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Register a new userSemi-relativistic meson exchange currents in (e,e) and (e,ep) reactions
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J.E. Amaro
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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 $(e,ep)$ 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.
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A comparison of impulse approximation calculations for the (e,ep) reaction, based on the Dirac equation and the Schrodinger one is presented. Trivial (kinematics) differences are indicated, as well as how to remove them from the standard nonrelativistic formalism. Signatures of the relativistic approach are found where the enhancement of the lower components (spinor distortion or negative energy contributions) modifies TL observables with respect to the nonrelativistic predictions, what seems to be confirmed by the experiment. Finally, the relativistic approach is used to analyze several experiments for the reaction 16O(e,ep)15N taken at values of Q^2 from 0.2 to 0.8 (GeV/c)^2, not finding a significant Q^2 dependence of the scale factors over this range.
The influence of short-range correlations (SRC) on the triple-coincidence (e,e$$pp) reactions is studied. The non-relativistic model uses a mean-field potential to account for the distortions that the escaping particles undergo. Apart from the SRC, that are implemented through a Jastrow ansatz with a realistic correlation function, we incorporate the contribution from pion exchange and intermediate $Delta _{33}$ currents. The (e,e$$pp) cross sections are predicted to exhibit a sizeable sensitivity to the SRC. The contribution from the two-nucleon breakup channel to the semi-exclusive $^{12}$C(e,e$$p) cross section is calculated in the kinematics of a recent NIKHEF-K experiment. In the semi-exclusive channel, a selective sensitivity in terms of the missing energy and momentum to the SRC is found.
The electron-target-asymmetries A_parallel and A_perpendicular with target spin parallel and perpendicular to the momentum transfer q were measured for both the two-- and three-body breakup of 3He in the 3He(e,ep)-reaction. Polarized electrons were scattered off polarized 3He in the quasielastic regime in parallel kinematics with the scattered electron and the knocked-out proton detected using the Three-Spectrometer-Facility at MAMI. The results are compared to Faddeev calculations which take into account Final State Interactions as well as Meson Exchange Currents. The experiment confirms the prediction of a large effect of Final State Interactions in the asymmetry of the three-body breakup and of an almost negligible one for the two-body breakup.
Short range correlated (SRC) nucleon-nucleon pairs in nuclei are typically studied using measurements of electron-induced hard nucleon-knockout reactions (e.g. $(e,ep)$ and $(e,epN)$), where the kinematics of the knocked-out nucleons are used to infer their initial state prior to the interaction. The validity of this inference relies on our understanding of the scattering reaction, most importantly how rescattering of the detected nucleons (final state interactions or FSI) distort their kinematical distributions. Recent SRC measurements on medium to heavy nuclei have been performed at high-$x_B$ (i.e., anti-parallel kinematics) where calculations of light nuclei indicate that such distortion effects are small. Here we study the impact of FSI on recent $^{12}$C$(e,ep)$ and $^{12}$C$(e,epp)$ measurements using a transport approach. We find that while FSI can significantly distort the measured kinematical distributions of SRC breakup events, selecting high-$x_B$ anti-parallel events strongly suppresses such distortions. In addition, including the effects of FSI improves the agreement between Generalized Contact Formalism-based calculations and data and can help identify those observables that have minimal sensitivity to FSI effects. This result helps confirm the interpretation of experimental data in terms of initial-state momentum distributions and provides a new tool for the study of SRCs using lepton-scattering reactions.
Since a long time electron scattering has been envisaged as a powerful and preferential tool to investigate nuclear properties. In particular, the (e,ep) knockout reaction has provided a wealth of information on the single particle (s.p.) aspects of nuclear structure, on the validity and the limit of the independent particle shell model. The work done for electron scattering is extremely useful also for the analysis and the interpretation of neutrino oscillation experiments, where nuclei are used as neutrino detectors and it is crucial that nuclear effects in neutrino-nucleus interactions are well under control. In this contribution it is discussed if and how the work done for (e,ep) can be exploited for the analysis of neutrino-nucleus scattering data.
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