Search Results (30 total)

We have examined the spin structure of the proton in the region of the nucleon resonances (1.085  GeV<W<1.910  GeV) at an average four momentum transfer of Q²=1.3  GeV². Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries A_∥ and A_⊥ to high precision, and extracted the asymmetries A₁ and A₂, and the spin structure functions g₁ and g₂. We found a notably nonzero A_⊥, significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.

The ratio of the proton's electric to magnetic form factor, G_E/G_M, can be extracted in elastic electron-proton scattering by measuring cross sections, beam-target asymmetry, or recoil polarization. Separate determinations of G_E/G_M by cross sections and recoil polarization observables disagree for Q²>1 (GeV/c)². Measurement by a third technique might uncover an unknown systematic error in either of the previous measurements. The beam-target asymmetry has been measured for elastic electron-proton scattering at Q² = 1.51 (GeV/c)² for target spin orientation aligned perpendicular to the beam momentum direction. This is the largest Q² at which G_E/G_M has been determined by a beam-target asymmetry experiment. The result, μG_E/G_M=0.884±0.027±0.029, is compared to previous world data.

Precise measurements of the deuteron vector analyzing power A_y^d and the tensor analyzing power A_yy of the ¹H (d→,γ)³He capture reaction have been performed at deuteron energies of 29 MeV and 45 MeV. The data have been compared to theoretical state-of-the-art calculations available today. Due to the large sensitivity of polarization observables and the precision of the data light could be shed on small effects present in the dynamics of the reaction.

We studied the reaction ¹²C(e,e^'p) in quasielastic kinematics at momentum transfers between 0.6 and 1.8 (GeV/c)² covering the single-particle region. From this the nuclear transparency factors are extracted using two methods. The results are compared to theoretical predictions obtained using a generalization of Glauber theory described in this paper. Furthermore, the momentum distribution in the region of the 1s-state up to momenta of 300 MeV/c is obtained from the data and compared to the correlated basis function theory and the independent-particle shell model.

We have carried out an (e,e^′p) experiment at high momentum transfer and in parallel kinematics to measure the strength of the nuclear spectral function S(k,E) at high nucleon momenta k and large removal energies E. This strength is related to the presence of short-range and tensor correlations, and was known hitherto only indirectly and with considerable uncertainty from the lack of strength in the independent-particle region. This experiment locates by direct measurement the correlated strength predicted by theory.

The electric form factor of the neutron was determined from measurements of the d→(e→,e^′n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (¹⁵ND₃) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G_Eⁿ=0.0526±0.0033(stat)±0.0026(sys) and 0.0454±0.0054±0.0037 at Q²=0.5 and 1.0  (GeV/c)², respectively.

The ³He and ⁴He longitudinal and transverse response functions are determined from an analysis of the world data on quasielastic inclusive electron scattering. The corresponding Euclidean response functions are derived and compared to those calculated with Green’s function Monte Carlo methods, using realistic interactions and currents. Large contributions associated with two-body currents are found, particularly in the ⁴He transverse response, in agreement with data. The contributions of the two-body charge and current operators in the ³He, ⁴He, and ⁶Li response functions are also studied via sum-rule techniques. A semiquantitative explanation for the observed systematics in the excess of transverse quasielastic strength, as function of mass number and momentum transfer, is provided. Finally, a number of model studies with simplified interactions, currents, and wave functions are carried out to elucidate the role played, in the full calculation, by tensor interactions and correlations.

We report the first measurement using a solid polarized target of the neutron electric form factor G_Eⁿ via d→(e→,e^′n)p. G_Eⁿ 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 G_Eⁿ  =  0.04632±0.00616(stat)±0.00341(syst) at Q²  =  0.495 (GeV/c)².

Inclusive electron scattering data are presented for ²H, C, Fe, and Au targets at an incident electron energy of 4.045 GeV for a range of momentum transfers from Q²=1 to 7 (GeV/c)². Data were taken at Jefferson Laboratory for low values of energy loss, corresponding to values of Bjorken x≳1. The structure functions do not show scaling in x in this range, where inelastic scattering is not expected to dominate the cross section. The data do show scaling, however, in the Nachtmann variable ξ. This scaling appears to be the result of Bloom- Gilman duality in the nucleon structure function combined with the Fermi motion of the nucleons in the nucleus. The resulting extension of scaling to larger values of ξ opens up the possibility of accessing nuclear structure functions in the high-x region at lower values of Q² than previously believed.

