Search Results (49 total)

We have measured the nuclear transparency of the A(e,e^′π⁺) process in ²H, ¹²C, ²⁷Al, ⁶³Cu, and ¹⁹⁷Au targets. These measurements were performed at the Jefferson Laboratory over a four momentum transfer squared range Q²=1.1 to 4.7  (GeV/c)². The nuclear transparency was extracted as the super-ratio of (σ_A/σ_H) from data to a model of pion-electroproduction from nuclei without π-N final-state interactions. The Q² and atomic number dependence of the nuclear transparency both show deviations from traditional nuclear physics expectations and are consistent with calculations that include the quantum chromodynamical phenomenon of color transparency.

We have measured the beam-normal single-spin asymmetry in elastic scattering of transversely polarized 3 GeV electrons from unpolarized protons at Q²=0.15, 0.25  (GeV/c)². The results are inconsistent with calculations solely using the elastic nucleon intermediate state and generally agree with calculations with significant inelastic hadronic intermediate state contributions. A_n provides a direct probe of the imaginary component of the 2γ exchange amplitude, the complete description of which is important in the interpretation of data from precision electron-scattering experiments.

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.

We report on a study of the longitudinal to transverse cross section ratio, R=σ_L/σ_T, at low values of x and Q², as determined from inclusive inelastic electron-hydrogen and electron-deuterium scattering data from Jefferson Laboratory Hall C spanning the four-momentum transfer range 0.06<Q²<2.8  GeV². Even at the lowest values of Q², R remains nearly constant and does not disappear with decreasing Q², as might be expected. We find a nearly identical behavior for hydrogen and deuterium.

We have examined the spin structure of the proton in the region of the nucleon resonances (1.085  GeV<W<1.910  GeV) at an average four momentum transfer of Q²=1.3  GeV². Using the Jefferson Lab polarized electron beam, a spectrometer, and a polarized solid target, we measured the asymmetries A_∥ and A_⊥ to high precision, and extracted the asymmetries A₁ and A₂, and the spin structure functions g₁ and g₂. We found a notably nonzero A_⊥, significant contributions from higher-twist effects, and only weak support for polarized quark-hadron duality.

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.

The ¹H(e,e^′π⁺)n cross section was measured at four-momentum transfers of Q²=1.60 and 2.45  GeV² at an invariant mass of the photon nucleon system of W=2.22  GeV. The charged pion form factor (F_π) was extracted from the data by comparing the separated longitudinal pion electroproduction cross section to a Regge model prediction in which F_π is a free parameter. The results indicate that the pion form factor deviates from the charge-radius constrained monopole form at these values of Q² by one sigma, but is still far from its perturbative quantum chromodynamics prediction.

The ratio of the proton's electric to magnetic form factor, G_E/G_M, can be extracted in elastic electron-proton scattering by measuring cross sections, beam-target asymmetry, or recoil polarization. Separate determinations of G_E/G_M by cross sections and recoil polarization observables disagree for Q²>1 (GeV/c)². Measurement by a third technique might uncover an unknown systematic error in either of the previous measurements. The beam-target asymmetry has been measured for elastic electron-proton scattering at Q² = 1.51 (GeV/c)² for target spin orientation aligned perpendicular to the beam momentum direction. This is the largest Q² at which G_E/G_M has been determined by a beam-target asymmetry experiment. The result, μG_E/G_M=0.884±0.027±0.029, is compared to previous world data.

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 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.

We studied the reaction ¹²C(e,e^'p) in quasielastic kinematics at momentum transfers between 0.6 and 1.8 (GeV/c)² covering the single-particle region. From this the nuclear transparency factors are extracted using two methods. The results are compared to theoretical predictions obtained using a generalization of Glauber theory described in this paper. Furthermore, the momentum distribution in the region of the 1s-state up to momenta of 300 MeV/c is obtained from the data and compared to the correlated basis function theory and the independent-particle shell model.

We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12≤Q²≤1.0  GeV². These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q² dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

We have carried out an (e,e^′p) experiment at high momentum transfer and in parallel kinematics to measure the strength of the nuclear spectral function S(k,E) at high nucleon momenta k and large removal energies E. This strength is related to the presence of short-range and tensor correlations, and was known hitherto only indirectly and with considerable uncertainty from the lack of strength in the independent-particle region. This experiment locates by direct measurement the correlated strength predicted by theory.

We report on precision measurements of the elastic cross section for electron-proton scattering performed in Hall C at Jefferson Lab. The measurements were made at 28 distinct kinematic settings covering a range in momentum transfer of 0.4<Q²<5.5  (GeV∕c)². These measurements represent a significant contribution to the world’s cross section data set in the Q² range, where a large discrepancy currently exists between the ratio of electric to magnetic proton form factors extracted from previous cross section measurements and that recently measured via polarization transfer in Hall A at Jefferson Lab. This data set shows good agreement with previous cross section measurements, indicating that if a heretofore unknown systematic error does exist in the cross section measurements, then it is intrinsic to all such measurements.

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.

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 quasielastic (e,e^′p) reaction was studied on targets of deuterium, carbon, and iron up to a value of momentum transfer Q² of 8.1 (GeV/c)². A nuclear transparency was determined by comparing the data to calculations in the plane-wave impulse approximation. The dependence of the nuclear transparency on Q² and the mass number A was investigated in a search for the onset of the color transparency phenomenon. We find no evidence for the onset of color transparency within our range of Q². A fit to the world’s nuclear transparency data reflects the energy dependence of the free-proton–nucleon cross section.

The first measurements of the d(γ,p)n differential cross section at forward angles and photon energies above 4 GeV were performed at the Thomas Jefferson National Accelerator Facility (JLab). The results indicate evidence of an angular dependent scaling threshold. Results at θ_cm  =  37° are consistent with the constituent counting rules for E_γ≳4 GeV, while those at 70° are consistent with the constituent counting rules for E_γ≳1.5 GeV.

We report the first measurement using a solid polarized target of the neutron electric form factor G_Eⁿ via d→(e→,e^′n)p. G_Eⁿ was determined from the beam-target asymmetry in the scattering of longitudinally polarized electrons from polarized deuterated ammonia ( ¹⁵ND₃). The measurement was performed in Hall C at Thomas Jefferson National Accelerator Facility in quasifree kinematics with the target polarization perpendicular to the momentum transfer. The electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle segmented detector. We find G_Eⁿ  =  0.04632±0.00616(stat)±0.00341(syst) at Q²  =  0.495 (GeV/c)².

Inclusive electron scattering data are presented for ²H, C, Fe, and Au targets at an incident electron energy of 4.045 GeV for a range of momentum transfers from Q²=1 to 7 (GeV/c)². Data were taken at Jefferson Laboratory for low values of energy loss, corresponding to values of Bjorken x≳1. The structure functions do not show scaling in x in this range, where inelastic scattering is not expected to dominate the cross section. The data do show scaling, however, in the Nachtmann variable ξ. This scaling appears to be the result of Bloom- Gilman duality in the nucleon structure function combined with the Fermi motion of the nucleons in the nucleus. The resulting extension of scaling to larger values of ξ opens up the possibility of accessing nuclear structure functions in the high-x region at lower values of Q² than previously believed.

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.

Tensor polarization observables ( t₂₀, t₂₁, and t₂₂) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)². The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q² the deuteron charge form factors G_C and G_Q. They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.