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E. Moya de Guerra

Publications and source records attributed to E. Moya de Guerra.

At least 37 records · Page 2Linked to original sources
arXiv2011

Scaling Function, Spectral Function and Nucleon Momentum Distribution in Nuclei

The link between the scaling function extracted from the analysis of (e,e') cross sections and the spectral function/momentum distribution in nuclei is revisited. Several descriptions of the spectral function based on the independent particle model are employed, together with the inclusion of nucleon correlations, and effects of the energy dependence arising from the width of the hole states are investigated. Although some of these approaches provide rough overall agreement with data, they are not found to be capable of reproducing one of the distinctive features of the experimental scaling function, namely its asymmetry. However, the addition of final-state interactions, incorporated in the present study using either relativistic mean field theory or via a complex optical potential, does lead to asymmetric scaling functions in accordance with data. The present analysis seems to indicate that final-state interactions constitute an essential ingredient and are required to provide a proper description of the experimental scaling function.

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arXiv2010

Relativistic description of 3He(e,e'p)2H

The Relativistic Distorted-Wave Impulse Approximation is used to describe the $^3$He($e,e^\prime p$)$^2$H process. We describe the $^3$He nucleus within the adiabatic hyperspherical expansion method with realistic nucleon-nucleon interactions. The overlap between the $^3$He and the deuteron wave functions can be accurately computed from a three-body calculation. The nucleons are described by solutions of the Dirac equation with scalar and vector (S-V) potentials. The wave function of the outgoing proton is obtained by solving the Dirac equation with a S-V optical potential fitted to elastic proton scattering data on the residual nucleus. Within this theoretical framework, we compute the cross section of the reaction and other observables like the transverse-longitudinal asymmetry, and compare them with the available experimental data measured at JLab.

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arXiv2010

Theoretical mean field and experimental occupation probabilities in the double beta decay system 76Ge to 76Se

Usual Woods-Saxon single particle levels with BCS pairing are not able to reproduce the experimental occupation probabilities of the proton and neutron levels 1p_{3/2}, 1p_{1/2}, 0f_{5/2}, 0g_{9/2} in the double-beta decay system 76Ge to 76Se. Shifting down the 0g_{9/2} level by hand can explain the data but it is not satisfactory. Here it is shown that a selfconsistent Hartree-Fock+BCS approach with experimental deformations for 76Ge and 76Se may decisively improve the agreement with the recent data on occupation probabilities by Schiffer et al. and Kay et al. Best agreement with available data on 76Ge and 76Se, as well as on neighbor isotopes, is obtained when the spin-orbit strength for neutrons is allowed to be larger than that for protons. The two-neutrino double-beta decay matrix element is also shown to agree with data.

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arXiv2009

Momentum distributions in medium and heavy exotic nuclei

We study nucleon momentum distributions of even-even isotopes of Ni, Kr, and Sn in the framework of deformed self-consistent mean-field Skyrme HF+BCS method, as well as of theoretical correlation methods based on light-front dynamics and local density approximation. The isotopic sensitivities of the calculated neutron and proton momentum distributions are investigated together with the effects of pairing and nucleon-nucleon correlations. The role of deformation on the momentum distributions in even-even Kr isotopes is discussed. For comparison, the results for the momentum distribution in nuclear matter are also presented.

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arXiv2009

Nuclear isospin mixing and elastic parity-violating electron scattering

The influence of nuclear isospin mixing on parity-violating elastic electron scattering is studied for the even-even, N=Z nuclei 12C, 24Mg, 28Si, and 32S. Their ground-state wave functions have been obtained using a self-consistent axially-symmetric mean-field approximation with density-dependent effective two-body Skyrme interactions. Some differences from previous shell-model calculations appear for the isovector Coulomb form factors which play a role in determining the parity-violating asymmetry. To gain an understanding of how these differences arise, the results have been expanded in a spherical harmonic oscillator basis. Results are obtained not only within the plane-wave Born approximation, but also using the distorted-wave Born approximation for comparison with potential future experimental studies of parity-violating electron scattering. To this end, for each nucleus the focus is placed on kinematic ranges where the signal (isospin-mixing effects on the parity-violating asymmetry) and the experimental figure-of-merit are maximized. Strangeness contributions to the asymmetry are also briefly discussed, since they and the isospin mixing contributions may play comparable roles for the nuclei being studied at the low momentum transfers of interest in the present work.

