Search Results (26 total)

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.

We measured angular distributions of differential cross section, beam analyzing power, and recoil polarization for neutral pion electroproduction at Q²=1.0 (GeV/c)² in 10 bins of 1.17≤W≤1.35 GeV across the Δ resonance. A total of 16 independent response functions were extracted, of which 12 were observed for the first time. Comparisons with recent model calculations show that response functions governed by real parts of interference products are determined relatively well near the physical mass, W=M_Δ≈1.232 GeV, but the variation among models is large for response functions governed by imaginary parts, and for both types of response functions, the variation increases rapidly with W>M_Δ. We performed a multipole analysis that adjusts suitable subsets of ℓ_π≤2 amplitudes with higher partial waves constrained by baseline models. This analysis provides both real and imaginary parts. The fitted multipole amplitudes are nearly model independent—there is very little sensitivity to the choice of baseline model or truncation scheme. By contrast, truncation errors in the traditional Legendre analysis of N→Δ quadrupole ratios are not negligible. Parabolic fits to the W dependence around M_Δ for the multiple analysis gives values for Re(S₁₊/M₁₊)=(-6.61±0.18)% and Re(E₁₊/M₁₊)=(-2.87±0.19)% for the pπ⁰ channel at W=1.232 GeV and Q²=1.0 (GeV/c)² that are distinctly larger than those from the Legendre analysis of the same data. Similarly, the multipole analysis gives Re(S₀₊/M₁₊)=(+7.1±0.8)% at W=1.232 GeV, consistent with recent models, while the traditional Legendre analysis gives the opposite sign because its truncation errors are quite severe.

We report new measurements of the parity-violating asymmetry A_PV in elastic scattering of 3 GeV electrons off hydrogen and ⁴He targets with ⟨θ_lab⟩≈6.0°. The ⁴He result is A_PV=(+6.40±0.23(stat)±0.12(syst))×10^-6. The hydrogen result is A_PV=(-1.58±0.12(stat)±0.04(syst))×10^-6. These results significantly improve constraints on the electric and magnetic strange form factors G_E^s and G_M^s. We extract G_E^s=0.002±0.014±0.007 at ⟨Q²⟩=0.077  GeV², and G_E^s+0.09G_M^s=0.007±0.011±0.006 at ⟨Q²⟩=0.109  GeV², providing new limits on the role of strange quarks in the nucleon charge and magnetization distributions.

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from ⁴He at an average scattering angle ⟨θ_lab⟩=5.7° and a four-momentum transfer Q²=0.091  GeV². From these data, for the first time, the strange electric form factor of the nucleon G_E^s can be isolated. The measured asymmetry of A_PV=(6.72±0.84_(stat)±0.21_(syst))×10^-6 yields a value of G_E^s=-0.038±0.042_(stat)±0.010_(syst), consistent with zero.

We measured angular distributions of recoil-polarization response functions for neutral pion electroproduction for W=1.23  GeV at Q²=1.0  (GeV/c)², obtaining 14 separated response functions plus 2 Rosenbluth combinations; of these, 12 have been observed for the first time. Dynamical models do not describe quantities governed by imaginary parts of interference products well, indicating the need for adjusting magnitudes and phases for nonresonant amplitudes. We performed a nearly model-independent multipole analysis and obtained values for Re (S₁₊/M₁₊)=-(6.84±0.15)% and Re (E₁₊/M₁₊)=-(2.91±0.19)% that are distinctly different from those from the traditional Legendre analysis based upon M₁₊ dominance and ℓ_π≤1 truncation.

First measurements of the Collins and Sivers asymmetries of charged hadrons produced in deep-inelastic scattering of muons on a transversely polarized ⁶LiD target are presented. The data were taken in 2002 with the COMPASS spectrometer using the muon beam of the CERN SPS at 160  GeV/c. The Collins asymmetry turns out to be compatible with zero, as does the measured Sivers asymmetry within the present statistical errors.

