arXiv subjects

J. E. Amaro

Publications and source records attributed to J. E. Amaro.

At least 145 records · Page 8Linked to original sources

arXiv2002

What Does Free Space Lambda-Lambda Interaction Predict for Lambda-Lambda Hypernuclei?

Data on Lambda-Lambda hypernuclei provide a unique method to learn details on the strangeness S =-2 sector of the baryon-baryon interaction. From the free space Bonn-Julich potentials, determined from data on baryon-baryon scattering in the S=0,-1 channels, we construct an interaction in the S =-2 sector to describe the experimentally known Lambda-Lambda hypernuclei. After including short--range (Jastrow) and RPA correlations, we find masses for these Lambda-Lambda hypernuclei in a reasonable agreement with data, taking into account theoretical and experimental uncertainties. Thus, we provide a natural extension, at low energies, of the Bonn-Julich OBE potentials to the S =-2 channel.

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arXiv2002

Gauge and Lorentz invariant one-pion exchange currents in electron scattering from a relativistic Fermi gas

A consistent analysis of relativistic pionic correlations and meson-exchange currents for electroweak quasielastic electron scattering from nuclei is carried out. Fully-relativistic one-pion-exchange electromagnetic operators are developed for use in one-particle emission electronuclear reactions within the context of the relativistic Fermi gas model. Then the exchange and pionic correlation currents are set up fully respecting the gauge invariance of the theory. Emphasis is placed on the self-energy current which, being infinite, needs to be renormalized. This is achieved starting in the Hartree-Fock framework and then expanding the Hartree-Fock current to first order in the square of the pion coupling constant to obtain a truly, gauge invariant, one-pion-exchange current. The model is applied to the calculation of the parity-conserving (PC) and parity-violating (PV) inclusive responses of nuclei. Interestingly, in the pionic correlations terms exist which arise uniquely from relativity, although their impact on the responses is found to be modest.

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arXiv2001

Electron helicity-dependence in (e,e'p) reactions with polarized nuclei and the fifth response function

The cross section for electron-induced proton knockout reactions with polarized beam and target is computed in a DWIA model. The analysis is focused on the electron helicity-dependent (HD) piece of the cross section. For the case of in-plane emission a new symmetry is found in the HD cross section, which takes opposite values for specific opposing pairs of nuclear orientations, explaining why the fifth response (involving polarized electrons, but unpolarized nuclei) is zero for this choice of kinematics. Even when this symmetry breaks down in the case of nucleon emission out of the scattering plane, it is shown that it is still present in the separate response functions. A physical explanation of these results is presented using a semi-classical model of the reaction based on the PWIA and on the concept of a nucleon orbit with its implied expectation values of position and spin.

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arXiv2001

Consistent one-pion exchange currents in electron scattering from a relativistic Fermi gas

A set of fully relativistic one-pion-exchange electromagnetic operators is developed for use in one-particle emission reactions induced by electrons. To preserve the gauge invariance of the theory additional pionic correlation operators are required beyond the usual meson-exchange current operators. Of these, in the present work emphasis is placed on the self-energy current which is infinite and needs to be renormalized. The renormalized current is expanded to first order to obtain a genuine one-pion-exchange contribution to the inclusive responses.

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arXiv2001

Effects of Short-Range Correlations in (e,e'p) reactions and nuclear overlap functions

A study of the effects of short-range correlations over the (e,e'p) reaction for low missing energy in closed shell nuclei is presented. We use correlated, quasi-hole overlap functions extracted from the asymptotic behavior of the one-body density matrix, containing central correlations of Jastrow type, up to first-order in a cluster expansion, and computed in the very high asymptotic region, up to 100 fm. The method to extract the overlap functions is checked in a simple shell model, where the exact results are known. We find that the single-particle wave functions of the valence shells are shifted to the right due to the short-range repulsion by the nuclear core. The corresponding spectroscopic factors are reduced only a few percent with respect to the shell model. However, the (e,e'p) response functions and cross sections are enhanced in the region of the maximum of the missing momentum distribution due to short-range correlations.

