Search Results (58 total)

This paper was published online on 20 May 2005 without several of the authors’ corrections incorporated. Equation (13) has been replaced. The captions of Figs. 16–18 have also been replaced. Typographical errors on pages 4, 6, 14, 15, 18, 19, 22, and 24 have all been corrected. The paper has been corrected as of 8 June 2005. The text is correct in the printed version of the journal.

The ratio of the proton elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic e→p→ep→reaction in the four-momentum transfer squared range of 0.5 to 3.5  GeV². In the single-photon exchange approximation, G_Ep/G_Mp is directly proportional to P_t/P_ℓ. The simultaneous measurement of P_t and P_ℓ in a polarimeter reduces systematic uncertainties. The results for G_Ep/G_Mp show a systematic decrease with increasing Q², indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously.

The physics program in Hall A at Jefferson Lab commenced in the summer of 1997 with a detailed investigation of the ¹⁶O(e,e^′p) reaction in quasielastic, constant (q,ω) kinematics at Q²≈0.8  (GeV∕c)², q≈1  GeV∕c, and ω≈445  MeV. Use of a self-calibrating, self-normalizing, thin-film waterfall target enabled a systematically rigorous measurement. Five-fold differential cross-section data for the removal of protons from the 1p-shell have been obtained for 0<p_miss<350  MeV∕c. Six-fold differential cross-section data for 0<E_miss<120  MeV were obtained for 0<p_miss<340  MeV∕c. These results have been used to extract the A_LT asymmetry and the R_L, R_T, R_LT, and R_L+TT effective response functions over a large range of E_miss and p_miss. Detailed comparisons of the 1p-shell data with Relativistic Distorted-Wave Impulse Approximation (RDWIA), Relativistic Optical-Model Eikonal Approximation (ROMEA), and Relativistic Multiple-Scattering Glauber Approximation (RMSGA) calculations indicate that two-body currents stemming from meson-exchange currents (MEC) and isobar currents (IC) are not needed to explain the data at this Q². Further, dynamical relativistic effects are strongly indicated by the observed structure in A_LT at p_miss≈300  MeV∕c. For 25<E_miss<50  MeV and p_miss≈50  MeV∕c, proton knockout from the 1s_1∕2-state dominates, and ROMEA calculations do an excellent job of explaining the data. However, as p_miss increases, the single-particle behavior of the reaction is increasingly hidden by more complicated processes, and for 280<p_miss<340  MeV∕c, ROMEA calculations together with two-body currents stemming from MEC and IC account for the shape and transverse nature of the data, but only about half the magnitude of the measured cross section. For 50<E_miss<120  MeV and 145<p_miss<340  MeV∕c, (e,e^′pN) calculations which include the contributions of central and tensor correlations (two-nucleon correlations) together with MEC and IC (two-nucleon currents) account for only about half of the measured cross section. The kinematic consistency of the 1p-shell normalization factors extracted from these data with respect to all available ¹⁶O(e,e^′p) data is also examined in detail. Finally, the Q²-dependence of the normalization factors is discussed.

We report a virtual Compton scattering study of the proton at low c.m. energies. We have determined the structure functions P_LL-P_TT/ϵ and P_LT, and the electric and magnetic generalized polarizabilities (GPs) α_E(Q²) and β_M(Q²) at momentum transfer Q²=0.92 and 1.76  GeV². The electric GP shows a strong falloff with Q², and its global behavior does not follow a simple dipole form. The magnetic GP shows a rise and then a falloff; this can be interpreted as the dominance of a long-distance diamagnetic pion cloud at low Q², compensated at higher Q² by a paramagnetic contribution from πN intermediate states.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from protons. Significant contributions to this asymmetry could arise from the contributions of strange form factors in the nucleon. The measured asymmetry is A=−15.05±0.98(stat)±0.56(syst)  ppm at the kinematic point ⟨θ_lab⟩=12.3° and ⟨Q²⟩=0.477  (GeV∕c)². Based on these data as well as data on electromagnetic form factors, we extract the linear combination of strange form factors G_E^s+0.392G_M^s=0.014±0.020±0.010, where the first error arises from this experiment and the second arises from the electromagnetic form factor data. This paper provides a full description of the special experimental techniques employed for precisely measuring the small asymmetry, including the first use of a strained GaAs crystal and a laser-Compton polarimeter in a fixed target parity-violation experiment.

