[en] Cross sections for semi-inclusive electroproduction of charged pions π^± from both proton and deuteron targets were measured for 0.2 < x < 0.5, 2 < Q² < 4 GeV², 0.3 < z < 1, and P_t² < 0.2 GeV². We find the azimuthal dependence to be small and consistent with zero, for P_t < 0.1 GeV. In the context of a simple fit, the initial transverse momenta of d quarks tends to be larger than for $u$ quarks, while the transverse momentum width of the favored fragmentation function is slightly larger than that of the unfavored function

[en] Parity violating electron scattering has become a well established tool which has been used, for example, to probe the Standard Model and the strange-quark contribution to the nucleon. While much of this work has focused on elastic scattering, the RES-Parity experiment, which has been proposed to take place at Jefferson Laboratory, would focus on inelastic scattering in the low-Q², low-W domain. RES-Parity would search for evidence of quark-hadron duality and resonance structure with parity violation in the resonance region. In terms of parity violation, this region is essentially unexplored, but the interpretation of other high-precision electron scattering experiments will rely on a reasonable understanding of scattering at lower energy and low-W through the effects of radiative corrections. RES-Parity would also study nuclear effects with the weak current. Because of the intrinsic broad band energy spectrum of neutrino beams, neutrino experiments are necessarily dependent on an untested, implicit assumption that these effects are identical to electromagnetic nuclear effects. RES-Parity is a relatively straight forward experiment. With a large expected asymmetry (∼ 0.5 x 10^-4) these studies may be completed with in a relatively brief period

[en] We describe a new aerogel threshold Cherenkov detector installed in the HMS spectrometer in Hall C at Jefferson Lab. The Hall C experimental program in 2003 required an improved particle identification system for better identification of π/K/p, which was achieved by installing an additional threshold Cherenkov counter. Two types of aerogel with n = 1.03 and n = 1.015 allow one to reach ∼10^-3 proton and 10^-2 kaon rejection in the 1-5 GeV/c momentum range with pion detection efficiency better than 99% (97%). The detector response shows no significant position dependence due to a diffuse light collection technique. The diffusion box was equipped with 16 Photonis XP4572 PMT's. The mean number of photoelectrons in saturation was ∼16 and ∼8, respectively. Moderate particle identification is feasible near threshold

[en] The role of resonance electroproduction at high Q² is discussed in the context of exclusive reactions, as well as the alternative theoretical models which are proposed to treat exclusive reactions in the few GeV²/c² region of momentum transfer. Jefferson Lab experiment 94-014, which measured the excitation of the Delta (1232) and S₁₁(1535) via the reactions p(e,e^'p)pi⁰ and p(e,e^'p)eta respectively at Q² ∼ 2.8 and 4 GeV²/c² is described, and the state of analysis reported

[en] Jefferson Lab experiment E94-014 measured the excitation of the Delta (1232) resonance via the reaction p(e,e(prime)p)π⁰ at Q² near 2.8 and 4 Gev². This is the highest Q² for which exclusive resonance electroproduction has ever been observed. Decay distributions of the Delta (1232) resonance into the ppi⁰ final state were measured over a wide range of barycentric decay angles and energies. The goal of this experiment is to assess the transition in Q² from the constituent quark model (CQM) to the regime where hard processes become important. At Q² ∼ few GeV²/c² the ratio E₁₊/M₁₊ depends dramatically on the theoretical description, varying from a few 10^-2 in the CQM limit, to about 1 in the pQCD limit. Preliminary analysis of the data shows that the ratio E₁₊/M₁₊ remains small at Q² up to 4 Gev²/c². After first pass analysis we obtain E₁₊/M₁₊ = -4.1 ± 1.2 at Q² = 2.8 Gev²/c² and E₁₊/M₁₊ = -7.9 ± 0.8 at Q² = 4 Gev²/c² (statistical errors only)

[en] The Qweak experiment at Jefferson Lab aims to make a 4% measurement of the parity-violating asymmetry in elastic scattering at very low Q² of a longitudinally polarized electron beam on a proton target. The experiment will measure the weak charge of the proton, and thus the weak mixing angle at low energy scale, providing a precision test of the Standard Model. Since the value of the weak mixing angle is approximately 1/4, the weak charge of the proton Q_w^p = 1-4 sin² θ_w is suppressed in the Standard Model, making it especially sensitive to the value of the mixing angle and also to possible new physics. The experiment is approved to run at JLab, and the construction plan calls for the hardware to be ready to install in Hall C in 2007. The theoretical context of the experiment and the status of its design are discussed

[en] The data analysis for the reaction H(e,e(prime) pi⁺)n, which was used to determine values for the charged pion form factor Fpi for values of Q2 = 0.6-1.6 (gEv/C)², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract Fpi from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of Fpi are presented

[en] Tensor polarization observables (t20, t21 and t22) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)². The experiment was performed at the Jefferson Laboratory in Hall C using the electron HMS Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q² the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with pQCD predictions

[en] Separated longitudinal and transverse cross sections for charged pion electroproduction from ¹H, ²H, and ³He were measured at Q² = 0.4 (GeV/c)² for two values of the invariant mass, (bar W) = 1.15 GeV and (bar W) = 1.60 GeV, in a search for a mass dependence which would signal the effect of nuclear pions. This is the first such study that includes recoil momenta significantly above the Fermi surface. The longitudinal cross section, if dominated by the pion-pole process, should be sensitive to nuclear pion currents. Comparisons of the longitudinal cross section target ratios to a quasifree calculation reveal a significant suppression in ^3>He at (bar W) = 1.60 GeV. The (bar W) = 1.15 GeV results are consistent with simple estimates of the effect of nuclear pion currents, but are also consistent with pure quasifree production

[en] We report the first measurement using a solid polarized target of the neutron electric form factor Gⁿ_E via d-vector(e-vector, e'n)p. Gⁿ_E was determined from the beam-target asymmetry in the scattering of longitudinally polarized electrons from polarized deuterated ammonia (¹⁵ND₃). The measurement was performed in Hall C at Thomas Jefferson National Accelerator Facility in quasifree kinematics with the target polarization perpendicular to the momentum transfer. The electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle segmented detector. We find Gⁿ_E = 0.04632±0.00616(stat)±0.00341(syst) at Q² = 0.495 (GeV/c)²