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

The ratio of the electric and magnetic form factors of the proton G(E(p))/G(M(p)), which is an image of its charge and magnetization distributions, was measured at the Thomas Jefferson National Accelerator Facility (JLab) using the recoil polarization technique. The ratio of the form factors is directly proportional to the ratio of the transverse to longitudinal components of the polarization of the recoil proton in the elastic e(-->)p---> e(-->)p reaction. The new data presented span the range 3.5< Q(2)< 5.6 GeV(2) and are well described by a linear Q(2) fit. Also, the ratio sqrt[Q(2)] F(2(p))/F(1(p)) reaches a constant value above Q(2) = 2 GeV(2).

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We report on the measurement of the γp → J/ψp cross section from Eγ = 11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section dσ/dt has an exponential slope of 1.67 ± 0.39 GeV −2 at 10.7 GeV average energy. The LHCb pentaquark candidates P + c can be produced in the s-channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions B(P + c → J/ψp).

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.