Exclusive electroproduction of π⁰ mesons on protons in the backward hemisphere has been studied at Q²=1.0  GeV² by detecting protons in the forward direction in coincidence with scattered electrons from the 4  GeV electron beam in Jefferson Lab’s Hall A. The data span the range of the total (γ*p) center-of-mass energy W from the pion production threshold to W=2.0  GeV. The differential cross sections σ_T+ϵσ_L, σ_TL, and σ_TT were separated from the azimuthal distribution and are presented together with the MAID and SAID parametrizations.

We measured the cross section and response functions for the quasielastic ¹⁶O(e,e^′p) reaction for missing energies 25≤E_m≤120 MeV at missing momenta P_m≤340 MeV/c. For 25<E_m<50 MeV and P_m≈60 MeV/c, the reaction is dominated by a single 1s_1/2 proton knockout. At larger P_m, the single-particle aspects are increasingly masked by more complicated processes. Calculations which include pion exchange currents, isobar currents, and short-range correlations account for the shape and the transversity, but for only half of the magnitude of the measured cross section.

High-precision ¹H(e,e^′p)π⁰ measurements at Q²  =  0.126 (GeV/c)² are reported, which allow the determination of quadrupole amplitudes in the γ^*N→Δ transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I  =  3/2) amplitude ratios, R_SM  =  (-6.5±0.2_stat+sys±2.5_mod)% and R_EM  =  (-2.1±0.2_stat+sys±2.0_mod)%, are dominated by model error. Previous R_SM and R_EM results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.

The recoil proton polarization has been measured in the p(e→,e^′p→)π⁰ reaction in parallel kinematics around W  =  1232  MeV, Q²  =  0.121  (GeV/c)², and ε  =  0.718 using the polarized cw electron beam of the Mainz Microtron. All three proton polarization components, P_x/P_e  =  (-11.4±1.3±1.4)%, P_y  =  (-43.1±1.3±2.2)%, and P_z/P_e  =  (56.2±1.5±2.6)%, could be measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio, CMR  =  (-6.4±0.7_stat±0.8_syst)%, was determined from P_x in the framework of the Mainz Unitary Isobar Model. The consistency among the reduced polarizations and the extraction of the ratio of longitudinal-to-transverse response is discussed.

The first (e→,e^′p→) polarization transfer measurements on a nucleus heavier than deuterium have been carried out at Jefferson Laboratory. Transverse and longitudinal components of the polarization of protons ejected in the reaction ¹⁶O(e→,e^′p→ ) were measured in quasielastic perpendicular kinematics at a Q² of 0.8 (GeV/c)². The data are in good agreement with state of the art calculations.

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 cross section for quasielastic 1p-shell proton knockout in the ¹⁶O(e,e^′p) reaction at ω  =  0.439 GeV and Q²  =  0.8 (GeV/c)² for missing momentum P_miss≤355 MeV/c. We have extracted the response functions R_L+TT, R_T, R_LT, and the left-right asymmetry, A_LT, for the 1p_1/2 and the 1p_3/2 states. The data are well described by relativistic distorted wave impulse approximation calculations. At large P_miss, the structure observed in A_LT indicates the existence of dynamical relativistic effects.

The ratio of the proton's elastic electromagnetic form factors, G_Ep/G_Mp, was obtained by measuring P_t and P_ℓ, the transverse and the longitudinal recoil proton polarization, respectively. For elastic e→p→ep→, G_Ep/G_Mp is proportional to P_t/P_ℓ. Simultaneous measurement of P_t and P_ℓ in a polarimeter provides good control of the systematic uncertainty. The results for the ratio G_Ep/G_Mp show a systematic decrease as Q² increases from 0.5 to 3.5 GeV², indicating for the first time a definite difference in the spatial distribution of charge and magnetization currents in the proton.

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.

The deuteron elastic structure function A(Q²) has been extracted in the range 0.7≤Q²≤6.0 (GeV/c)² from cross section measurements of elastic electron-deuteron scattering in coincidence using the Hall A Facility of Jefferson Laboratory. The data are compared to theoretical models, based on the impulse approximation with the inclusion of meson-exchange currents, and to predictions of quark dimensional scaling and perturbative quantum chromodynamics.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The kinematic point [ 〈θ_lab〉  =  12.3° and 〈Q²〉  =  0.48 (GeV/c)²] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor G_E^s. The result, A  =  -14.5±2.2 ppm, is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies G_E^s+0.39G_M^s  =  0.023±0.034(stat)±0.022(syst)±0.026(δG_Eⁿ), where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor.

We present a measurement of the induced proton polarization P_n in π⁰ electroproduction on the proton around the Δ resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of W=1231 MeV and Q²=0.126 GeV²/c², respectively. We measured a large induced polarization, P_n=-0.397±0.055±0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model.

