Search Results (28 total)

Kaon electroproduction from light nuclei and hydrogen, using ¹H, ²H, ³He, ⁴He, and carbon targets has been measured at the Thomas Jefferson National Accelerator Facility. The quasifree angular distributions of Λ and Σ hyperons were determined at Q²=0.35 (GeV/c)² and W=1.91 GeV. Electroproduction on hydrogen was measured at the same kinematics for reference.

The data analysis for the reaction ¹H(e,e^'π⁺)n, which was used to determine values for the charged pion form factor F_π for values of Q²= 0.6–1.6 GeV², has been repeated with careful inspection of all steps and special attention to systematic uncertainties. Also the method used to extract F_π from the measured longitudinal cross section was critically reconsidered. Final values for the separated longitudinal and transverse cross sections and the extracted values of F_π are presented.

Cross-section values for Compton scattering on the proton were measured at 25 kinematic settings over the range s=5–11 and -t=2–7  GeV² with a statistical accuracy of a few percent. The scaling power for the s dependence of the cross section at fixed center-of-mass angle was found to be 8.0±0.2, strongly inconsistent with the prediction of perturbative QCD. The observed cross-section values are in fair agreement with the calculations using the handbag mechanism, in which the external photons couple to a single quark.

A large data set of charged-pion (π^±) electroproduction from both hydrogen and deuterium targets has been obtained spanning the low-energy residual-mass region. These data conclusively show the onset of the quark-hadron duality phenomenon, as predicted for high-energy hadron electroproduction. We construct several ratios from these data to exhibit the relation of this phenomenon to the high-energy factorization ansatz of electron-quark scattering and subsequent quark→pion production mechanisms.

A pioneering experiment in Λ hypernuclear spectroscopy, undertaken at the Thomas Jefferson National Accelerator Facility (JLab), was recently reported. The experiment used the high precision, continuous electron beam at JLab, and a special arrangement of spectrometer magnets to measure the hypernuclear spectrum from C and ⁷Li targets using the (e,e'K⁺) reaction. The _Λ¹²B spectrum found in this investigation was previously published, but is reported here in more detail, with improved resolution. In addition, the results of a _Λ⁷He spectrum also obtained in the experiment, are shown. This latter spectrum indicates the need for a more detailed few-body calculation of the hypernucleus and the reaction process. The success of the experiment demonstrates the potential of the (e,e'K⁺) reaction for high resolution spectroscopy of hypernuclear spectra.

We present the first measurement of the Q² dependence of the neutron spin structure function g₂ⁿ at five kinematic points covering 0.57  (GeV/c)²≤Q²≤1.34  (GeV/c)² at x≃0.2. Though the naive quark-parton model predicts g₂=0, nonzero values occur in more realistic models of the nucleon which include quark-gluon correlations, finite quark masses, or orbital angular momentum. When scattering from a noninteracting quark, g₂ⁿ can be predicted using next-to-leading order fits to world data for g₁ⁿ. Deviations from this prediction provide an opportunity to examine QCD dynamics in nucleon structure. Our results show a positive deviation from this prediction at lower Q², indicating that contributions such as quark-gluon interactions may be important. Precision data obtained for g₁ⁿ are consistent with next-to-leading order fits to world data.

Compton scattering from the proton was investigated at s=6.9  GeV² and t=-4.0  GeV² via polarization transfer from circularly polarized incident photons. The longitudinal and transverse components of the recoil proton polarization were measured. The results are in disagreement with a prediction of perturbative QCD based on a two-gluon exchange mechanism, but agree well with a prediction based on a reaction mechanism in which the photon interacts with a single quark carrying the spin of the proton.

We report the results of a new Rosenbluth measurement of the proton electromagnetic form factors at Q² values of 2.64, 3.20, and 4.10  GeV². Cross sections were determined by detecting the recoiling proton, in contrast to previous measurements which detected the scattered electron. Cross sections were determined to 3%, with relative uncertainties below 1%. The ratio μ_pG_E/G_M was determined to 4%–8% and showed μ_pG_E/G_M≈1. These results are consistent with, and much more precise than, previous Rosenbluth extractions. They are inconsistent with recent polarization transfer measurements of similar precision, implying a systematic difference between the techniques.

