The instrumentation in Hall A at the Thomas Jefferson National Accelerator Facility was designed to study electro-and photo-induced reactions at very high luminosity and good momentum and angular resolution for at least one of the reaction products. The central components of Hall A are two identical high resolution spectrometers, which allow the vertical drift chambers in the focal plane to provide a momentum resolution of better than 2 x 10(-4). A variety of Cherenkov counters, scintillators and lead-glass calorimeters provide excellent particle identification. The facility has been operated successfully at a luminosity well in excess of 10(38) CM-2 s(-1). The research program is aimed at a variety of subjects, including nucleon structure functions, nucleon form factors and properties of the nuclear medium. (C) 2003 Elsevier B.V. All rights reserved

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We present measurements of the ratio of the proton elastic electromagnetic form factors, p G Ep /G M p . The Jefferson Lab Hall A Focal Plane Polarimeter was used to determine the longitudinal and transverse components of the recoil proton polarization in ep elastic scattering; the ratio of these polarization components is proportional to the ratio of the two form factors. These data reproduce the observation of Jones et al. ͓Phys. Rev. Lett. 84, 1398 ͑2000͔͒, that the form factor ratio decreases significantly from unity above Q 2 ϭ1 GeV 2 .

We investigated simultaneously the 12C(e,e'p) and 12C(e,e'pp) reactions at Q2=2 (GeV/c)2, xB=1.2, and in an (e, e'p) missing-momentum range from 300 to 600 MeV/c. At these kinematics, with a missing momentum greater than the Fermi momentum of nucleons in a nucleus and far from the delta excitation, short-range nucleon-nucleon correlations are predicted to dominate the reaction. For (9.5+/-2)% of the 12C(e,e'p) events, a recoiling partner proton was observed back-to-back to the 12C(e,e'p) missing-momentum vector, an experimental signature of correlations.

The ratio of the proton elastic electromagnetic form factors, GEp/GMp, was obtained by measuring Pt and P ℓ , the transverse and longitudinal recoil proton polarization components, respectively, for the elastic ep → e p reaction in the four-momentum transfer squared range of 0.5 to 3.5 GeV 2 . In the single-photon exchange approximation, the ratio GEp/GMp is directly proportional to the ratio Pt/P ℓ . The simultaneous measurement of Pt and P ℓ in a polarimeter reduces systematic uncertainties. The results for the ratio GEp/GMp show a systematic decrease with increasing Q 2 , indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been re-analyzed and systematic uncertainties have become significantly smaller than previously published results.