arXiv subjects

J. E. Amaro

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

At least 127 records · Page 7Linked to original sources

arXiv2005

Semi-relativistic description of quasielastic neutrino reactions and superscaling in a continuum shell model

The so-called semi-relativistic expansion of the weak charged current in powers of the initial nucleon momentum is performed to describe charge-changing, quasielastic neutrino reactions $(ν_μ,μ^-)$ at intermediate energies. The quality of the expansion is tested by comparing with the relativistic Fermi gas model using several choices of kinematics of interest for ongoing neutrino oscillation experiments. The new current is then implemented in a continuum shell model together with relativistic kinematics to investigate the scaling properties of $(e,e')$ and $(ν_μ,μ^-)$ cross sections.

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arXiv2005

Equivalence between local Fermi gas and shell models in inclusive muon capture from nuclei

Motivated by recent studies of inclusive neutrino nucleus processes and muon capture within a correlated local Fermi gas model (LFG), we discuss the relevance of nuclear finite size effects in these reactions at low energy, in particular for muon capture. To disentangle these effects from others coming from the reaction dynamics we employ here a simple uncorrelated shell model that embodies the typical finite size content of the problem. The integrated decay widths of muon atoms calculated with this shell model are then compared for several nuclei with those obtained within the uncorrelated LFG, using in both models exactly the same theoretical ingredients and parameters. We find that the two predictions are in quite good agreement, within 1--7%, when the shell model density and the correct energy balance is used as input in the LFG calculation. The present study indicates that, despite the low excitation energies involved in the reaction, integrated inclusive observables, like the total muon capture width, are quite independent of the fine details of the nuclear wave functions.

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arXiv2005

Inclusive Quasi-Elastic Neutrino Reactions

The Quasi-Elastic contribution of the nuclear inclusive electron scattering model developed in Nucl. Phys. A 627 (1997) 543 is extended to the study of electroweak Charged Current induced nuclear reactions, at intermediate energies of interest for future neutrino oscillation experiments. The model accounts for long range nuclear (RPA) correlations, Final State Interaction and Coulomb corrections. RPA correlations are shown to play a crucial role in the whole range of neutrino energies, up to 500 MeV, studied in this work. Predictions for inclusive muon capture for different nuclei, and for the reactions $^{12}$C$(ν_μ,μ^-)X$ and $^{12}$C$(ν_e,e^-)X$ near threshold are also given.

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arXiv2005

A Heavy Quark Symmetry Approach to Baryons

We evaluate different properties of baryons with a heavy c or b quark. The use of Heavy Quark Symmetry (HQS) provides with an important simplification of the non relativistic three body problem which can be solved by means of a simple variational approach. This scheme is able to reproduce previous results obtained with more involved Faddeev calculations. The resulting wave functions are parametrized in a simple manner, and can be used to calculate further observables.

hep-ph↗

arXiv2004

Using Electron Scattering Superscaling to predict Charge-changing Neutrino Cross Sections in Nuclei

Superscaling analyses of few-GeV inclusive electron scattering from nuclei are extended to include not only quasielastic processes, but now also into the region where $Δ$-excitation dominates. It is shown that, with reasonable assumptions about the basic nuclear scaling function extracted from data and information from other studies of the relative roles played by correlation and MEC effects, the residual strength in the resonance region can be accounted for through an extended scaling analysis. One observes scaling upon assuming that the elementary cross section by which one divides the residual to obtain a new scaling function is dominated by the $N\toΔ$ transition and employing a new scaling variable which is suited to the resonance region. This yields a good representation of the electromagnetic response in both the quasielastic and $Δ$ regions. The scaling approach is then inverted and predictions are made for charge-changing neutrino reactions at energies of a few GeV, with focus placed on nuclei which are relevant for neutrino oscillation measurements. For this a relativistic treatment of the required weak interaction vector and axial-vector currents for both quasielastic and $Δ$-excitation processes is presented.

