arXiv subjects

G. Co'

Publications and source records attributed to G. Co'.

53 records · Page 3Linked to original sources

arXiv2004

Weak response of nuclei

We discuss some differences and similarities between electron and neutrino scattering off atomic nuclei. We find that, in the giant resonance region, the two processes excite different nuclear modes, therefore the weak and the electromagnetic nuclear responses are rather different. In any case, the scattering of electrons and photons is the best guide we have to test the validity of our nuclear models and their prediction power. The experience in describing electromagnetic excitations of the nucleus, suggests that, when the nucleus is excited in the continuum, the re-interaction between the emitted nucleon and the remaining nucleus should not be neglected. A simple model taking into account this final state interaction is proposed, and applied to the neutrino scattering off 16O nucleus.

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arXiv2002

Nuclear re-interaction effects in quasi-elastic neutrino nucleus scattering

The quasi-elastic neutrino-nucleus cross section has been calculated by using a Fermi gas model corrected to consider the re-scattering between the emitted nucleon and the rest nucleus. As an example of the relevance of this effect we show results for the muon production cross section on 16O target.

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arXiv2002

Short-range correlations and meson exchange currents in photonucleon emission

One-nucleon emission processes induced by photon absorption are studied by considering short-range correlations effects. At energies above the giant resonance region the validity of the direct knock-out model has been tested by comparison with continuum Random Phase Approximation results. Nucleon re-scattering effects have been considered by using an optical potential. The role of the electromagnetic convection, magnetization and meson exchange currents has been investigated as a function of both excitation energy and momentum transfer. The short-range correlation effects have been studied by using various correlation functions. We found that the nucleon photo-emission cross section is rather sensitive to the presence of short-range correlations at large values of nucleon emission angle. In this region, however, the effects of meson exchange currents are even larger than those produced by short-range correlations.

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arXiv2001

Short-range correlations in semi-exclusive electron scattering experiments

One-nucleon emission electron scattering experiments are studied with a model that considers short--range correlations up to the first order in the number of correlation lines. The proper normalization of the many-body wave functions requires the evaluation of two- and three-point diagrams, the last ones usually neglected in the literature. When all these diagrams are included the effects of the short-range correlations are rather small. The results of our calculations are compared with experimental data taken on $^{16}$O.

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arXiv2001

A correlated model for lambda-hypernuclei

We study the properties of hypernuclei containing one lambda hyperon in the framework of the correlated basis function theory with Jastrow correlations. Fermi hypernetted chain integral equations are derived and used to evaluate energies and one-body densities of lambda hypernuclei having a doubly closed shell nucleonic core in the jj coupling scheme, from Carbon to Lead. We also study hypernuclei having the least bound neutron substituted by the lambda particle. The semi-realistic Afnan and Tang nucleon-nucleon potential and Bodmer and Usmani lambda-nucleon potential are adopted. The effect of many-body forces are considered by means either of a three body lambda-nucleon-nucleon potential of the Argonne type or of a density dependent modification of the lambda-nucleon interaction, fitted to reproduce the lambda binding energy in nuclear matter. While Jastrow correlations underestimate the attractive contribution of the three body $\la$ interaction, the density dependent potential provides a good description of the lambda binding energies over all the nuclear masses range, in spite of the relative simplicity of the model.

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arXiv2000

One Body Density Matrix, Natural Orbits and Quasi Hole States in 16O and 40Ca

The one body density matrix, momentum distribution, natural orbits and quasi hole states of 16O and 40Ca are analyzed in the framework of the correlated basis function theory using state dependent correlations with central and tensor components. Fermi hypernetted chain integral equations and single operator chain approximation are employed to sum cluster diagrams at all orders. The optimal trial wave function is determined by means of the variational principle and the realistic Argonne v8' two-nucleon and Urbana IX three-nucleon interactions. The correlated momentum distributions are in good agreement with the available variational Monte Carlo results and show the well known enhancement at large momentum values with respect to the independent particle model. Diagonalization of the density matrix provides the natural orbits and their occupation numbers. Correlations deplete the occupation number of the first natural orbitals by more than 10%. The first following ones result instead occupied by a few percent. Jastrow correlations lower the spectroscopic factors of the valence states by a few percent (~1-3%) and an additional ~8-12% depletion is provided by tensor correlations. It is confirmed that short range correlations do not explain the spectroscopic factors extracted from (e,e'p) experiments. 2h-1p perturbative corrections in the correlated basis are expected to provide most of the remaining strength, as in nuclear matter.

