Search Results (97 total)

We report values for the neutron electric to magnetic form factor ratio, G_En/G_Mn, deduced from measurements of the neutron's recoil polarization in the quasielastic ²H(e→,e^'n→)¹H reaction, at three Q² values of 0.45, 1.13, and 1.45 (GeV/c)². The data at Q²=1.13 and 1.45 (GeV/c)² are the first direct experimental measurements of G_En employing polarization degrees of freedom in the Q²>1 (GeV/c)² region and stand as the most precise determinations of G_En for all values of Q².

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.

Electroproduction of the ω meson was investigated in the ¹H(e,e^′p)ω reaction. The measurement was performed at a four-momentum transfer Q²≈0.5  GeV². Angular distributions of the virtual photon-proton center-of-momentum cross sections have been extracted over the full angular range. These distributions exhibit a strong enhancement over t-channel parity exchange processes in the backward direction. According to a newly developed electroproduction model, this enhancement provides significant evidence of resonance formation in the γ*p→ωp reaction channel.

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.

Double-differential cross sections d²σ∕(dΩdE_x) and spectra of analyzing powers, A_y, for the ⁴He(p⃗,n) reaction were measured at T_p≈100 and 200  MeV at forward angles between θ_lab=0° and 44°, and were analyzed using wave functions from the recoil-corrected continuum shell model (RCCSM). The steep rise of the cross-section spectra just above the p+³He rest energy threshold results from the excitation of the 2⁻ and 1⁻ resonances in ⁴Li and is well reproduced by the RCCSM at both incident energies. At higher excitation energies, the predicted cross sections fall off faster than the experimental values at forward angles. The absolute cross sections from the RCCSM were renormalized by factors of 1.3 and 1.6 at 100  MeV and 200  MeV, respectively, in order to fit the cross section summed over the lowest 10  MeV of excitation at the peak of the angular distribution. The 100  MeV A_y, averaged over the lowest 10  MeV of excitation, are well reproduced by the RCCSM but not the 200  MeV A_y, nor are the details of the A_y spectra well described at either energy.

We report the results from a systematic study of the quasielastic (e,e^′p) reaction on ¹²C,  ⁵⁶Fe, and ¹⁹⁷Au performed at Jefferson Lab. We have measured nuclear transparency and extracted spectral functions (corrected for radiation) over a Q² range of 0.64–3.25  (GeV∕c)² for all three nuclei. In addition, we have extracted separated longitudinal and transverse spectral functions at Q² of 0.64 and 1.8  (GeV∕c)² for these three nuclei (except for ¹⁹⁷Au at the higher Q²). The spectral functions are compared to a number of theoretical calculations. The measured spectral functions differ in detail but not in overall shape from most of the theoretical models. In all three targets the measured spectral functions show considerable excess transverse strength at Q²=0.64  (GeV∕c)², which is much reduced at 1.8  (GeV∕c)².

We report new measurements of the ratio of the electric form factor to the magnetic form factor of the neutron, G_Eⁿ/G_Mⁿ, obtained via recoil polarimetry from the quasielastic ²H(e→,e^′n→)¹H reaction at Q² values of 0.45, 1.13, and 1.45   (GeV/c)² with relative statistical uncertainties of 7.6% and 8.4% at the two higher Q² points, which points have never been achieved in polarization measurements.

We report measurements of cross sections for the reaction ¹H(e,e^′K⁺)Y, for both the Λ and Σ⁰ hyperon states, at an invariant mass of W=1.84 GeV and four-momentum transfers 0.5<Q²<2 (GeV/c)². Data were taken for three values of virtual photon polarization ε, allowing the decomposition of the cross sections into longitudinal and transverse components. The Λ data are a revised analysis of prior work, whereas the Σ⁰ results have not been previously reported.

A complete set of polarization-transfer observables has been measured at 197 MeV in the quasifree region for the (p→,n→) reactions on C, Ca, and Pb targets. Data have been obtained at laboratory scattering angles of 13°, 24°, 37°, and 48°, which span an energy-loss range up to 150 MeV, with a corresponding momentum-transfer range q=0.75–2.4 fm^-1. The empirical results are compared to the observables obtained from the free nucleon-nucleon data base. Derived spin-longitudinal and spin-transverse responses for Ca are compared with those previously obtained at 346 and 495 MeV incident energies.

In this paper, we present a complete set of polarization-transfer observables measured at 197 MeV in the quasifree region for (p→,n→) reactions on ²H and ^3,4He targets. Data were obtained at laboratory scattering angles of 13°, 24°, 37°, and 48° for the ²H target and at 13° and 37° for the ^3,4He targets. The data span an energy-loss range up to 150 MeV, with a corresponding momentum-transfer range q=0.75–2.4 fm^-1. The polarization-transfer observables are used to calculate the center-of-mass polarization observables. The empirical results for the ²H target are compared to the observables obtained from the free nucleon-nucleon database and with Faddeev-type calculations. No discernible differences are observed in either the values of polarization-transfer observables or in the center-of-mass polarization observables among the three nuclei studied in this paper.

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.

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.

A separation of the longitudinal and transverse ¹²C(e,e^′p) cross sections in the quasifree region has been performed in parallel kinematics at Q² of 0.64 and 1.8 GeV² for initial proton momentum <80 MeV. The separated transverse and longitudinal spectral functions at Q²=0.64 GeV² show significant differences for missing energy between 25 and 60 MeV indicating a breakdown in the single nucleon knockout picture. The transverse spectral functions exhibit definite momentum transfer dependence.

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 differential cross section for ²H(γ,d)π⁰ has been measured at deuteron center-of-mass angles of 90° and 136°. This work reports the first data for this reaction above a photon energy of 1 GeV, and permits a test of the apparent constituent counting rule and reduced nuclear amplitude behavior as observed in elastic ed scattering. Measurements were performed up to a photon energy of 4.0 GeV, and are in good agreement with previous lower energy measurements. Overall, the data are inconsistent with both constituent-counting rule and reduced nuclear amplitude predictions.

The differential cross section for the process p(e,e^′p)η has been measured at Q²=2.4 and 3.6 (GeV/c)² at center-of-mass energies encompassing the S₁₁(1535) resonance. The latter point is the highest-Q² exclusive measurement of this process to date. The resonance width and the helicity-1/2 transition amplitude are extracted from the data, and evidence for the possible onset of scaling in this reaction is shown. A lower bound of ≈0.45 is placed on the S₁₁(1535)→pη branching fraction.

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 studied the electroproduction of the Δ(1232) resonance via the reaction p(e,e^′p)π⁰ at four-momentum transfers Q²  =  2.8 and 4.0 GeV². This is the highest Q² for which exclusive resonance electroproduction has ever been observed. Decay angular distributions for Δ→pπ⁰ were measured over a wide range of barycentric energies covering the resonance. The N–Δ transition form factor G_M^* and ratios of resonant multipoles E₁₊/M₁₊ and S₁₊/M₁₊ were extracted from the decay angular distributions. These ratios remain small, indicating that perturbative QCD is not applicable for this reaction at these momentum transfers.