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arXiv2004

Final-state interaction and recoil polarization in (e,e'p) reactions: comparison with the polarized target case

A study of the total cross section for polarized proton knockout in $(e,e'{\vec p})$ reactions is carried out for the closed-shell nucleus $^{40}$Ca. The dependence of FSI effects on polarization observables viewed as functions of the nucleon polarization angles is analyzed and interpreted within the basis of a semi-classical model for the orbit of the struck nucleon and trajectory of the ejected nucleon. A comparison with the case of a $^{39}$K polarized target and unpolarized protons is performed.

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arXiv2004

Skewed recoil polarization in (e,e'p) reactions from polarized nuclei

The general formalism describing $\vec{A}(\vec{e},e'\vec{p})B$ reactions, involving polarization of the electron beam, target and ejected proton, is presented within the framework of the relativistic plane wave impulse approximation for medium and heavy nuclei. It is shown that the simultaneous measurement of the target and ejected proton polarization can provide new information which is not contained in the separate analysis of the $\vec{A}(\vec{e},e'p)B$ and $A(\vec{e},e'\vec{p})B$ reactions. The polarization transfer mechanism in which the electron interacts with the initial nucleon carrying the target polarization, making the proton exit with a fractional polarization in a different direction, is referred to here as ``skewed polarization''. The new observables characterizing the process are identified, and written in terms of polarized response functions and asymmetries which are of tensor nature. The corresponding half-off-shell single-nucleon responses are analyzed using different prescriptions for the electromagnetic vertex and for different kinematics. Numerical predictions are presented for selected perpendicular and parallel kinematics in the case of $^{39}$K as polarized target.

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arXiv2004

Inclusive Quasi-Elastic Charged-Current Neutrino-Nucleus Reactions

The Quasi-Elastic (QE) contribution of the nuclear inclusive electron scattering model developed in Nucl. Phys. A627 (1997) 543 is extended to the study of electroweak Charged Current (CC) induced nuclear reactions, at intermediate energies of interest for future neutrino oscillation experiments. The model accounts for, among other nuclear effects, long range nuclear (RPA) correlations, Final State Interaction (FSI) and Coulomb corrections. Predictions for the inclusive muon capture in $^{12}$C and the reaction $^{12}$C $(ν_μ,μ^-)X$ near threshold are also given. RPA correlations are shown to play a crucial role and their inclusion leads to one of the best existing simultaneous description of both processes, with accuracies of the order of 10-15% per cent for the muon capture rate and even better for the LSND measurement.

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arXiv2004

Nuclear Many-Body Theory of Electroweak Interactions with Nuclei at Intermediate Energies

The Quasi-Elastic (QE) contribution of the nuclear inclusive electron model developed in reference \cite{GNO97} is extended to the study of electroweak Charged Current (CC) induced nuclear reactions at intermediate energies of interest for future neutrino oscillation experiments. Long range nuclear (RPA) correlations, Final State Interaction (FSI) and Coulomb corrections are included within the model. RPA correlations are shown to play a crucial role in the whole range of neutrino energies, up to 500 MeV, studied in this work. Predictions for inclusive muon capture for different nuclei through the Periodic Table and for the reactions $^{12}$C $(ν_μ,μ^-)X$ and $^{12}$C $(ν_e,e^-)X$ near threshold are also given.

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arXiv2004

Induced Nucleon Polarization and Meson-Exchange Currents in (e,e'p) Reactions

Nucleon recoil polarization observables in $(e,e'\vec{p})$ reactions are investigated using a semi-relativistic distorted-wave model which includes one- and two-body currents with relativistic corrections. Results for the induced polarization asymmetry are shown for closed-shell nuclei and a comparison with available experimental data for $^{12}$C is provided. A careful analysis of meson exchange currents shows that they may affect significantly the induced polarization for high missing momentum.

