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
We use distorted wave electron scattering calculations to extract the weak charge form factor F W (q), the weak charge radius R W , and the point neutron radius R n of 208 Pb from the Lead Radius Experiment (PREX) parity-violating asymmetry measurement. The form factor is the Fourier transform of the weak charge density at the average momentum transferq = 0.475 fm −1 . We find F W (q) = 0.204 ± 0.028 (exp) ± 0.001 (model). We use the Helm model to infer the weak radius from F W (q). We find R W = 5.826 ± 0.181 (exp) ± 0.027 (model) fm. Here the experimental error includes PREX statistical and systematic errors, while the model error describes the uncertainty in R W from uncertainties in the surface thickness σ of the weak charge density. The weak radius is larger than the charge radius, implying a "weak charge skin" where the surface region is relatively enriched in weak charges compared to (electromagnetic) charges. We extract the point neutron radius R n = 5.751 ± 0.175 (exp) ± 0.026 (model) ± 0.005 (strange) fm from R W . Here there is only a very small error (strange) from possible strange quark contributions. We find R n to be slightly smaller than R W because of the nucleon's size. Finally, we find a neutron skin thickness of R n − R p = 0.302 ± 0.175 (exp) ± 0.026 (model) ± 0.005 (strange) fm, where R p is the point proton radius.
The neutron longitudinal and transverse asymmetries A n 1 and A n 2 have been extracted from deep inelastic scattering of polarized electrons by a polarized 3 He target at incident energies of 19.42, 22.66 and 25.51 GeV. The measurement allows for the determination of the neutron spin structure functions g n 1 (x; Q 2 ) and g n 2 (x; Q 2 ) over the range 0:03 < x < 0:6 at an average Q 2 of 2 (GeV=c) 2 . The data are used for the evaluation of the Ellis-Ja e and Bjorken sum rules. The neutron spin structure function g n 1 (x; Q 2 ) is small and negative within the range of our measurement, yielding an integral R 0:6 0:03 g n 1 (x)dx = 0:028 0:006 (stat) 0:006 (syst). Assuming Regge behavior at low x, we extract n 1 =
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