Search Results (58 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.

In a global analysis of the latest parity-violating electron scattering measurements on nuclear targets, we demonstrate a significant improvement in the experimental knowledge of the weak neutral-current lepton-quark interactions at low energy. The precision of this new result, combined with earlier atomic parity-violation measurements, places tight constraints on the size of possible contributions from physics beyond the standard model. Consequently, this result improves the lower-bound on the scale of relevant new physics to ∼1  TeV.

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.

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.

Parity-odd asymmetries in the electromagnetic decays of compound nuclei can sometimes be amplified above values expected from simple dimensional estimates by the complexity of compound nuclear states. Using a statistical approach, we estimate the root-mean-square of the distribution of expected parity-odd correlations s→_n·k→_γ, where s→_n is the neutron spin and k→_γ is the momentum of the γ, in the integrated γ spectrum from the capture of cold polarized neutrons on Al, Cu, and In. We present measurements of the asymmetries in these and other nuclei. Based on our calculations, large enhancements of asymmetries were not predicted for the studied nuclei and the statistical estimates are consistent with our measured upper bounds on the asymmetries.

The complete world set of parity-violating electron scattering data up to Q²∼0.3  GeV² is analyzed. We extract the current experimental determination of the strange electric and magnetic form factors of the proton, as well as the weak axial form factors of the proton and neutron, at Q²=0.1  GeV². Within experimental uncertainties, we find that the strange form factors are consistent with zero, as are the anapole contributions to the axial form factors. Nevertheless, the correlation between the strange and anapole contributions suggest that there is only a small probability that these form factors all vanish simultaneously.

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 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.

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 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.

The electric form factor of the neutron was determined from measurements of the d→(e→,e^′n)p reaction for quasielastic kinematics. Polarized electrons were scattered off a polarized deuterated ammonia (¹⁵ND₃) target in which the deuteron polarization was perpendicular to the momentum transfer. The scattered electrons were detected in a magnetic spectrometer in coincidence with neutrons in a large solid angle detector. We find G_Eⁿ=0.0526±0.0033(stat)±0.0026(sys) and 0.0454±0.0054±0.0037 at Q²=0.5 and 1.0  (GeV/c)², respectively.

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.

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 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.

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.