Search Results (56 total)

We have measured the transverse asymmetry A_T^' in the quasielastic ³He→(e→,e^') process with high precision at Q² values from 0.1 to 0.6  (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted at Q² values of 0.1 and 0.2  (GeV/c)² using a nonrelativistic Faddeev calculation which includes both final-state interactions (FSI) and meson-exchange currents (MEC). Theoretical uncertainties due to the FSI and MEC effects were constrained with a precision measurement of the spin-dependent asymmetry in the threshold region of ³He→(e→,e^'). We also extracted the neutron magnetic form factor G_Mⁿ at Q² values of 0.3 to 0.6  (GeV/c)² based on plane wave impulse approximation calculations.

The differential cross sections for the γn→π^-p and the γp→π⁺n processes were measured at Jefferson Lab. The photon energies ranged from 1.1 to 5.5 GeV, corresponding to center-of-mass energies from 1.7 to 3.4 GeV. The pion center-of-mass angles varied from 50^° to 110^°. The π^- and π⁺ photoproduction data both exhibit a global scaling behavior at high energies and high transverse momenta, consistent with the constituent counting rule prediction and the existing π⁺ data. The data suggest possible substructure of the scaling behavior, which might be oscillations around the scaling value. The data show an enhancement in the scaled cross section at center-of-mass energy near 2.2 GeV. The differential cross section ratios [dσ/dt(γn→π^-p)/dσ/dt(γp→π⁺n)] at high energies and high transverse momenta can be described by calculations based on one-hard-gluon-exchange diagrams.

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 have measured the nuclear transparency of the fundamental process γn→π^-p in ⁴He. These measurements were performed at Jefferson Lab in the photon energy range of 1.6–4.5 GeV and at θ_cm^π=70° and 90°. These measurements are the first of their kind in the study of nuclear transparency in photoreactions. They also provide a benchmark test of Glauber calculations based on traditional models of nuclear physics. The transparency results suggest deviations from the traditional nuclear physics picture. The momentum transfer dependence of the measured nuclear transparency is consistent with Glauber calculations that include the quantum chromodynamics phenomenon of color transparency.

We have measured the differential cross section for the γn→π^-p and γp→π⁺n reactions at θ_c.m.=90° in the photon energy range from 1.1 to 5.5 GeV at Jefferson Lab (JLab). The data at E_γ≳3.3   GeV exhibit a global scaling behavior for both π^- and π⁺ photoproduction, consistent with the constituent counting rule and the existing π⁺ photoproduction data. Possible oscillations around the scaling value are suggested by these new data. The data show enhancement in the scaled cross section at a center-of-mass energy near 2.2 GeV. The cross section ratio of exclusive π^- to π⁺ photoproduction at high energy is consistent with the prediction based on one-hard-gluon-exchange diagrams.

A high precision measurement of the transverse spin-dependent asymmetry A_T^′ in ³He→(e→,e^′) quasielastic scattering was performed in Hall A at Jefferson Lab at values of the squared four-momentum transfer, Q², between 0.1 and 0.6 (GeV/c)². A_T^′ is sensitive to the neutron magnetic form factor, G_Mⁿ. Values of G_Mⁿ at Q²=0.1 and 0.2 (GeV/c)², extracted using Faddeev calculations, were reported previously. Here, we report the extraction of G_Mⁿ for the remaining Q² values in the range from 0.3 to 0.6 (GeV/c)² using a plane-wave impulse approximation calculation. The results are in good agreement with recent precision data from experiments using a deuterium target.

We present the first precision measurement of the spin-dependent asymmetry in the threshold region of ³H→e(e→,e^′) at Q² values of 0.1 and 0.2 (GeV/c)². The agreement between the data and nonrelativistic Faddeev calculations which include both final-state interactions and meson-exchange current effects is very good at Q²  =  0.1 (GeV/c)², while a small discrepancy at Q²  =  0.2 (GeV/c)² is observed.

We have measured the transverse asymmetry A_T^′ in ³He→(e→,e^′) quasielastic scattering in Hall A at Jefferson Laboratory with high precision for Q² values from 0.1 to 0.6 (GeV/c)². The neutron magnetic form factor G_Mⁿ was extracted based on Faddeev calculations for Q²  =  0.1 and 0.2 (GeV/c)² with an experimental uncertainty of less than 2%.

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.

