Final-state interactions in (e,e′p) reactions with polarized nuclei

The cross section for coincidence, quasielastic proton knock-out by electrons from a polarized ³⁹K nucleus is computed in DWIA using an optical potential in describing the wave function of the ejected nucleon. The dependence of the FSI on the initial polarization angles of the nucleus is analyzed and explained in a new, semi-classical picture of the reaction in which the nuclear transparency decreases as a function of the amount of nuclear matter that the proton has to cross, thus providing a method for obtaining detailed information on its mean free path in finite nuclei. We propose a procedure to find the best initial kinematical conditions for minimizing the FSI which will be useful as a guide for future experiments with polarized nuclei.     

Effects of the form factor on exclusive (e, e′p) reactions

For the exclusive (e, e′p) reaction, we discuss the possibility of observing effects due to swollen nucleons in the nuclear medium, such as ⁴⁰Ca and ²⁰⁸Pb, by introducing form factors of the nucleon in the nuclear medium. These form factors include effectively the change of nucleon properties in the nuclear medium. This calculation is performed by using a Dirac-Hartree single particle model for a bound state and a relativistic optical model for a continuum state with inclusion of the electron Coulomb distortion. The effect of the form factor, which increases with higher momentum transfer (q ≥ 400MeV/c) is too small to be discerned from the errors on the available experimental data. But it affects the determination of spectroscopic factors to some extent.     

Correlations and the cross section of exclusive (e,e′p) reactions for 16O

The reduced cross section for exclusive (e, e′p) reactions has been studied in DWIA for the example of the nucleus ¹⁶O using a spectral function containing effects of correlations. The spectral function is evaluated directly for the finite nucleus starting from a realistic nucleon-nucleon interaction within the framework of the Green's function approach. The emphasis is focused on the correlations induced by excitation modes at low energies described within a model-space of shell-model configurations including states up to the sdg shell. Cross sections for the p-wave quasi-hole transitions at low missing energies are presented and compared with the most recent experimental data. In the case of the so-called perpendicular kinematics the reduced cross section derived in DWIA shows an enhancement at high missing momenta as compared to the PWIA result. Furthermore the cross sections for the s- and d-wave quasi-hole transitions are presented and compared to available data at low missing momenta. Also in these cases, which cannot be described in a model without correlations, a good agreement with the experiment is obtained.     

Study of relativistic bound state wave functions in quasielastic (e,e′ p) reactions

The unpolarized response functions of the quasielastic ¹⁶O(e,e′ p)¹⁵N reaction are calculated for three different types of relativistic bound-state wave functions. The wave functions are obtained from relativistic Hartree, relativistic Hartree-Fock and density-dependent relativistic Hartree calculations that reproduce the experimental rms charge radius of ¹⁶O. The sensitivity of the unpolarized response functions to the single-particle structure of the different models is investigated in the relativistic plane-wave impulse approximation. Redistributions of the momentum dependence in the longitudinal and transverse response function can be related to the binding energy of the single-particle states. The interference responses R_LT and R_TT reveal a strong sensitivity to the small component of the relativistic bound-state wave function.     

3He(e,e′pp) and 3He(e,e′pn) reactions at AmPS and MAMI

This paper reports on measurements of the ³He(e,e′pp) reaction measured at AmPS and the ³He(e,e′pn) reaction measured at MAMI. The measurements were performed in similar kinematics to allow a comparison to be made between the cross sections of the two reactions. Results are shown for both reactions together with a comparison for the ³He(e,e′pp) data with continuum Faddeev calculations.     

Multi-scattering effects in (e,e′p) knockout

Multistep direct scattering of the ejectile proton is considered in (e,e′p) reactions in the quasi-free region as an improvement over the usual treatment of final-state interactions by means of an optical potential. The theory is applied to ⁴⁰Ca(e,e′p) as a case example. Important contributions of two- and three-step processes are found at high missing energy and momentum in agreement with the experimental trend.     

Comparison of Monte Carlo simulation and direct multistep scattering theory in (e,e′p) nuclear reactions

Two methods to deal with final state interactions in (e,e′p) reactions in nuclei are compared. One of them uses a Monte Carlo semiclassical approach while the other uses a statistical quantum mechanical approach. The comparison serves to give support to both approaches, showing at the same time their limitations.     

Exclusive (e,e′pp) knockout reactions

Exclusive cross sections of the ¹⁶O(e,e′pp) ¹⁴C knockout reaction are calculated for transitions to the low-lying discrete final states of the residual nucleus. Short-range correlations and two-body currents, due to the excitation of a Δ resonance in the intermediate state, are included in the calculations. Final-state interactions are taken into account by means of phenomenological spin-dependent optical potentials. Recoil-momentum distributions for transitions to states with different angular momentum exhibit different shapes, which are basically determined by the cm orbital angular momentum of the initial proton pair.     

