Search Results (30 total)

We have made the first measurements of the virtual Compton scattering (VCS) process via the H(e, e^'p)γ exclusive reaction in the nucleon resonance region, at backward angles. Results are presented for the W-dependence at fixed Q²=1 GeV² and for the Q² dependence at fixed W near 1.5 GeV. The VCS data show resonant structures in the first and second resonance regions. The observed Q² dependence is smooth. The measured ratio of H(e, e^'p)γ to H(e, e^'p)π⁰ cross sections emphasizes the different sensitivity of these two reactions to the various nucleon resonances. Finally, when compared to real Compton scattering (RCS) at high energy and large angles, our VCS data at the highest W (1.8-1.9 GeV) show a striking Q² independence, which may suggest a transition to a perturbative scattering mechanism at the quark level.

We present a measurement of the spin-dependent cross sections for the ³He→(e→,e^′)X reaction in the quasielastic and resonance regions at a four-momentum transfer 0.1≤Q²≤0.9  GeV². The spin-structure functions have been extracted and used to evaluate the nuclear Burkhardt-Cottingham and extended Gerasimov-Drell-Hearn sum rules for the first time. The data are also compared to an impulse approximation calculation and an exact three-body Faddeev calculation in the quasielastic region.

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 generalized forward spin polarizabilities γ₀ and δ_LT of the neutron have been extracted for the first time in a Q² range from 0.1 to 0.9  GeV². Since γ₀ is sensitive to nucleon resonances and δ_LT is insensitive to the Δ resonance, it is expected that the pair of forward spin polarizabilities should provide benchmark tests of the current understanding of the chiral dynamics of QCD. The new results on δ_LT show significant disagreement with chiral perturbation theory calculations, while the data for γ₀ at low Q² are in good agreement with a next-to-leading-order relativistic baryon chiral perturbation theory calculation. The data show good agreement with the phenomenological MAID model.

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.

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.

Precision measurements of the relative analyzing powers of five electron beam polarimeters, based on Compton, Møller, and Mott scattering, have been performed using the CEBAF accelerator at the Thomas Jefferson National Accelerator Facility (Jefferson Laboratory). A Wien filter in the 100 keV beam line of the injector was used to vary the electron spin orientation exiting the injector. High statistical precision measurements of the scattering asymmetry as a function of the spin orientation were made with each polarimeter. Since each polarimeter receives beam with the same magnitude of polarization, these asymmetry measurements permit a high statistical precision comparison of the relative analyzing powers of the five polarimeters. This is the first time a precise comparison of the analyzing powers of Compton, Møller, and Mott scattering polarimeters has been made. Statistically significant disagreements among the values of the beam polarization calculated from the asymmetry measurements made with each polarimeter reveal either errors in the values of the analyzing power or failure to correctly include all systematic effects. The measurements reported here represent a first step toward understanding the systematic effects of these electron polarimeters. Such studies are necessary to realize high absolute accuracy (ca. 1%) electron polarization measurements, as required for some parity violation measurements planned at Jefferson Laboratory. Finally, a comparison of the value of the spin orientation exiting the injector that provides maximum longitudinal polarization in each experimental hall leads to an independent and very precise (better than 10^-4) absolute measurement of the final electron beam energy.

We have measured the spin structure functions g₁ and g₂ of ³He in a double-spin experiment by inclusively scattering polarized electrons at energies ranging from 0.862 to 5.058 GeV off a polarized ³He target at a 15.5° scattering angle. Excitation energies covered the resonance and the onset of the deep inelastic regions. We have determined for the first time the Q² evolution of Γ₁(Q²)=∫₀¹g₁(x,Q²)dx, Γ₂(Q²)=∫₀¹g₂(x,Q²)dx, and d₂(Q²)=∫₀¹x²[2g₁(x,Q²)+3g₂(x,Q²)]dx for the neutron in the range 0.1≤Q²≤0.9  GeV² with good precision. Γ₁(Q²) displays a smooth variation from high to low Q². The Burkhardt-Cottingham sum rule holds within uncertainties and d₂ is nonzero over the measured range.

We present data on the inclusive scattering of polarized electrons from a polarized ³He target at energies from 0.862 to 5.06 GeV, obtained at a scattering angle of 15.5°. Our data include measurements from the quasielastic peak, through the nucleon resonance region, and beyond, and were used to determine the virtual photon cross-section difference σ_1/2-σ_3/2. We extract the extended Gerasimov-Drell-Hearn integral for the neutron in the range of four-momentum transfer squared Q² of 0.1–0.9   GeV².

