Search Results (15 total)

Quadrupole amplitudes in the γ^*N→Δ transition are associated with the issue of nucleon deformation. A search for these small amplitudes has been the focus of a series of measurements undertaken at Bates/MIT by the OOPS Collaboration. We report on results from H(e,e^′p)π⁰ data obtained at Q²=0.070 (GeV/c)² and invariant mass of W=1155 MeV using the out-of-plane detection technique with the OOPS spectrometers. The σ_LT and σ_T+εσ_L response functions were isolated. These results, along with those of previous measurements at W=1172 MeV and Q²=0.127 (GeV/c)², aim in elucidating the interplay between resonant and nonresonant amplitudes.

Measurements of the ²H(e→,e^′p)n reaction were performed with the out-of-plane magnetic spectrometers (OOPS) at the MIT-Bates Linear Accelerator. The longitudinal-transverse, f_LT and f_LT^′, and the transverse-transverse, f_TT, interference responses at a missing momentum of 210 MeV/c were simultaneously extracted in the dip region at Q²  =  0.15  (GeV/c)². In comparison to models of deuteron electrodisintegration, the data clearly reveal strong effects of relativity and final-state interactions and the importance of two-body meson-exchange currents and isobar configurations. We demonstrate that such effects can be disentangled by extracting these responses using the novel out-of-plane technique.

High-precision ¹H(e,e^′p)π⁰ measurements at Q²  =  0.126 (GeV/c)² are reported, which allow the determination of quadrupole amplitudes in the γ^*N→Δ transition; they simultaneously test the reliability of electroproduction models. The derived quadrupole-to-dipole ( I  =  3/2) amplitude ratios, R_SM  =  (-6.5±0.2_stat+sys±2.5_mod)% and R_EM  =  (-2.1±0.2_stat+sys±2.0_mod)%, are dominated by model error. Previous R_SM and R_EM results should be reconsidered after the model uncertainties associated with the method of their extraction are taken into account.

The first measurements of f_LT^′, known as the fifth response function, have been made for the ²H(e→,e^′p) and ¹²C(e→,e^′p) reactions. This response is directly related to the imaginary part of the interference between the transverse and longitudinal nuclear electromagnetic currents. Its observation requires longitudinally polarized electron beams and out-of-plane detection, the latter made possible by the newly developed out-of-plane spectrometer system. The initial measurements were made by using a 560-MeV polarized electron beam and quasielastic kinematics at Q²=3.3 fm^-2. The development of the methodology for out-of-plane physics, and the analysis of the data from the initial experiments are described in detail. The measured fifth response and the related asymmetry in the coincidence cross section are in agreement, albeit with large statistical errors, with the theoretical predictions. Future extensions of the out-of-plane program are also discussed.

We report on a measurement of the tensor analyzing power T₂₀ in elastic electron-deuteron scattering in the range of four-momentum transfer from 1.8 to 3.2 fm^-1. Electrons of 704 MeV were scattered from a polarized deuterium internal target. The tensor polarization of the deuterium nuclei was determined with an ion-extraction system, allowing an absolute measurement of T₂₀. The data are described well by a nonrelativistic calculation that includes the effects of meson-exchange currents.

We report on a first measurement of tensor analyzing powers in quasielastic electron-deuteron scattering at an average three-momentum transfer of 1.7 fm^-1. Data sensitive to the spin-dependent nucleon density in the deuteron were obtained for missing momenta up to 150 MeV/c with a tensor polarized ²H target internal to an electron storage ring. The data are well described by a calculation that includes the effects of final-state interaction, meson-exchange and isobar currents, and leading-order relativistic contributions.

We present a measurement of the induced proton polarization P_n in π⁰ electroproduction on the proton around the Δ resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of W=1231 MeV and Q²=0.126 GeV²/c², respectively. We measured a large induced polarization, P_n=-0.397±0.055±0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model.

The coincidence cross section and the interference structure function, R_LT, were measured for the ¹²C(e,e^′p) ¹¹B reaction at quasielastic kinematics and central momentum transfer of |q→|=400 MeV/c. The measurement was at an opening angle of θ_pq=11°, covering a range in missing energy of E_m=0 to 65 MeV. The R_LT structure function is found to be consistent with zero for E_m>50 MeV, confirming an earlier study which indicated that R_L vanishes in this region. The integrated strengths of the p- and s-shell are compared with a distorted wave impulse approximation (DWIA) calculation. The s-shell strength and shape are also compared with a Hartree Fock–random phase approximation (HF-RPA) calculation. The DWIA calculation does not succeed in giving a consistent description of both the cross section data and the extracted R_LT response for either shell. The HF-RPA calculation describes the data more consistently, which may be due to the inclusion of 2-body currents in this calculation.