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arXiv2009

Longitudinal and Transverse Scaling Functions within the Coherent Density Fluctuation Model

We extend our previous description of the superscaling phenomenon in inclusive electron scattering within the Coherent Density Fluctuation Model (CDFM). This model is a natural extension to finite nuclei of the Relativistic Fermi Gas Model (RFG) within which the scaling variable $ψ^{\prime}$ was introduced. In this work we propose a new modified CDFM approach to calculate the total, longitudinal and transverse scaling functions built up from the hadronic tensor and the longitudinal and transverse response functions in the RFG. We test the superscaling behavior of the new CDFM scaling functions by calculating the cross sections of electron scattering (in QE- and $Δ$-region for nuclei with $12\leq A\leq208$ at different energies and angles) and comparing to available experimental data. The new modified CDFM approach is extended to calculate charge-changing neutrino and antineutrino scattering on $^{12}$C at 1 GeV incident energy.

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arXiv2008

Single and low-lying states dominance in two-neutrino double-beta decay

A theoretical analysis of the single-state dominance hypothesis for the two-neutrino double-beta decay rates is performed on the examples of the double-beta decays of 100Mo, 116Cd, and 128Te. We also test the validity of an extended low-lying-state dominance that takes into account the contributions of the low-lying excited states in the intermediate nucleus to the double-beta decay rates. This study has been accomplished for all the double-beta emitters for which we have experimental information on their half-lives. The theoretical framework is a proton-neutron quasiparticle random-phase approximation based on a deformed Skyrme Hartree-Fock mean field with pairing correlations. Our calculations indicate that there are not clear evidences for single- or low-lying-state dominance in the two-neutrino double-beta decay. Finally, we investigate the single electron energy distributions of the outgoing electrons in the double-beta decay processes with an exact treatment of the energy denominators, which could help to a more comprehensive analysis of NEMO-3 data.

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arXiv2008

Remarks on monopole charge properties within the Generalized Coherent State Model

The Generalized Coherent State Model, proposed previously for a unified description of magnetic and electric collective properties of nuclear systems, is used to study the ground state band charge density as well as the E0 transitions from $0^+_β$ to $0^+_g$. The influence of the nuclear deformation and of angular momentum projection on the charge density is investigated. The monopole transition amplitude has been calculated for ten nuclei. The results are compared with some previous theoretical studies and with the available experimental data. Our results concerning angular momentum projection are consistent with those of previous microscopic calculations for the ground state density. The calculations for the E0 transitions agree quite well with the experimental data. Issues like how the shape transitions or shape coexistence are reflected in the $ρ(E0)$ behavior are also addressed.

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arXiv2008

Phenomenological description of the states $0^+$ and $2^+$ in some even-even nuclei

A sixth-order quadrupole boson Hamiltonian is used to describe the states $0^+$ and $2^+$ identified in several nuclei by various types of experiments. Two alternative descriptions of energy levels are proposed. One corresponds to a semi-classical approach of the model Hamiltonian while the other one provides the exact eigenvalues. Both procedures yield close formulas for energies. The first procedure involves four parameters, while the second involves a compact formula with five parameters. In each case the parameters are fixed by a least-square fit procedure. Applications are performed for eight even-even nuclei. Both methods yield results which are in a surprisingly good agreement with the experimental data. We give also our predicted reduced transition probabilities within the two approaches, although the corresponding experimental data are not yet available.

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arXiv2008

Properties of Nucleon Resonances by means of a Genetic Algorithm

We present an optimization scheme that employs a Genetic Algorithm (GA) to determine the properties of low-lying nucleon excitations within a realistic photo-pion production model based upon an effective Lagrangian. We show that with this modern optimization technique it is possible to reliably assess the parameters of the resonances and the associated error bars as well as to identify weaknesses in the models. To illustrate the problems the optimization process may encounter, we provide results obtained for the nucleon resonances $Δ$(1230) and $Δ$(1700). The former can be easily isolated and thus has been studied in depth, while the latter is not as well known experimentally.

