[en] The three-body photodisintegration of ³He has been measured, at energies below the pion-production threshold, in kinematic regions where contributions from three-body forces are expected to be maximized, and two-body mechanisms are suppressed. The measured differential cross sections show an enhancement over calculations using one-body and two-body photo-absorption mechanisms only. The inclusion of a two-pion-exchange three-body absorption mechanisms is needed to describe the data. (orig.)

[en] We study coherent η photoproduction from nuclei in a relativistic impulse approximation approach. For the elementary production amplitude we use a standard relativistic parametrization based on a set of four Lorentz- and gauge-invariant amplitudes. The photonuclear amplitude is evaluated without recourse to a nonrelativistic reduction; the full relativistic structure of the amplitude is maintained. On general arguments we show that the coherent process is sensitive to only one of the elementary amplitudes. Moreover, we show that the nuclear structure information is fully contained in the ground-state tensor density. The tensor density is evaluated in a mean-field approximation to the Walecka model and it is shown to be sensitive to relativistic effects. Distortion effects are incorporated through an η-nucleus optical potential that is computed in a simple 'tρ' approximation. copyright 1997 The American Physical Society

[en] We study the nuclear (or A) dependence of the coherent η photoproduction reaction in a relativistic impulse approximation approach. We use a standard relativistic parametrization of the elementary amplitude, based on a set of four Lorentz- and gauge-invariant amplitudes, to calculate the coherent production cross section from ⁴He, ¹²C, and ⁴⁰Ca. In contrast to nonrelativistic treatments, our approach maintains the full relativistic structure of the process. The nuclear structure affects the process through the ground-state tensor density. This density is sensitive to relativistic effects and depends on A in a different manner than the vector density used in nonrelativistic approaches. This peculiar dependence results in ⁴He having a cross section significantly smaller than that of ¹²C emdash in contrast to existent nonrelativistic calculations. Distortion effects are incorporated through an η-nucleus optical potential that is computed in a simple open-quotes tρclose quotes approximation. copyright 1998 The American Physical Society

[en] We study the coherent photoproduction of pseudoscalar mesons particularly of neutral pions placing special emphasis on the various sources that put into question earlier nonrelativistic-impulse-approximation calculations. These include final-state interactions, relativistic effects, off-shell ambiguities, and violations to the impulse approximation. We establish that while distortions play an essential role in the modification of the coherent cross section, the uncertainty in our results due to the various choices of optical-potential models is relatively small (at most 30%). By far the largest uncertainty emerges from the ambiguity in extending the many on-shell-equivalent representations of the elementary amplitude off the mass shell. Indeed, relativistic impulse-approximation calculations that include the same pionic distortions, the same nuclear-structure model, and two sets of elementary amplitudes that are identical on-shell, lead to variations in the magnitude of the coherent cross section by up to factors of 5. Finally, we address qualitatively the assumption of locality implicit in most impulse-approximation treatments, and suggest that the coherent reaction probes in addition to the nuclear density the polarization structure of the nucleus. copyright 1999 The American Physical Society

[en] The three-body photodisintegration of ³He has been measured at photon energies ranging from 90 to 250 MeV, in kinematic regions where three-body forces effects are expected to be maximized, and two-body mechanisms suppressed. The differential cross sections as a function of neutron momentum demonstrate that calculations using only one-body and two-body photoabsorption mechanisms cannot describe the data and that a two-pion-exchange, three-body absorption mechanism is needed to adequately describe the data

[en] We describe the design and commissioning of the photon tagging beamline installed in experimental Hall B at the Thomas Jefferson National Accelerator Facility (JLab). This system can tag photon energies over a range from 20% to 95% of the incident electron energy, and is capable of operation with beam energies up to 6.1 GeV. A single dipole magnet is combined with a hodoscope containing two planar arrays of plastic scintillators to detect energy-degraded electrons from a thin bremsstrahlung radiator. The first layer of 384 partially overlapping small scintillators provides photon energy resolution, while the second layer of 61 larger scintillators provides the timing resolution necessary to form a coincidence with the corresponding nuclear interaction triggered by the tagged photon. The definitions of overlap channels in the first counter plane and of geometric correlation between the two planes are determined using digitized time information from the individual counters. Auxiliary beamline devices are briefly described, and performance results to date under real operating conditions are presented. The entire photon-tagging system has met or exceeded its design goals

[en] Accurate measurements of the p(e→,e'p)π⁰ reaction were performed at Q²=0.127 (GeV/c)² in the Δ resonance energy region. The experiments at the MIT-Bates Linear Accelerator used an 820 MeV polarized electron beam with the out-of-plane magnetic spectrometer system (OOPS). In this Letter we report the first simultaneous determination of both the TL and TL' ('fifth' or polarized) cross sections at low Q² where the pion cloud contribution is predicted to dominate the quadrupole amplitudes (E2 and C2). These are the real and imaginary parts of the transverse-longitudinal interference amplitudes and provide a sensitive determination of the Coulomb quadrupole amplitude and a test of reaction calculations. Comparisons with model calculations are presented. The empirical MAID calculation gives the best overall agreement with this accurate data. The parameters of this model for the values of the resonant multipoles are vertical bar M₁₊(I=3/2) vertical bar =(40.9±0.3)x10^-3/m_π, CMR=C2/M1=-6.5±0.3%, EMR=E2/M1=-2.2±0.9%, where the errors are due to the experimental uncertainties

[en] New, high-precision measurements of the He³(e,e^'p) reaction using the A1 Collaboration spectrometers at the Mainz microtron MAMI are presented. These were performed in antiparallel kinematics at energy transfers below the quasielastic peak, and at a central momentum transfer of 685 MeV/c. Cross sections and distorted momentum distributions were extracted and compared to theoretical predictions and existing data. The longitudinal and transverse behavior of the cross section was also studied. Sizable differences in the cross-section behavior from theoretical predictions based on the plane wave impulse approximation were observed in both the two- and three-body breakup channels. Full Faddeev-type calculations account for some of the observed excess cross-section, but significant differences remain

[en] Over the past decade, experiments which have measured (e,e'p) cross sections out to high missing energies, in kinematics near the quasielastic ridge, have shown an excess strength above and beyond that expected from a DWIA framework. While some of this excess has been shown to be accounted for by meson-exchange currents, there remains a substantial amount of data in which the excess is not yet accounted for and which may be a signature of multinucleon processes. In this talk, the results from many of the recent high missing energy (e,e'p) measurements will be reviewed, and possible interpretations discussed. Some focus will be given to an ongoing experimental program at Mainz to systematically study the high missing energy regime for ³He and ⁴He (MAMI Experiments A1/1-93 and A1/3-96). The future plans for continuation of such studies, including reference to two-nucleon knockout, will be briefly presented in conclusion. (authors)

[en] We have measured the ⁴He(e,e'p)³H reaction at missing momenta of 130-300 MeV/cusing the three-spectrometer facility at the Mainz microtron MAMI. Data were taken in perpendicular kinematics to allow us to determine the response function R_LT and the asymmetry term A_TL. The data are compared to both relativistic and non-relativistic calculations. (orig.)