[en] Measurements and systematics errors of the 4-momentum transfer squared (Q²) and backgrounds for the 2005 HAPPEX runs are described

[en] We present the first measurement of the Q²-dependence of the neutron spin structure function g₂ⁿ at five kinematic points covering 0.57 (GeV/c)² (le) Q² (le) 1.34 (GeV/c)² at x ∼ 0.2. Though the naive quark-parton model predicts g₂=0, non-zero values for g₂ occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses or orbital angular momentum. The sensitivity of g₂ to physics beyond the simple quark-parton model provides a unique opportunity to examine QCD dynamics in nucleon structure. When scattering occurs from a non-interacting quark, a prediction for g₂ⁿ can be calculated using next-to-leading order fits to world data for g₁ⁿ. Our results show a positive deviation from this prediction indicating that contributions such as quark-gluon interactions may be important. Data were also obtained for g₁ⁿ and are consistent with a next-to-leading order fit to world data evolved to our kinematic range

[en] We report on measurements of the neutron spin asymmetries A_1,2ⁿ and polarized structure functions g_1,2ⁿ at three kinematics in the deep inelastic region, with x = 0.33, 0.47 and .60 and Q₂ = 2.7, 3.5 and 4.8 (GeV/c)², respectively. These measurements were performed using a 5.7 GeV longitudinally-polarized electron beam and a polarized ³He target. The results for A₁ⁿ and g₁ⁿ at x = 0.33 are consistent with previous world data and, at the two higher x points, have improved the precision of the world data by about an order of magnitude. The new A₁ⁿ data show a zero crossing around x = 0.47 and the value at x = 0.60 is significantly positive. These results agree with a next-to-leading order QCD analysis of previous world data. The trend of data at high x agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (pQCD) assuming hadron helicity conservation. Results for A₂ⁿ and g₂ⁿ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment d₂ⁿ was evaluated and the new result has improved the precision of this quantity by about a factor of two. When combined with the world proton data, polarized quark distribution functions were extracted from the new g₁ⁿ/F₁ⁿ values based on the quark parton model. While results for Δu/u agree well with predictions from various models, results for Δd/d disagree with the leading-order pQCD prediction when hadron helicity conservation is imposed

[en] We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ⁴He at an average scattering angle θ_lab = 5.7 degrees and a four-momentum transfer Q² = 0.091 GeV². From these data, for the first time, the strange electric form factor of the nucleon G_E^s can be isolated. The measured asymmetry of A_PV = 6.72 ± 0.84 (stat) ± 0.21 (syst) parts per million yields a value of G_E^s = -0.038 ± 0.042 (stat) ± 0.010 (syst), consistent with zero

[en] We present the first measurements of (rvec e)p → epγ cross section in the deep virtual Compton scattering (DVCS) regime and the valence quark region (x_Bj = 0.36). From JLab E00-110, we extract the imaginary part of the Bethe-Heitler (BH)--DVCS interference terms, to order twist-3 for Q² = 1.5, 1.9, and 2.3 GeV², and the real part of the BH-DVCS interference terms at Q²2 = 2.3 GeV². We present the first model-independent measurement of linear combinations of generalized parton distributions (GPDs) and GPD integrals up to twist-3 approximation. The validity of this approximation is strongly supported by the absence of Q²-variation of the extracted terms--thereby constraining the size of higher twist contributions to our observables