Determination of the neutron electric form factor in the D(e,e′n)p reaction and the influence of nuclear binding

The electric form factor of the neutron G _E,n has been determined at the Mainz Microtron MAMI at the low momentum transfer Q ²= 0.15 (GeV/c)² in a measurement of the recoil polarisation ratio P _x/P_z in the quasifree reaction D(e,e′n)p. At this Q ² the influence of the nuclear binding is strong. A purely kinematical model is used to get some insight into the effect of the initial Fermi momentum distribution of the neutron. The influence of the final state interaction is determined quantitatively by a model of Arenh?vel et al.. After the corresponding corrections a value of G _E,n(0.15 (GeV/c)²) = 0.0481±0.0065_stat±0.0053_syst is obtained. Received: 12 April 1999     

Inclusive (e,e′ N), (e,e′ NN), (e,e′ π), …v reactions in nuclei

We study the inclusive (e, e′ N), (e, e′ NN), (e, e′ π), (e, e′ πN) reactions in nuclei using a Monte Carlo simulation method to treat the multichannel problem of the final state. The input consists of reaction probabilities for the different steps evaluated using microscopical many body methods. We obtain a good agreement with experiment in some channels where there is data and make predictions for other channels which are presently under investigation in several electron laboratories. The comparison of the theoretical results with experiment for several kinematical conditions and diverse channels can serve to learn about different physical processes occurring in the reaction. The potential of this theoretical tool to make prospections for possible experiments, aiming at pinning down certain reaction probabilities, is also emphasized.     

Analysis of factorization in (e,e′p) reactions: A survey of the relativistic plane wave impulse approximation

The issue of factorization within the context of coincidence quasi-elastic electron scattering is revisited. Using a relativistic formalism for the entire reaction mechanism and restricting ourselves to the case of plane waves for the outgoing proton, we discuss the role of the negative-energy components of the bound nucleon wave function.     

Longitudinal-transverse interference structure function in the reaction2H(e,e′ p)

The longitudinal-transverse interference structure functionf ₀₁ has been measured in deuterium electrodisintegration experiments at various values of the four-momentum transferQ ². Data taken at different laboratories and at roughly the same value ofQ ² are shown to be consistent. When compared to non-relativistic impulse-approximation calculations mostf ₀₁ data show an enhancement. A relativistic treatment of the reaction²H(e, e p) removes the discrepancy. More definite evidence for relativistic effects in deuterium electrodisintegration can be obtained at higher values of the recoil momentum. The experimental implications for such²H(e, e p) experiments are discussed as well.Dedicated to Prof. Werner Sandhas on the occasion of his 60th birthday     

Towards spin degrees of freedom in (e,e′p)

The expeprimental (e, ep) program at Mainz is presented. The reaction (e, ep) without spin degrees of freedom is compared with the case when polarization observables are considered. It is demonstrated on some examples how the new information can be used to extend the knowledge of the reaction mechanism and nucleon/nuclear structure. Construction and operation of a focal-plane proton polarimeter at Mainz is described.Lectures held at the Indian-Summer School on Electromagnetic and Weak Interactions of Particles with Nuclei, Sázava, Czechoslovakia, 6–11 September 1992.     

Complication effects in highly-excited state structure of the (γ, n), (e,e′) and (Μ−, γ) reactions

The complication effects in the highly-excited state structure (impurity of the complex configurations of the 2p2h type) of the (, n), (e, e) and (^–, ) nuclear reactions with the⁴⁰Ca nucleus have been studied in terms of the ph-approximation including the following effects: (a) the presence of states of the two particles — two holes type, (b) rigidity loss by nucleus in excited state.     

Relaxation of the analyzing power of the (p, τX) and (p, αX) reactions in the continuum

The observed continuous decrease of the analyzing power for the reactions ($\text{p}$, τX) and ($\text{p}$, αX) on ⁹⁰Zr and ²⁰⁹Bi at E_p = 72 MeV as a function of the excitation energy of the final nucleus is described by a phenomenological relaxation process.     

The influence of the spin-orbital bond on direct photonuclear reactions (γ, n) and (γ, p)

The Courant model for direct photonuclear reactions is modified by using the wave functions of a shell model with spin-orbital bond. It is shown that the introduction of spin-orbital interaction does not influence the cross-section. The changes caused by the spin-orbital bond are apparent only on the angular distribution, which has the Courant forma+bsin² , but with a different value ofb/a, i.e. the anisotropy coefficient. New selection lawsj j, j j±1 are found. The introduction of spin-orbital interaction permits the negative value of the anisotropy coefficient, found in some experimental papers, to be explained in a natural way.

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- - (, n) (, )

: - . , . , - , , a+bsin² , , b/a, . . . j j, j j±1. - , .

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Excerpt from diploma work done at Faculty of Technical and Nuclear Physics in Prague.

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In conclusion the author thanks lecturer J. Kvasnica for the choice of subject and valuable remarks.     

Model independent determination of GEn from the d(e′n)p reaction

Electrodisintegration of vector polarized deuterium by longitudinally polarized electrons offers a possibility for measuring the neutron electric form factor G_En. In principle, the electron asymmetry is essentially proportional to G_En when quasi-free kinematics are employed. We investigate the model dependence of this result by employing several NN force models both with and without meson-exchange and isobar corrections. We find the asymmetry to be remarkably model independent near the neutron quasi-elastic peak.