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arXiv2000

RPA quasi-elastic responses in infinite and finite nuclear systems

Quasi-elastic responses in nuclear matter and in $^{12}$C and $^{40}$Ca nuclei are calculated in ring approximation to investigate the finite size effects on the electromagnetic quasi-elastic responses. A method to simulate these effects in infinite systems calculations is proposed. The sensitivity of the results to the various terms of the residual interaction is studied. The results of nuclear matter RPA calculations are compared with those obtained in ring approximation to evidence the importance of the exchange terms.

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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

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

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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arXiv1997

Ground state of N=Z doubly closed shell nuclei in CBF theory

The ground state properties of N=Z doubly closed shell nuclei are studied within correlated basis function theory. A truncated version of the Urbana v14 realistic potential, with spin, isospin and tensor components, is adopted, together with state dependent correlations. Fermi hypernetted chain integral equation and single operator chain approximation are used to evaluate density, distribution function and ground state energy of 16O and 40Ca. The results favourably compare with the available, variational MonteCarlo estimates and provide a first substantial check of the accuracy of the cluster summation method for state dependent correlations. We achieve in finite nuclei at least the same level of accuracy in the treatment of non central interactions and correlations as in nuclear matter. This opens the way for a microscopic study of medium heavy nuclei ground state using present days realistic hamiltonians.

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arXiv1996

Effects of state dependent correlations on nucleon density and momentum distributions

The proton momentum and density distributions of closed shell nuclei are calculated within a model treating short--range correlations up to first order in the cluster expansion. The validity of the model is verified by comparing the results obtained with purely scalar correlations with those produced by finite nuclei Fermi Hypernetted Chain calculations. State dependent correlations are used to calculate momentum and density distributions of 12C, 16O, 40Ca, and 48Ca, and the effects of their tensor components are studied.

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arXiv1996

Model calculations of doubly closed shell nuclei in CBF theory III. j-j coupling and isospin dependence

Correlated Basis Function theory and Fermi Hypernetted Chain technique are extended to study medium-heavy, doubly closed shell nuclei in j-j coupling scheme, with different single particle wave functions for protons and neutrons and isospin dependent two-body correlations. Central semirealistic interactions are used. Ground state energies, one-body densities, distribution functions and momentum distributions are calculated for 12C, 16O, 40Ca, 48Ca and 208Pb nuclei. The values of the ground state energies provided by isospin dependent correlations are lower than those obtained with isospin independent correlations. In finite nuclear systems, the two--body Euler equations provide correlation functions variationally more effective than those obtained with the same technique in infinite nuclear matter.

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arXiv1996

Mean-field calculations of quasi-elastic responses in 4He

We present calculations of the quasi-elastic responses functions in 4He based upon a mean-field model used to perform analogous calculations in heavier nuclei. The meson exchange current contribution is small if compared with the results of calculations where short-range correlations are explicitly considered. It is argued that the presence of these correlations in the description of the nuclear wave functions is crucial to make meson exchange current effects appreciable.

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arXiv1995

Short-range Correlations in a CBF description of closed-shell nuclei

The Correlated Basis Function theory (CBF) provides a theoretical framework to treat on the same ground mean-field and short-range correlations. We present, in this report, some recent results obtained using the CBF to describe the ground state properties of finite nuclear systems. Furthermore we show some results for the excited state obtained with a simplified model based on the CBF theory.

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arXiv1995

Mean-field description of nuclear charge density distributions.

We propose a method to extract nuclear charge distributions from elastic electron scattering data based upon a mean field approach. The nuclear charge distributions are generated by solving the Schroedinger equation with a mean-field potential expanded in terms of Hermite functions. The coefficients of the potential are changed in order to obtain the best fit with the cross section data. Applications to the $^{12}C$, $^{16}O$ $^{40}Ca$ and $^{208}Pb$ nuclei are presented.

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arXiv1994

Nuclear Charge Density Distributions from Elastic Electron Scattering Data

The model independent procedure of extracting charge density distributions from elastic electron scattering data is investigated. The charge density distributions are expanded on an orthonormal basis and the parameters of the expansions are fixed by the comparison with the experimental data. Two bases with different analytical properties (Fourier-Bessel and Hermite) are used. This allows us to disentangle the uncertainties coming from the choice of the expansion basis from those intrinsic to the extraction procedure. We design a set of tests to select the number of the expansion coefficients adequate for a proper description of the data. The procedure is applied to elastic data measured on $^{12}$C, $^{40}$Ca and $^{208}$Pb nuclei.

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