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arXiv2001

Relativistic pionic effects in quasielastic electron scattering

The impact of relativistic pionic correlations and meson-exchange currents on the response functions for electromagnetic quasielastic electron scattering from nuclei is studied in detail. Results in first-order perturbation theory are obtained for one-particle emission electronuclear reactions within the context of the relativistic Fermi gas model. Improving upon previous analyses where non-relativistic reductions of the currents were performed, here a fully relativistic analysis in which both forces and currents are treated consistently is presented. Lorentz covariance is shown to play a crucial role in enforcing the gauge invariance of the theory. Effects stemming uniquely from relativity in the pionic correlations are identified and, in particular, a comprehensive study of the self-energy contributions and of the currents associated with the pion is presented. First- and second-kind scaling for high momentum transfer is investigated.

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arXiv1999

Final-State Interaction in Electron Scattering by Polarized Nuclei

The cross section for the reaction $^{39}\vec{\rm K}(e,e'p)^{38}$Ar is computed, and the dependence of the FSI on the initial polarization angles of the nucleus $^{39}$K is analyzed. The results are explained in a semi-classical picture of the reaction. This procedure allows us to find the best initial kinematical conditions for minimizing the FSI.

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arXiv1999

Electron scattering with a short-range correlation model

The inclusive electromagnetic responses in the quasi-elastic region are calculated with a model which considers the terms of the cluster expansion containinga single correlation line. The validity of this model is studied by comparing, in nuclear matter, its results with those of a complete calculation. Results in finite nuclei for both one-and two-nucleon emission are presented.

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arXiv1999

Nuclear currents based on the integral form of the continuity equation

We present an approach to obtain new forms of the nuclear electromagnetic current, which is based on an integral form of the continuity equation. The procedure can be used to restore current conservation in model calculations in which the continuity equation is not verified. Besides, it provides, as a particular result, the so-called Siegert's form of the nuclear current, first obtained by Friar and Fallieros by extending Siegert's theorem to arbitrary values of the momentum transfer. The new currents are explicitly conserved and permit a straightforward analysis of their behavior at both low and high momentum transfers. The results are illustrated with a simple nuclear model which includes a harmonic oscillator mean potential.

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arXiv1999

Quaiselastic scattering from relativistic bound nucleons: Transverse-Longitudinal response

Predictions for electron induced proton knockout from the $p_{1/2}$ and $p_{3/2}$ shells in $^{16}$O are presented using various approximations for the relativistic nucleonic current. Results for the differential cross section, transverse-longitudinal response ($R_{TL}$) and left-right asymmetry $A_{TL}$ are compared at $|Q^2|=0.8$ (GeV/c)$^2$ corresponding to TJNAF experiment 89-003. We show that there are important dynamical and kinematical relativistic effects which can be tested by experiment.

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arXiv1999

Relativistic effects in electromagnetic nuclear responses in the quasi-elastic delta region

A new non-relativistic expansion in terms of the nucleon's momentum inside nuclear matter of the current for isobar electro-excitation from the nucleon is performed. Being exact with respect to the transferred energy and momentum, this yields new current operators which retain important aspects of relativity not taken into account in the traditional non-relativistic reductions. The transition current thus obtained differs from the leading order of the traditional expansion by simple multiplicative factors. These depend on the momentum and energy transfer and can be easily included together with relativistic kinematics in non-relativistic, many-body models of isobar electro-excitation in nuclei. The merits of the new current are tested by comparing with the unexpanded electromagnetic nuclear responses in the isobar peak computed in a relativistic Fermi gas framework. The sensitivity of the relativistic responses to the isobar's magnetic, electric and Coulomb form factors and the finite width of the isobar is analyzed.