Deuterium molecules were obtained by recombination, on a copper surface, of deuterium atoms prepared in specific hyperfine states. The molecules were stored for about 5 ms in an open-ended cylindrical cell, placed in a 23 mT magnetic field, and their tensor polarization was measured by elastic scattering of 704 MeV electrons. The results of the measurements are consistent with the deuterium molecules retaining the tensor polarization of the initial atoms.

We report an absolute measurement of the tensor analyzing powers T₂₀ and T₂₂ in elastic electron-deuteron scattering at a momentum transfer of 1.6 fm^-1. The novel approach of this measurement is the use of a tensor polarized ²H target internal to an electron storage ring, with in situ measurement of the polarization of the target gas. Scattered electrons and recoil deuterons were detected in coincidence with two large acceptance nonmagnetic detectors. The techniques demonstrated have broad applicability to further measurements of spin-dependent electron scattering.

We have separated the longitudinal ( f₀₀), transverse ( f₁₁), and longitudinal-transverse interference ( f₀₁) structure functions in the ²H(e,e^′p)n reaction at |q→|≈400 MeV/c and ω≈110 MeV. A nonrelativistic calculation which includes effects due to final state interactions, meson exchange currents, and isobar configurations agrees with the measured f₁₁ and f₀₁ but overpredicts f₀₀ by 25% ( 2σ). The data are also compared to the results of previous structure function measurements.

We have measured the coincidence d(e→,e′p) reaction in quasielastic scattering, detecting the proton in a noncoplanar geometry. The electron helicity asymmetry A_e and the imaginary part of the longitudinal-transverse interference structure function f_LT^′ have been determined at a four-momentum transfer Q²=3.3 fm^-2. The results are compared with theoretical calculations which use realistic potentials for the NN interaction.

We have measured a new electron scattering observable, the “fifth” structure function, f₀₁^′, which is the imaginary part of the transverse-longitudinal interference response. Its observation requires a longitudinally polarized beam and coincident, out-of-plane particle detection. f₀₁^′ arises from interference between reaction channels and provides an additional means for their disentanglement. In the quasielastic C¹²(e→,e′p) measurements reported here, f₀₁^′ is driven by the interference of the direct knockout and rescattering amplitudes.

The giant multipole resonances in ¹¹⁶Sn have been studied using the (e,e’n) reaction. Data were taken at effective momentum transfers of 0.37, 0.45, and 0.55 fm^-1 and a multipole analysis of the data was performed. The inferred multipole strength functions identify the E2 and E0 resonances as distinct peaks at 12.2 and 17.9 MeV, respectively. The energy-weighted sum-rule strengths for the E2 and E0 resonances, obtained using a Lorentzian fit to the data, are 34±13% and 93±37%. When compared with results from alpha scattering and pion scattering the sum-rule strengths exhibit approximate agreement, but the E0 strength identified in this measurement lies at higher excitation energy, consistent with the trend observed in heavier nuclei. The (e,e’n) data are compared with a continuum random phase approximation (RPA) calculation of the E2 and E0 strengths, and with an open-shell RPA calculation of the E2 strength. Both calculations disagree with the data in the region of the E2 resonance.

The real and imaginary parts of the complex refractive index, ñ(ω)=n(ω)+ik(ω), of arc-evaporated carbon films have been obtained over the range of photon energies ħω from 0.5 to 62.0 eV. Values of k(ω) obtained from transmission measurements in this energy range were combined with values of k(ω) from the literature in the infrared and soft-x-ray regions. A Kramers-Kronig analysis then yielded the values of n(ω). The density of the arc-evaporated carbon films was found to be 1.90±0.05 g cm^-3 by the ‘‘sink-float’’ method, and their thicknesses were determined optically. A sum-rule calculation yielded the effective numbers of valence and core electrons to be 4.2 and 1.8, respectively. The experimental values determined for ñ(ω) have been used to estimate values of the inelastic mean free path Λ(E) for electrons of energy E from 200 to 3000 eV in amorphous carbon. Good agreement is found between Λ(E) and experimentally determined values of electron attenuation length L(E) from the literature.