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arXiv2008

Superscaling and Charge-Changing Neutrino Scattering from Nuclei in the $\boldsymbol Δ$-Region beyond the Relativistic Fermi Gas Model

The superscaling analysis using the scaling function obtained within the coherent density fluctuation model is extended to calculate charge-changing neutrino and antineutrino scattering on $^{12}$C at energies from 1 to 2 GeV not only in the quasielastic but also in the delta excitation region. The results are compared with those obtained using the scaling functions from the relativistic Fermi gas model and from the superscaling analysis of inclusive scattering of electrons from nuclei.

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arXiv2008

Crossing symmetry and phenomenological widths in effective Lagrangian models of the pion photoproduction process

We investigate the importance of crossing symmetry in effective field models and the effects of phenomenological nucleon resonance widths on the paradigmatic case of pion photoproduction. We use reaction models containing four star resonances up to 1.8 Gev ($Δ$(1232), N(1440), N(1520), N(1535), $Δ$(1620), N(1650), $Δ$(1700), and N(1720)) with different prescriptions for crossed terms and widths, to fit the latest world database on pion photoproduction. We compare $χ^2$ results from selected multipoles and fits. The $χ^2$ is highly dependent on the fulfillment of crossing symmetry and the inclusion of $u$ channels.

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arXiv2007

Nuclear skin emergence in Skyrme deformed Hartree-Fock calculations

A study of the charge and matter densities and the corresponding rms radii for even-even isotopes of Ni, Kr, and Sn has been performed in the framework of deformed self-consistent mean field Skyrme HF+BCS method. The resulting charge radii and neutron skin thicknesses of these nuclei are compared with available experimental data, as well as with other theoretical predictions. The formation of a neutron skin, which manifests itself in an excess of neutrons at distances greater than the radius of the proton distribution, is analyzed in terms of various definitions. Formation of a proton skin is shown to be unlikely. The effects of deformation on the neutron skins in even-even deformed nuclei far from the stability line are discussed.

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arXiv2007

Eta Photoproduction as a Test of the Extended Chiral Symmetry

We analyze the $γp \to ηp$ process from threshold up to 1.2 GeV, employing an effective Lagrangian approach that allows for a mixing of eta couplings of pseudoscalar and pseudovector nature. The mixing ratio of the couplings may serve as a quantitative estimation of the $SU_L(3)\times SU_R(3)$ extended chiral symmetry violation in this energy regime. The data analyzed (differential cross sections and asymmetries) show a preference for the pseudoscalar coupling -- 91% of pseudoscalar coupling component for the best fit. We stress that a more conclusive answer to this question requires a more complete electromagnetic multipole database than the presently available one.

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arXiv2007

Superscaling and Neutral Current Quasielastic Neutrino-Nucleus Scattering beyond the Relativistic Fermi Gas Model

The superscaling analysis is extended to include quasielastic (QE) scattering via the weak neutral current of neutrinos and antineutrinos from nuclei. The scaling function obtained within the coherent density fluctuation model (used previously in calculations of QE inclusive electron and charge-changing (CC) neutrino scattering) is applied to neutral current neutrino and antineutrino scattering with energies of 1 GeV from $^{12}$C with a proton and neutron knockout (u-channel inclusive processes). The results are compared with those obtained using the scaling function from the relativistic Fermi gas model and the scaling function as determined from the superscaling analysis (SuSA) of QE electron scattering.

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arXiv2007

Superscaling in dilute Fermi gas and its relation to general properties of the nucleon momentum distribution in nuclei

The superscaling observed in inclusive electron scattering is described within the dilute Fermi gas model with interaction between the particles. The comparison with the relativistic Fermi gas (RFG) model without interaction shows an improvement in the explanation of the scaling function $f(ψ')$ in the region $ψ' < -1$, where the RFG result is $f(ψ') = 0$. It is found that the behavior of $f(ψ')$ for $ψ' < -1$ depends on the particular form of the general power-law asymptotics of the momentum distribution $n(k)\sim 1/ k^{4+m}$ at large $k$. The best agreement with the empirical scaling function is found for $m\simeq 4.5$ in agreement with the asymptotics of $n(k)$ in the coherent density fluctuation model where $m = 4$. Thus, superscaling gives information about the asymptotics of $n(k)$ and the NN forces.

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