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.

Contribution of higher order effects to the one-photon exchange approximation were studied in the first excited state of ¹²C by comparing inclusive inelastic scattering cross sections of electrons and positrons obtained at the Saclay Linear Accelerator. The data were compared to a distorted wave Born approximation (DWBA) calculation. The results indicate an effect less than 2% within 2σ, compatible with what was observed in recent elastic scattering measurements.

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.

Absolute differential cross sections for the reaction ep→epγ have been measured at a four-momentum transfer with virtuality Q²  =  0.33 GeV² and polarization ε  =  0.62 in the range 33.6 to 111.5 MeV/c for the momentum of the outgoing photon in the photon-proton center of mass frame. The experiment has been performed with the high-resolution spectrometers at the Mainz Microtron MAMI. From the photon angular distributions, two structure functions which are a linear combination of the generalized polarizabilities have been determined for the first time.

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.

Positron and electron quasielastic cross sections on ¹²C and ²⁰⁸Pb were measured at the Saclay linear accelerator (ALS) to test Coulomb corrections to the plane-wave Born approximation. The positron and electron response functions at the same effective kinematics were found to be equal within a 3% experimental uncertainty, which allows us to discriminate between different theoretical models of Coulomb distortions. Our result strongly supports the effective momentum approximation, even for nuclei as heavy as ²⁰⁸Pb.

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.

Dispersive effects have been investigated by comparing elastic scattering of electrons and positrons from ¹²C at the Saclay Linear Accelerator. The results demonstrate that dispersive effects at energies of 262 MeV and 450 MeV are less than 2% below the first diffraction minimum [0.95<q_eff (fm^-1)<1.66] in agreement with the prediction of Friar and Rosen. At the position of this minimum (q_eff=1.84 fm^-1), the deviation between the positron scattering cross section and the cross section derived from the electron results is -44%±30%.

³He(e,e^′p)X cross sections were measured in parallel kinematics for recoil momenta around 260 MeV/c. A Rosenbluth decomposition of the cross section was made and transverse and longitudinal spectral functions were extracted. The longitudinal spectral function is strongly quenched relative to the transverse one, the ratio of the integral of the longitudinal to transverse spectral functions over the continuum channel being 0.175±0.046±0.049. A model which takes into account final state interactions and meson exchange currents predicts a ratio of 0.43, it can therefore explain only part ot the quenching.

Cross sections for the ⁴He(e,e’p)R reaction have been measured for recoil momenta between 225 and 500 MeV/c. We find (1) that in the continuum channel a peak in the missing energy spectrum appears which shifts with increasing recoil momentum in a predictable way, and (2) that for the highest recoil momenta (above 300 or 350 MeV/c) overall agreement is obtained with microscopic calculations in which the scattering on nucleon pairs predominates. These results strongly suggest that the highest recoil momenta are generated by the scattering on one of a pair of nucleons. This interpretation is reinforced by an observed continuum momentum distribution which, for high recoil momenta, resembles the momentum distribution of deuterium. We infer that the high momentum part of the momentum distributioon of a nucleon pair appears unmodified when embedded in a nuclear medium.

Longitudinal, transverse, and longitudinal-transverse structure functions for the ²H(e,e’p) reaction have been determined. Measurements of the cross sections were made in-plane in nearly quasielastic kinematics spanning momentum transfers between 200 and 670 MeV/c and recoil momenta between 0 and 150 MeV/c. In addition, cross sections at momentum transfers above 800 MeV/c were measured at backward scattering angles in aligned kinematics where the response is predominantly transverse. We compared our data with both relativistic and nonrelativistic models. Our results are not consistently in agreement with any of the calculations based on these models. The disagreement between our data and these calculations ranges from 0% to 30%; on average it is about 10%. Notable discrepancies arise in describing relativistic effects for the longitudinal-transverse structure function, at a momentum transfer of 400 MeV/c as well as for the transverse structure function at momentum transfer above 800 MeV/c.

The cross section for exclusive π⁺ electroproduction on the proton has been measured near threshold for the first time at two different values of the virtual photon polarization (ɛ∼0.2 and ɛ∼0.7). Using the low energy theorem for this reaction we deduce the axial and pseudoscalar weak form factors G_A and G_P at ‖t‖=0.073, 0.139, and 0.179 (GeV/c)². The slope of G_A agrees with the value obtained in neutrino experiments. G_P satisfies the pion pole dominance hypothesis, which is thus verified for the first time in this range of transfer.