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 _Λ^3,4H and _Λ⁴H hypernuclear bound states have been observed for the first time in kaon electroproduction on ^3,4He targets. The production cross sections have been determined at Q²=0.35   GeV² and W=1.91   GeV. For either hypernucleus the nuclear form factor is determined by comparing the angular distribution of the ^3,4He(e,e^′K⁺)_Λ^3,4H processes to the elementary cross section ¹H(e,éK⁺)Λ on the free proton, measured during the same experiment.

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.

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 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.

High-energy, cw electron beams at new accelerator facilities allow electromagnetic production and precision study of hypernuclear structure, and we report here on the first experiment demonstrating the potential of the (e,e^′K⁺) reaction for hypernuclear spectroscopy. This experiment is also the first to take advantage of the enhanced virtual photon flux available when electrons are scattered at approximately zero degrees. The observed energy resolution was found to be ≈900  keV for the _Λ¹²B spectrum, and is substantially better than any previous hypernuclear experiment using magnetic spectrometers. The positions of the major excitations are found to be in agreement with a theoretical prediction and with a previous binding energy measurement, but additional structure is also observed in the core excited region, underlining the future promise of this technique.

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.

The coherent ³He(e,e^′π⁺)³H reaction was measured at Q²=0.4 (GeV/c)² and W=1.6 GeV for two values of the virtual photon polarization, ε, allowing the separation of longitudinal and transverse cross sections. The results from the coherent process on ³He were compared to H(e,e^′π⁺)n data taken at the same kinematics. This marks the first direct comparison of these processes. At these kinematics (p_π=1.1 GeV/c), pion rescattering from the spectator nucleons in the ³He(e,e^′π⁺)³H process is expected to be small, simplifying the comparison to π⁺ production from the free proton.

Separated longitudinal and transverse cross sections for charged pion electroproduction from ¹H, ²H, and ³He were measured at Q²  =  0.4 (GeV/c)² for two values of the invariant mass, W  =  1.15 GeV and W  =  1.60 GeV, in a search for a mass dependence which would signal the effect of nuclear pions. This is the first such study that includes recoil momenta significantly above the Fermi surface. The longitudinal cross section, if dominated by the pion-pole process, should be sensitive to nuclear pion currents. Comparisons of the longitudinal cross section target ratios to a quasifree calculation reveal a significant suppression in ³He at W  =  1.60 GeV. The W  =  1.15 GeV results are consistent with simple estimates of the effect of nuclear pion currents, but are also consistent with pure quasifree production.

Separated longitudinal and transverse structure functions for the reaction ¹H(e,e^′π⁺)n were measured in the momentum transfer region Q²  =  0.6–1.6 (GeV/c)² at a value of the invariant mass W  =  1.95 GeV. New values for the pion charge form factor were extracted from the longitudinal cross section by using a recently developed Regge model. The results indicate that the pion form factor in this region is larger than previously assumed and is consistent with a monopole parametrization fitted to very low Q² elastic data.

A newly obtained sample of inclusive electron-nucleon scattering data has been analyzed for precision tests of quark-hadron duality. The data are in the nucleon resonance region, and span the range 0.3<Q²<5.0 (GeV/c)². Duality is observed both in limited and extended regions around the prominent resonance enhancements. Higher twist contributions to the F₂ structure function are found to be small on average, even in the low Q² regime of ≈0.5 (GeV/c)². Using duality, an average scaling curve is obtained. In all cases, duality appears to be a nontrivial property of the nucleon structure function.

A newly obtained data sample of inclusive electron-nucleon scattering from both hydrogen and deuterium targets is analyzed. These JLab data span the nucleon resonance region up to four-momentum transfers of 5 (GeV/c)². The data are found to follow an average scaling curve. The inclusion of low-momentum transfer data yields a scaling curve resembling deep inelastic neutrino-nucleus scattering data, suggesting a sensitivity to valencelike structure only.

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.

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.

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.