A Reply to the Comment by F. Klein et al.

The first measurements of the induced proton polarization P_n for the ¹²C(e,e^′p→) reaction are reported. The experiment was performed at quasifree kinematics for energy and momentum transfer (ω,q)≈ (294 MeV, 756 MeV/c) and sampled a missing momentum range of 0–250 MeV/c. The induced polarization arises from final-state interactions and for these kinematics is dominated by the real part of the spin-orbit optical potential. The distorted-wave impulse approximation provides good agreement with data for the 1p_3/2 shell. The data for the continuum suggest that both the 1s_1/2 shell and underlying ℓ>1 configurations contribute.

Recoil proton polarization observables were measured for both the p(e→,e^′p→) and d(e→,e^′p→)n reactions at two values of Q² using a newly commissioned proton focal plane polarimeter at the MIT-Bates Linear Accelerator Center. The hydrogen and deuterium spin-dependent observables D_ℓℓ and D_ℓt, the induced polarization P_n, and the form factor ratio G_E^p/G_M^p were measured under identical kinematics. The deuterium and hydrogen results are in good agreement with each other and with the plane-wave impulse approximation (PWIA).

The compound La_1-xSr_xMnO₃ undergoes structural and magnetic phase transitions as a function of temperature and pressure. The temperature at which the structural phase transition from a high-temperature rhombohedral phase to a low-temperature orthorhombic phase takes place is very sensitive to the level of doping (x) and pressure, the magnetic transition temperature to a ferromagnetically ordered state less so. Recent magnetostriction and resistivity measurements have suggested that the structural transition may also be driven by the application of a magnetic field. We have performed single-crystal neutron-diffraction experiments which confirm that by carefully controlling the level of doping the structural phase transition can be induced at constant temperature by application of a magnetic field.

The reaction ³He(e,e^′d)p was measured at four-momentum transfer q  =  420 MeV, for proton recoil momentum P_r  =  22, 54, and 62 MeV/c, in parallel deuteron kinematics. Longitudinal (L) and transverse (T) response functions were obtained by means of Rosenbluth separations. The separated response functions are, respectively, found to be dominated by the contributions of isoscalar and isovector two-body currents. The L/T ratios are about 3.5. This is an order of magnitude lower than that for elastic scattering on a free deuteron, or what would be expected for a purely isoscalar transition, but approximately agrees with what is theoretically expected for isoscalar plus isovector contributions.

Electrochemical coloration mechanisms of electrochromic (EC), amorphous tungsten and iridium oxide films have been investigated by direct elemental analysis of their compositions in oxidized and reduced states. Combined Rutherford backscattering and nuclear-reaction-analysis techniques were employed to determine the identities and concentrations of ions injected as charge compensators from the contacting electrolyte during EC redox reactions. In aqueous electrolytes, the cathodic coloration of an anodically formed, hydrous tungsten oxide film (composition ∼WO₃·H₂O), is accompanied by injection of protons, in accordance with mechanisms determined for anhydrous WO₃. The anodic coloration and coloration bleaching of an anodically formed, hydrous iridium oxide (IROX) film, which has a varying degree of hydration throughout its thickness, is accompanied by ejection and injection of H⁺ and H₃O⁺ ions. In nonaqueous aprotic electrolytes, the slow but reversible insertion of Li⁺ (r=0.60 Å) and Na⁺ (r=0.95 Å) ions into IROX films has been successfully demonstrated for the first time. Li⁺ ions were found to exchange with H⁺ ions in these hydrous films. No evidence could be found for insertion of K⁺ (r=1.33 Å), F^- (r=1.36 Å), or OH^- (r=1.55 Å) ions as charge compensators. These results provide a measure of the size and concentration of vacancies and defects in amorphous IROX films.

Neutron resonances in ²⁴Na from E_r=0 to 1 MeV are studied with the ²³Na(d, p)²⁴Na reaction at E_d=11.0 MeV using a biased quadrupole spectrometer. Fractionated single-particle states are identified via their forward angle cross sections, using distorted wave Born-approximation predictions with resonance state form factors for the residual ²⁴Na system. Spectroscopic results are compared with those from (n, n) and/or (n, γ) studies over the same region of excitation.

NUCLEAR REACTIONS ²³Na(d, p), E_d=11 MeV, measured σ(θ). ²⁴Na deduced levels, l, j, π, spectroscopic factors. DWBA analysis, resolution 12 keV; comparison to neutron total σ, ²³Na.