Abstract:
A study of the effects of meson-exchange currents and isobar configurations in $A(\vec{e},e'\vec{p})B$ reactions is presented. We use a distorted wave impulse approximation (DWIA) model where final-state interactions are treated through a phenomenological optical potential. The model includes relativistic corrections in the kinematics and in the electromagnetic one- and two-body currents. The full set of polarized response functions is analyzed, as well as the transferred polarization asymmetry. Results are presented for proton knock-out from closed-shell nuclei, for moderate to high momentum transfer.

Abstract:
The response functions for the unpolarized $(e,e'p)$ and polarized $(\vec{e},e'p)$ reaction are calculated for medium-heavy nuclei under quasifree conditions. The formalism presented here incorporates two-body currents related to meson-exchange and the $\Delta (1232)$ excitation. The final-state interaction of the outgoing nucleon with the residual nucleus is handled in a HF-RPA formalism. The sensitivity of the results to the two-body currents is discussed for the five structure functions in quasielastic $(\vec{e},e'p)$ scattering off the target nuclei $^{16}O$ and $^{40}Ca$. A selective sensitivity to the two-body currents is obtained in the longitudinal-transverse interference term $W_{LT}$ where two-body currents can explain part of the discrepancy between the impulse-approximation calculations and the data.

Abstract:
The effect of meson and isobar degrees of freedom in A($\vec{e},e'\vec{p}$) and A(e,e'n) is studied for four-momentum transfers Q^2 in the range between 0.2 and 0.8 (GeV/c)^2. The calculations are performed in a non-relativistic framework with explicit (N,\Delta,\pi) degrees-of-freedom. For the whole range of momentum transfers under investigation the relative effect of the meson-exchange and isobar degrees of freedom is significant. At low missing momenta and quasi-elastic conditions, a tendency to reduce the (e,e'p) and (e,e'n) differential cross sections is noticed. The greatest sensitivity is found in the interference structure functions $W_{LT}$ and $W_{TT}$. The recoil polarization observables, on the other hand, are moderately affected by the meson-exchange and $\Delta$-isobar currents.

Abstract:
Nucleon recoil polarization observables in $(e,e'\vec{p})$ reactions are investigated using a semi-relativistic distorted-wave model which includes one- and two-body currents with relativistic corrections. Results for the induced polarization asymmetry are shown for closed-shell nuclei and a comparison with available experimental data for $^{12}$C is provided. A careful analysis of meson exchange currents shows that they may affect significantly the induced polarization for high missing momentum.

Abstract:
Differential (e,e'pp) measurements are presently recognized as a way of studying short-range correlations in finite nuclei. The ($\vec{e},e'\vec{p}p$) and ($\vec{e},e'\vec{p}n$) differential cross section and polarization observables are studied in a microscopic model that accounts for the short-range correlations, outgoing-nucleon distortions, meson-exchange and $\Delta$-isobar currents. It is pointed out that polarization observables represent an attractive alternative for absolute electronuclear two-nucleon knockout measurements. In the polarization transfer $P'_t$ for ($\vec{e},e'\vec{p}p$), the effect of central short-range correlations is predicted to be large while at the same time the final-state interaction effects are small.

Abstract:
Nucleon recoil polarization in electromagnetic reactions with two-nucleon emission is discussed for both (${\vec e},e'{\vec N}N$) and (${\vec \gamma},{\vec N}N$). Numerical results are given for exclusive two-nucleon knockout reactions from $^{16}$O in a theoretical model where final-state interactions, one-body and two-body currents, and the effect of correlations in the initial pair wave function are included.

Abstract:
At present there exists a great interest in the search for evidence of possible modification of the nucleon form factors inside the nuclear medium. Recent theoretical work predict changes in the form factors within the experimental limits. Importantly, the longitudinal to sideways transferred polarization ratio has been identified as being ideally suited for such studies, as these polarization observables are believed to be the least sensitive to most standard nuclear structure uncertainties while their ratio shows a high sensitivity to the ratio of the electric to magnetic form factors. The kinematic regime where the measurements have been undertaken is at relatively high energy and it is clear that relativistic effects in wave functions and operators are essential. In this work we focus on the analysis of polarized $^{16}O(\vec{e},e'\vec{p})^{15}N$ observables. Our aim is to explore a selected set of model dependences that could contaminate any attempt to infer medium modifications, mainly related to the description of FSI and to the role played by relativity.

Abstract:
Measurements of the ${^2}H(\vec{e},e^{\prime}p)n$ reaction were performed using an 800-MeV polarized electron beam at the MIT-Bates Linear Accelerator and with the out-of-plane magnetic spectrometers (OOPS). The longitudinal-transverse, $f_{LT}$ and $f_{LT}^{\prime}$, and the transverse-transverse, $f_{TT}$, interference responses at a missing momentum of 210 MeV/c were simultaneously extracted in the dip region at Q$^2$=0.15 (GeV/c)$^2$. On comparison to models of deuteron electrodisintegration, the data clearly reveal strong effects of relativity and final-state interactions, and the importance of the two-body meson-exchange currents and isobar configurations. We demonstrate that these effects can be disentangled and studied by extracting the interference response functions using the novel out-of-plane technique.

Abstract:
We present the first precision measurement of the spin-dependent asymmetry in the threshold region of $^3\vec{\rm He}(\vec{e},e')$ at $Q^2$-values of 0.1 and 0.2 (GeV/c)$^2$. The agreement between the data and non-relativistic Faddeev calculations which include both final-state interactions (FSI) and meson-exchange currents (MEC) effects is very good at $Q^2$ = 0.1 (GeV/c)$^2$, while a small discrepancy at $Q^2$ = 0.2 (GeV/c)$^2$ is observed.

Abstract:
Electron-induced one-nucleon knock-out observables are computed for moderate to high momentum transfer making use of semi-relativistic expressions for the one-body and two-body meson-exchange current matrix elements. Emphasis is placed on the semi-relativistic form of the $\Delta$-isobar exchange current and several prescriptions for the dynamical-equivalent form of the $\Delta$-propagator are analyzed. To this end, the inclusive transverse response function, evaluated within the context of the semi-relativistic approach and using different prescriptions for the $\Delta$-propagator, is compared with the fully relativistic calculation performed within the scheme of the relativistic Fermi gas model. It is found that the best approximation corresponds to using the traditional static $\Delta$-propagator. These semi-relativistic approaches, which contain important aspects of relativity, are implemented in a distorted wave analysis of quasielastic $(e,e'p)$ reactions. Final state interactions are incorporated through a phenomenological optical potential model and relativistic kinematics is assumed when calculating the energy of the ejected nucleon. The results indicate that meson exchange currents may modify substantially the $TL$ asymmetry for high missing momentum.