Search Results (20 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.

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.

Unpolarized cross sections and vector target analyzing powers for the ⁷Li→(π⁺,π^+′p) proton knockout reaction were measured using a vector polarized ⁷LiH target and a 240 MeV π⁺ beam at the πM1 channel of PSI. Typical target polarizations were >30% for ⁷Li. Coincident π⁺-p data are presented for three emitted pion angles (60°, 85°, and 108°). For each π⁺ angle coincident data with adequate statistics were obtained for three proton angles near the quasifree π⁺-p angle. The π⁺ angles were chosen to emphasize and isolate contributions to the target analyzing powers from the two-body π-nucleon interaction with a polarized nucleon whose polarization resulted from either the target polarization or from the distortion-induced effective polarization. The data are compared with factorized-amplitude distorted-wave impulse approximation (DWIA) calculations. The unpolarized cross sections are rather well described by these calculations. However, for all three angles the target analyzing powers are substantially reduced from predictions of conventional DWIA calculations. This result suggests a rather strong spin dependence in the Δ-nucleus spreading potential.

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.

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 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.

Cross-section measurements of the ⁹Be,¹²C(α,2α) reaction at 580 MeV bombarding energy are presented. The data are compared with distorted-wave impulse approximation calculations. The agreement between theory and experiment suggests a dominance of the quasifree knockout mechanism. The extracted α-particle spectroscopic factors are in reasonable agreement with theory and proton-induced knockout reactions, unlike measurements for the (α,2α) reaction at energies <~140 MeV, but still show significant angular dependence.

We report on a precision measurement of the neutron spin structure function g₁ⁿ using deep inelastic scattering of polarized electrons by polarized ³He. For the kinematic range 0.014<x<0.7 and 1<Q²<17 (GeV/c)², we obtain ∫0.014^0.7g₁ⁿ(x) dx  =  -0.036±0.004(stat)±0.005(syst) at an average Q²  =  5 (GeV/c)². We find relatively large negative values for g₁ⁿ at low x. The results call into question the usual Regge theory method for extrapolating to x  =  0 to find the full neutron integral ∫1g₁ⁿ(x) dx, needed for testing the quark-parton model and QCD sum rules.

We have measured the analyzing power for elastic scattering of π⁺ from a target of polarized ¹H. Data were taken for incident pion beam energies of 165 and 240 MeV at several pion scattering angles. The current data generally agree with previously existing measurements of A_y for this reaction and also with results of the SAID phase-shift analysis program. In most cases the new data are of higher precision than previously existing data. © 1996 The American Physical Society.

We present distorted-wave impulse approximation calculations for the polarization of outgoing protons resulting from (π⁺,2p) reactions using a quasideuteron absorption model with phenomenological amplitudes for the πNN vertex. As an example of the interesting interference and distorted-wave effects that can be observed from such reactions, we study the case of a ¹²C target. Calculations are also compared to the recently published data for the nuclei ^3,4He.

Angular distributions and total cross sections for the ⁴He(π⁺,pp)²H reaction have been measured with small relative uncertainties at incident energies of T_π⁺=64, 87, 114, 162, 217, 278, and 327 MeV. The results strongly support the quasideuteron absorption model for pion absorption on two nucleons, which is found to contribute only ≊50% to the total absorption cross section near the Δ resonance. All essential reaction channels of pion absorption near the Δ resonance on heavier nuclei seem to be present in ⁴He, but not in ³He. Any nuclear-density-related increase of pion absorption in ⁴He relative to ²H is <50% and no binding energy effect is found.

The availability of polarized nuclear targets for nuclear reaction studies offers new research directions. In this paper we present the distorted wave impulse approximation formalism and calculations for two-nucleon pion absorption on polarized nuclear targets. We assume that the pion absorbs on a ³S₁ n-p pair and use phenomenological amplitudes to describe the πNN vertex. The effects of the distortions on the incoming pion and outgoing protons, in contrast to the elementary πNN vertex, are delineated for certain special cases in which the formalism simplifies. Predictions of cross sections and vector and tensor analyzing powers are made for polarized target nuclei ⁶Li, ⁷Li, ¹³C, and ¹⁴N at pion incident energies of 115, 165, and 255 MeV.

Differential and integrated cross sections for the reaction ⁴He(π⁺,pp) at T_π=114 and 162 MeV are reported. These are the first results for this reaction using a large solid angle detector covering ≊55% of 4π sr. Directly measured integrated cross sections for absorption on two-nucleon pairs are σ_abs=25.7±1.8 mb at 114 MeV and σ_abs=21.0±1.5 mb at 162 MeV. These cross sections, when corrected by a factor of 1.4 final state interaction effects, account for about 50% of the total absorption cross section for all final-state reaction channels at both energies. The data are compared with Monte Carlo simulations of the reaction process using a quasideuteron absorption model for the two-nucleon absorption process in a plane wave impulse approximation. The consequences for the extraction of the two-nucleon absorption cross section by imposing different conditions on the data are explored.

Electromagnetic dissociation of ⁵⁹Co and ¹⁹⁷Au target nuclei and their subsequent deexcitation by two-neutron emission was observed. Beams of relativistic ¹²C, ²⁰Ne, ⁴⁰Ar, ⁵⁶Fe, and ¹³⁹La ions were used. The measured electromagnetic dissociation cross sections become large for high-Z targets and projectiles, reaching 335 mb for the ¹⁹⁷Au(¹³⁹La,X)¹⁹⁵Au reaction with 1.26 GeV/nucleon ¹³⁹La projectiles. The experimental cross sections in excess of the estimated nuclear contributions are in reasonable agreement with a calculation of the electromagnetic dissociation contribution using the Weizsäcker-Williams method for estimating the virtual photon spectrum.

The electromagnetic dissociation of ⁵⁹Co and ¹⁹⁷Au target nuclei by 1.26 GeV/nucleon ¹³⁹La projectiles was inferred from measurement of cross sections for the one-neutron removal reaction. The corresponding electromagnetic dissociation cross sections are large, reaching 0.28±0.04 and 1.97±0.13 barns for the ⁵⁹Co(¹³⁹La,X)⁵⁸CO and ¹⁹⁷Au(¹³⁹La,X)^196Au reactions, respectively. The experimental cross sections in excess of the estimated nuclear contributions are generally well described by use of the Weizsäcker-Williams method for calculating the electromagnetic dissociation contributions, but they appear to increase more slowly as the projectile charge is increased, consistent with a trend observed earlier for lower-Z projectiles.