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 A(Q²) structure function in elastic electron-deuteron scattering was measured at six momentum transfers Q² between 0.66 and 1.80 (GeV/c)² in Hall C at Jefferson Laboratory. The scattered electrons and recoil deuterons were detected in coincidence, at a fixed deuteron angle of 60.5°. These new precise measurements resolve discrepancies between older sets of data. They put significant constraints on existing models of the deuteron electromagnetic structure, and on the strength of isoscalar meson exchange currents.

The ( e,e^′p) reaction was studied on targets of C, Fe, and Au at momentum transfers squared Q² of 0.6, 1.3, 1.8, and 3.3 GeV² in a region of kinematics dominated by quasifree electron-proton scattering. Missing energy and missing momentum distributions are reasonably well described by plane wave impulse approximation calculations with Q² and A dependent corrections that measure the attenuation of the final state protons.

Differential cross sections for quasielastic electron scattering on ⁴⁰Ca have been measured at laboratory scattering angles of 45.5°, 90°, and 140° with bombarding energies ranging from 130 to 840 MeV. Transverse and longitudinal response functions have been extracted for momentum transfers from 300 to 500 MeV/c. Contrary to some previously reported results, the total observed longitudinal strength agrees with the relativistic Fermi gas prediction to within ±18%.

Absolute differential cross sections for the threshold electrodisintegration of the deuteron with good resolution were measured at a laboratory scattering angle of 160° for five values of Q² ranging from 8.66 to 42.4 fm^-2. Comparisons of the data averaged over E_np from 0 to 3 MeV and from 0 to10 MeV are made with nonrelativistic meson exchange calculations. These calculations are sensitive to the nucleon electromagnetic form factors, nucleon-nucleon potential, and relativistic effects. The data are also compared with a hybrid quark-hadron model calculation that describes the deuteron as a six-quark cluster for the short range part of the interaction. Some of these calculations can describe the data reasonably well over certain ranges of Q²; however, none of these calculations can accurately describe the data over the entire measured Q² range.

We propose a mechanism for the previously unexplained formation of spanwise cells observed in the vortex street wake of finite-length cylindrical bluff bodies at low Reynolds numbers. It involves the spatial growth of the secondary Eckhaus instability of oblique vortex shedding, controlled by end effects, and leads to a new interpretation of observed wake frequency relations. Predictions of cell size variations and wake patterns are obtained from the phenomenological Ginzburg-Landau model, and good agreement is found with experimental observations.

The tensor polarization of the recoil deuteron in elastic electron-deuteron scattering has been measured at the Bates Linear Accelerator Center at three values of four-momentum transfer Q=3.78, 4.22, and 4.62 fm^-1, corresponding to incident electron energies of 653, 755, and 853 MeV. The scattered electrons and the recoil deuterons were detected in coincidence. The recoil deuterons were transported to a liquid hydrogen target to undergo a second scattering. The angular distribution of the d→-p scattering was measured using a polarimeter. The polarimeter was calibrated in an auxiliary experiment using a polarized deuteron beam at the Laboratoire National Saturne. A Monte Carlo procedure was used to generate interpolated calibration data because the energy spread in the deuteron energies in the Bates experiment spanned the range of deuteron energies in the calibration experiment. The extracted values of t₂₀ are compared to predictions of different theoretical models of the electromagnetic form factors of the deuteron: nonrelativistic and relativistic nucleon-meson dynamics, Skyrme model, quark models, and perturbative quantum chromodynamics. Along with the world data the structure functions A(Q) and B(Q) are used to separate the charge monopole and charge quadrupole form factors of the deuteron. A node in the charge monopole form factor is observed at Q=4.39±0.16 fm^-1.

Inclusive inelastic electron scattering cross sections for ³H and ³He were measured for excitation energies below 18 MeV. Longitudinal and transverse response functions were determined from these cross sections for six values of the three-momentum transfer q in the range 0.88 <q<2.87 fm^-1. A previously observed C0 multipole enhancement near threshold in the two-body channel for ³He is confirmed but is not observed for ³H. The experimental data are found to be in good agreement with two recent calculations of the longitudinal and transverse response functions. The first uses variational ground-state wave functions and the orthogonal correlated states method to describe the two- and three-body breakup channels. The second uses bound and continuum Faddeev wave functions obtained for a simple central potential. Agreement with the data is good for the first model and better for the second. The inclusion of final-state interactions (FSI) in the Faddeev continuum is found to be very important in these threshold breakup kinematics; in many cases inclusion of FSI changes the response functions by factors of two or three giving excellent agreement with the data. The transverse response functions are well reproduced, even though neither model includes meson exchange currents. Ratios of the response functions for the two nuclei are also well described.