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 High Momentum 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 perturbative QCD predictions.

We have measured the neutron electric form factor G_en via ³He→(e→,e^′n)pp at Q²  =  0.67 (GeV/c)² using the 3-spectrometer facility of the A1 Collaboration at the Mainz Microtron and a dedicated neutron detector. High pressure polarized ³He→ gas was used as a target of polarized neutrons. G_en is determined from the ratio of the asymmetries A_⊥/A_∥ measured in quasifree kinematics with the target spin perpendicular and parallel to the momentum transfer. We find G_en  =  0.052±0.011±0.005.

Inclusive electron scattering is measured with 4.045 GeV incident beam energy from C, Fe, and Au targets. The measured energy transfers and angles correspond to a kinematic range for Bjorken x>1 and momentum transfers from Q²  =  1–7 (GeV/c)². When analyzed in terms of the y-scaling function the data show for the first time an approach to scaling for values of the initial nucleon momenta significantly greater than the nuclear matter Fermi momentum (i.e., >0.3 GeV/c).

The A(Q²) structure function in elastic electron-deuteron scattering was measured at six momentum transfers Q² between 0.66 and 1.80 (GeV/c)² in Hall C at Jefferson Laboratory. The scattered electrons and recoil deuterons were detected in coincidence, at a fixed deuteron angle of 60.5°. These new precise measurements resolve discrepancies between older sets of data. They put significant constraints on existing models of the deuteron electromagnetic structure, and on the strength of isoscalar meson exchange currents.

A Comment on the Letter by E. E. W. Bruins, et al., Phys. Rev. Lett. 75, 21 (1995). The authors of the Letter offer a Reply.

We report measurements of the proton form factors G_E^p and G_M^p extracted from elastic scattering in the range 1≤Q²≤3 (GeV/c)² with total uncertainties < 15% in G_E^p and < 3% in G_M^p. Comparisons are made to theoretical models, including those based on perturbative QCD, vector-meson dominance, QCD sum rules, and diquark constituents in the proton. The results for G_E^p are somewhat larger than indicated by most theoretical parametrizations, and the ratios of the Pauli and Dirac form factors Q²(F₂^p / F₁^p) are lower in value and demonstrate a weaker Q² dependence than those predictions. A global extraction of the elastic form factors from several experiments in the range 0.1 0.1<Q²<10 (GeV/c)² is also presented.

We report results on a precision measurement of the ratio R=σ_L / σ_T in deep inelastic electron-nucleon scattering in the kinematic range 0.2≤x≤0.5 and 1≤Q²≤10 (GeV/c)². Our results show, for the first time, a clear falloff of R with increasing Q². Our R results are in agreement with QCD predictions only when corrections for target mass effects and some additional higher twist effects are included. At small x, the data on R favor structure functions with a large gluon contribution. We also report results on the differences R_A-R_D and the cross section ratio σ^A / σ^D between Fe and Au nuclei and the deuteron. Our results for R_A-R_D are consistent with zero for all x, Q² indicating that possible contributions to R from nuclear higher twist effects and spin-0 constituents in nuclei are not different from those in nucleons. The ratios σ^A / σ^D from all recent experiments, at all x, Q² values, are now in agreement.