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arXiv2004

Charmed and Bottom Baryons: a Variational Approach based on Heavy Quark Symmetry

The use of Heavy Quark Symmetry to study bottom and charmed baryons leads to important simplifications of the non-relativistic three body problem, which turns out to be easily solved by a simple variational ansatz. Our simple scheme reproduces previous results (baryon masses, charge and mass radii, $...$) obtained by solving the Faddeev equations with simple non-relativistic quark--quark potentials, adjusted to the light and heavy--light meson spectra. Wave functions, parameterized in a simple manner, are also given and thus they can be easily used to compute further observables. Our method has been also used to find the predictions for strangeness-less baryons of the SU(2) chirally inspired quark-quark interactions. We find that the one pion exchange term of the chirally inspired interactions leads to relative changes of the $Λ_b$ and $Λ_c$ binding energies as large as 90%.

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arXiv2003

Meson Exchange Currents in (\vec{e},e'\vec{p}) observables

Our main aim in this work is to study the effects of meson-exchange currents (MEC) over asymmetries and recoil polarization observables in A(e,e'p)B reactions. This DWIA+MEC model takes care of relativistic degrees of freedom by using semi-relativistic operators for the one-body current as well as for the two-body MEC. These SR currents are obtained by a direct Pauli reduction of the corresponding relativistic operators by an expansion in terms of missing momentum over the nucleon mass, while the current dependence on the transferred energy and momentum is treated exactly. Also some comparisons with experimental data are made.

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arXiv2003

Charm- and Bottom- Baryons: A Variational Approach Using Heavy Quark Symmetry

We evaluate masses of bottom and charmed baryons using several non-relativistic quark potentials which parameters have been adjusted to the meson spectra. Heavy Quark Symmetry leads to important simplifications of the three body problem, which turns out to be easily solved by a simple variational ansatz. The wave functions obtained can be readily used to compute further observables as mass densities or form factors. The quark-quark potentials explored so far, show an overall good agreement with the experimental masses.

nucl-th↗

arXiv2003

Meson Exchange Currents in (e,e'p) recoil polarization observables

A study of the effects of meson-exchange currents and isobar configurations in $A(\vec{e},e'\vec{p})B$ reactions is presented. We use a distorted wave impulse approximation (DWIA) model where final-state interactions are treated through a phenomenological optical potential. The model includes relativistic corrections in the kinematics and in the electromagnetic one- and two-body currents. The full set of polarized response functions is analyzed, as well as the transferred polarization asymmetry. Results are presented for proton knock-out from closed-shell nuclei, for moderate to high momentum transfer.

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arXiv2003

Semi-relativistic meson exchange currents in (e,e') and (e,e'p) reactions

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 $Δ$-isobar exchange current and several prescriptions for the dynamical-equivalent form of the $Δ$-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 $Δ$-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 $Δ$-propagator. These semi-relativistic approaches, which contain important aspects of relativity, are implemented in a distorted wave analysis of quasielastic $(e,e'p)$ 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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arXiv2003

Delta-isobar relativistic meson exchange currents in quasielastic electron scattering

We study the role of the $Δ$-isobar current on the response functions for high energy inclusive quasielastic electron scattering from nuclei. We consider a general Lagrangian which is compatible with contact invariance and perform a fully relativistic calculation in first-order perturbation theory for one-particle emission. The dependence of the responses upon off-shell parametrizations is analyzed and found to be mild. A discussion of scaling behaviour and a comparison with various non-relativistic approaches are also presented.

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arXiv2003

Pionic decay of Lambda Hypernuclei in a Continuum Shell Model

We evaluate pionic decay widths of Lambda hypernuclei using a shell model for, both the nuclear bound and the continuum nucleon wave functions in the final state, and distorted waves for the outgoing pion. An excellent agreement with the recent KEK measurement of $π^-$-decay widths of $^{12}_Λ$C and $^{28}_Λ$Si is found. Besides, results for $^{56}_Λ$Fe are consistent with the existing upper bound.

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arXiv2002

Momentum distribution of relativistic nuclei with Hartree-Fock mesonic correlations

The impact of Hartree-Fock correlations on the nuclear momentum distribution is studied in a fully relativistic one boson exchange model. Hartree-Fock equations are exactly solved to first order in the coupling constants. The renormalization of the Dirac spinors in the medium is shown to affect the momentum distribution, as opposed to what happens in the non-relativistic case. The unitarity of the model is shown to be preserved by the present renormalization procedure.

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