods to measure the nucleon electromagnetic form factors, in presence of two-photon exchange. We show that the measurements of the differential cross section for electron and positron nucleon elastic scattering, in the same kinematical conditions, allows to extract the nucleon electromagnetic form factors. The same is correct for the polarization method, with the measurement of the Px,z components of the final nucleon polarization (for the scattering of longitudinally polarized electrons and positrons). An alternative way, in absence of positron beam, is to measure a definite set of T-odd polarization observables, which includes three different experiments or only T-even observables, with five independent experiments. In both cases, the ratio GE(Q2)/GM(Q2) is related to quantities of the order of α and requires different polarization experiments with very high accuracy.