We report on measurements of the neutron spin asymmetries A_1,2ⁿ and polarized structure functions g_1,2ⁿ at three kinematics in the deep inelastic region, with x=0.33, 0.47, and 0.60 and Q²=2.7, 3.5, and 4.8  (GeV∕c)², respectively. These measurements were performed using a 5.7  GeV longitudinally polarized electron beam and a polarized ³He target. The results for A₁ⁿ and g₁ⁿ at x=0.33 are consistent with previous world data and, at the two higher-x points, have improved the precision of the world data by about an order of magnitude. The new A₁ⁿ data show a zero crossing around x=0.47 and the value at x=0.60 is significantly positive. These results agree with a next-to-leading-order QCD analysis of previous world data. The trend of data at high x agrees with constituent quark model predictions but disagrees with that from leading-order perturbative QCD (PQCD) assuming hadron helicity conservation. Results for A₂ⁿ and g₂ⁿ have a precision comparable to the best world data in this kinematic region. Combined with previous world data, the moment d₂ⁿ was evaluated and the new result has improved the precision of this quantity by about a factor of 2. When combined with the world proton data, polarized quark distribution functions were extracted from the new g₁ⁿ∕F₁ⁿ values based on the quark-parton model. While results for Δu∕u agree well with predictions from various models, results for Δd∕d disagree with the leading-order PQCD prediction when hadron helicity conservation is imposed.

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.

We present a measurement of the longitudinal spin cross section asymmetry for deep-inelastic muon-nucleon interactions with two high transverse momentum hadrons in the final state. Two methods of event classification are used to increase the contribution of the photon-gluon fusion process to above 30%. The most effective one, based on a neural network approach, provides the asymmetries A_p^lN→lhhX=0.030±0.057(stat)±0.010(syst) and A_d^lN→lhhX=0.070±0.076(stat)±0.010(syst). From these values we derive an averaged gluon polarization ΔG/G=-0.20±0.28(stat)±0.10(syst) at an average fraction of nucleon momentum carried by gluons 〈η〉=0.07.

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 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 have measured the neutron spin asymmetry A₁ⁿ with high precision at three kinematics in the deep inelastic region at x=0.33, 0.47, and 0.60, and Q²=2.7, 3.5, and 4.8   (GeV/c)², respectively. Our results unambiguously show, for the first time, that A₁ⁿ crosses zero around x=0.47 and becomes significantly positive at x=0.60. Combined with the world proton data, polarized quark distributions were extracted. Our results, in general, agree with relativistic constituent quark models and with perturbative quantum chromodynamics (PQCD) analyses based on the earlier data. However they deviate from PQCD predictions based on hadron helicity conservation.

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.

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.

We present the results of the spin asymmetries A₁ of the proton and the deuteron in the kinematic region extending down to x=6×10^-5 and Q²=0.01 GeV². The data were taken with a dedicated low x trigger, which required hadron detection in addition to the scattered muon, so as to reduce the background at low x. The results complement our previous measurements and the two sets are consistent in the overlap region. No significant spin effects are found in the newly explored region.

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 next-to-leading order QCD analysis of the presently available data on the spin structure function g₁ including the final data from the Spin Muon Collaboration. We present results for the first moments of the proton, deuteron, and neutron structure functions, and determine singlet and nonsinglet parton distributions in two factorization schemes. We also test the Bjorken sum rule and find agreement with the theoretical prediction at the level of 10%.

We present the final results of the spin asymmetries A₁ and the spin structure functions g₁ of the proton and the deuteron in the kinematic range 0.0008<x<0.7 and 0.2<Q²<100 GeV². For the determination of A₁, in addition to the usual method which employs inclusive scattering events and includes a large radiative background at low x, we use a new method which minimizes the radiative background by selecting events with at least one hadron as well as a muon in the final state. We find that this hadron method gives smaller errors for x<0.02, so it is combined with the usual method to provide the optimal set of results.