The tensor polarization of the recoil deuteron in elastic electron-deuteron scattering has been measured at the Bates Linear Accelerator Center at three values of four-momentum transfer Q=3.78, 4.22, and 4.62 fm^-1, corresponding to incident electron energies of 653, 755, and 853 MeV. The scattered electrons and the recoil deuterons were detected in coincidence. The recoil deuterons were transported to a liquid hydrogen target to undergo a second scattering. The angular distribution of the d→-p scattering was measured using a polarimeter. The polarimeter was calibrated in an auxiliary experiment using a polarized deuteron beam at the Laboratoire National Saturne. A Monte Carlo procedure was used to generate interpolated calibration data because the energy spread in the deuteron energies in the Bates experiment spanned the range of deuteron energies in the calibration experiment. The extracted values of t₂₀ are compared to predictions of different theoretical models of the electromagnetic form factors of the deuteron: nonrelativistic and relativistic nucleon-meson dynamics, Skyrme model, quark models, and perturbative quantum chromodynamics. Along with the world data the structure functions A(Q) and B(Q) are used to separate the charge monopole and charge quadrupole form factors of the deuteron. A node in the charge monopole form factor is observed at Q=4.39±0.16 fm^-1.

The differential cross section for the reaction ²H(γ,p)n has been measured at several center-of-mass angles ranging from 50° to 143° for photon energies between 0.8 and 1.8 GeV. The experiment was performed at the SLAC-NPAS facility with the use of the 1.6 GeV/c spectrometer to detect the high energy protons produced by a bremsstrahlung beam directed at a liquid deuterium target. Contributions from concurrent disintegration by the residual electron beam were determined by measuring the proton yield without the Cu photon radiator. At angles not very far from 90°, the energy dependence of the cross sections is consistent with predictions of scaling using counting rules for constituent quarks. At least one theoretical calculation based on a meson-baryon picture of the reaction is able to reproduce the magnitude and energy dependence of the 90° cross section. The angular distribution exhibits a large enhancement at backward angles at the higher energies.

The response function of nuclei in the quasielastic region at large momentum transfer (q≤10 fm^-1) is measured for a series of nuclei, ⁴He, ¹²C, ²⁷Al, ⁵⁶Fe, and ¹⁹⁷Au, up to large values of the Bjorken scaling variables x<2.5.

Inclusive electron scattering cross sections for ^3,4He have been measured in the quasielastic region at electron energies between 0.9 and 4.3 GeV, and scattering angles of 15° and 85°. Longitudinal (R_L) and transverse (R_T) response functions have been extracted using a Rosenbluth separation at constant ‖q‖ of 1.050 GeV/c. The ratio of the longitudinal to the transverse reduced response functions in the negative y region reaches unity.

Nuclear structure functions are extracted for high-energy electron scattering from nuclei at large values of the kinematic variable x and Q² in the range 1–4 (GeV/c)². At the highest Q², the data for x>1 begin to display a scaling indicative of local duality.

The tensor polarization t₂₀ of the recoil deuteron in elastic e-d scattering has been measured for three values of four-momentum transfer, q=3.78, 4.22, and 4.62 fm^-1. The data have been used to locate the first node in the charge monopole form factor of the deuteron at q=4.39±0.16 fm^-1. The results for t₂₀ are in reasonable agreement with expectations based on the nucleon-meson description of nuclear dynamics.

Inclusive electron-scattering cross sections have been measured for ⁵⁶Fe in the quasielastic region at electron energies between 0.9 and 4.3 GeV, at scattering angles of 15° and 85°. Longitudinal and transverse response functions at a q of 1.14 GeV/c have been extracted using a Rosenbluth separation. The experimental Coulomb sum has been obtained with aid of an extrapolation. The longitudinal response function, after correction for Coulomb distortion, is lower than quasifree-scattering-model predictions at the quasielastic peak and on the high-ω side.

Results of measurements of the transverse polarization transfer coefficients K_y^y′(0°) for H²(p→,n→)pp at 54 and 71 MeV are presented. The magnitude and energy dependence of this parameter have been determined with sufficient precision (4%) to permit the use of this reaction as a source of nearly monoenergetic polarized neutrons in precise measurements. K_y^y′(0°) is found to have a significant dependence on excitation energy. The results at low excitation energy are in agreement with calculations in the impulse approximation using the nucleon-nucleon phase shifts obtained from the Bonn or Paris potentials. A substantial difference from the results obtained using the empirical phase shifts is found. Results of measurements of the longitudinal polarization transfer coefficient K_z^z′(0°) at 54 MeV are also presented.

The response function of nuclear matter is determined from experimental data on inclusive electron scattering from finite nuclei.