Proton propagation in nuclei was studied using the (e,e’p) reaction in the quasifree region. The coincidence (e,e’p) cross sections were measured at an electron angle of 50.4° and proton angles of 50.1°, 58.2°, 67.9°, and 72.9° for ¹²C, ²⁷Al, ⁵⁸Ni, and ¹⁸¹Ta targets at a beam energy of 779.5 MeV. The average outgoing proton energy was 180 MeV. The ratio of the (e,e’p) yield to the simultaneously measured (e,e’) yield was compared to that calculated in the plane-wave impulse approximation and an experimental transmission defined. These experimental transmissions are considerably larger (a factor of ∼2 for ¹⁸¹Ta) than those one would calculate from the free N-N cross sections folded into the nuclear density distribution. A new calculation that includes medium effects (N-N correlations, density dependence of the N-N cross sections and Pauli suppression) accounts for this increase.

The threshold electrodisintegration of the deuteron was measured with good energy resolution to a maximum four-momentum transfer squared of 42 fm^-2. The data are compared with meson exchange calculations that show sensitivity to the choice of exchange-current form factor, the nucleon electromagnetic form factors, and the nucleon-nucleon potential. While conventional theories can give a reasonable description of the data, comparison of the data with quark cluster calculations shows poor agreement.

The tensor polarization t₂₀ of the recoil deuteron in elastic e-d scattering has been measured for three values of four-momentum transfer, q=3.78, 4.22, and 4.62 fm^-1. The data have been used to locate the first node in the charge monopole form factor of the deuteron at q=4.39±0.16 fm^-1. The results for t₂₀ are in reasonable agreement with expectations based on the nucleon-meson description of nuclear dynamics.

The A dependence of the (e,e’p) reaction in the quasifree region has been measured at an average Q² of 0.33 (GeV/c)² for targets of ¹²C, ²⁷Al, ⁵⁸Ni, and ¹⁸¹Ta. The outgoing proton kinetic energy was 180±30 MeV. By comparing the ratio of (e,e’p) coincidence to (e,e’) singles yields, average proton transmissions are obtained for each target. The resulting ‘‘mean free path’’ or, more precisely, the attenuation length for protons in the nucleus is significantly longer than expectations based on the free nucleon-nucleon cross section.

A detailed investigation of the excited states of ¹³C below 10 MeV excitation energy by means of high-resolution electron scattering at momentum transfers between 0.4 and 2.4 fm^-1 has been performed. Additional data taken for excitation energies between 10 and 22 MeV are also selectively discussed. Up to an excitation energy of 12 MeV the data are compared with the results of a shell-model calculation which uses the full 0ħω and 1ħω model spaces together with effective one-body transition operators.

Longitudinal response functions for ³H and ³He have been obtained from inclusive electron scattering cross sections, for 200≤q≤550 MeV/c. The response functions are compared with several theoretical calculations, all of which use exact three-body ground-state wave functions. The response functions are found to be more similar than the calculations predict. When a direct calculation of the Coulomb sum rule is compared with our data, the agreement is good and evidence for two-proton correlations in ³He is clear.

Proton and neutron transition densities in ³⁰Si are examined by a combination of intermediate energy (e,e’) and (p,p’) reactions and mirror electromagnetic transition rates. This analysis is performed for the 2₁⁺ and 2₂⁺ states at 2.234 and 3.499 MeV in ³⁰Si. Electron scattering data are presented for these states. Shell-model calculations for the proton and neutron transition matrix elements and proton transition densities are compared with the electromagnetic results. The proton transition densities are reasonably predicted for the 2₁⁺ state but are not adequately predicted for the 2₂⁺ state. A microscopic coupled-channel calculation of 650 MeV (p,p’) is used to test the shell-model isoscalar transition densities. Given the uncertainties present in the reaction calculation and interaction, the isoscalar density for the 2₁⁺ state is found to be adequate, but the density for the 2₂⁺ state is less accurate. The coupled-channel effect is shown to be important for the 2₂⁺ state. This dependence increases with energy but should be taken into account for an accurate description of (p,p’) reactions at all energies.