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arXiv1999

Analysis of Meson Exchange and Isobar Currents in (e,e'p) Reactions from O-16

An analysis of the effects of meson exchange and isobar currents in exclusive (e,e'p) processes from O-16 under quasi-free kinematics is presented. A model that has probed its feasibility for inclusive quasi-elastic (e,e') processes is considered. Sensitivity to final state interactions between the outgoing proton and the residual nucleus is discussed by comparing the results obtained with phenomenological optical potentials and a continuum nuclear shell-model calculation. The contribution of the meson-exchange and isobar currents to the response functions is evaluated and compared to previous calculations, which differ notably from our results. These two-body contributions cannot solve the puzzle of the simultaneous description of the different responses experimentally separated. Copyright 1999 by The American Physical Society

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arXiv1999

Mean field description of electron induced quasi-elastic excitation in nuclei

The validity of the approximations done in the mean field description of the quasi-elastic excitation of medium-heavy nuclei is discussed. A test of the reliability of the plane wave Born approximation is presented. The uncertainty related to the choice of the electromagnetic nucleon form factors is discussed. The effects produced by the meson exchange currents generated by the exchange of a single pion are studied, as well as the need of including relativistic corrections to the one-body currents. The results of the continuum shell model and those of the Fermi gas model are compared and the need of treating the emitted nucleon within a relativistic framework is studied. We analyze the results of Random Phase Approximation calculations not only in terms of their effects on the mean field responses, but also in terms of the theoretical consistency in the choice of the effective residual interaction. The role of the final state interaction is investigated. A microscopic justification of the use of complex optical potential to describe the final state interaction is provided. The effects of the final state interaction on the mean field responses are studied with a sum rule conserving model. A comparison with the experimental data measured in the C12 and Ca40 nuclei is shown.

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arXiv1998

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 K39 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.

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arXiv1998

Relativistic Effects in Electromagnetic Meson-Exchange Currents for One-Particle Emission Reactions

Following recent studies of non-relativistic reductions of the single-nucleon electromagnetic current operator, here we extend the treatment to include meson exchange current operators. We focus on one-particle emission electronuclear reactions. In contrast to the traditional scheme where approximations are made for the transferred momentum, transferred energy and momenta of the initial-state struck nucleons, we treat the problem exactly for the transferred energy and momentum, thus obtaining new current operators which retain important aspects of relativity not taken into account in the traditional non-relativistic reductions. We calculate the matrix elements of our current operators between the Fermi sphere and a particle-hole state for several choices of kinematics. We present a comparison between our results using approximate current operators and those obtained using the fully-relativistic operators, as well as with results obtained using the traditional non-relativistic current operators.

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arXiv1998

Inclusive quasielastic scattering of polarized electrons from polarized nuclei

The inclusive quasielastic response functions that appear in the scattering of polarized electrons from polarized nuclei are computed and analyzed for several closed-shell-minus-one nuclei with special attention paid to 39K. Results are presented using two models for the ejected nucleon --- when described by a distorted wave in the continuum shell model or by a plane wave in PWIA with on- and off-shell nucleons. Relativistic effects in kinematics and in the electromagnetic current have been incorporated throughout. Specifically, the recently obtained expansion of the electromagnetic current in powers only of the struck nucleon's momentum is employed for the on-shell current and the effects of the first-order terms (spin-orbit and convection) are compared with the zeroth-order (charge and magnetization) contributions. The use of polarized inclusive quasielastic electron scattering as a tool for determining near-valence nucleon momentum distributions is discussed.

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arXiv1998

A model of short-range correlations in the charge response

The validity of a model treating the short-range correlations up to the first order is studied by calculating the charge response of an infinite system and comparing the obtained results with those of a Fermi Hypernetted Chain calculation.

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arXiv1998

Spin Observables in Coincidence Electron Scattering from Nuclei I: Reduced Response Functions

A theoretical description of nucleon knockout reactions initiated by polarized electron scattering from polarized nuclei is presented. Explicit expressions for the complete set of reduced response functions (independent of the polarization angle) that can be experimentally obtained assuming plane waves for the electron are given in a general multipole expansion. The formalism is applied to the particular case of closed-shell-minus-one nuclei using two models for the ejected nucleon, including the final-state interaction phenomenologically with a complex optical potential and in the factorized plane-wave impulse approximation. Relativistic effects in the kinematics and in the electromagnetic current are incorporated throughout --- specifically a new expansion of the electromagnetic current in powers only of the struck nucleon momentum is employed. Results are presented for the nucleus 39K.

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