arXiv subjects

J. R. Vignote

Publications and source records attributed to J. R. Vignote.

8 recordsLinked to original sources

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

Polarization Transfer in the 4He(e,e'p)3H Reaction at Q^2 = 0.8 and 1.3 (GeV/c)^2

Proton recoil polarization was measured in the quasielastic 4He(e,e'p)3H reaction at Q^2 = 0.8 (GeV/c)^2 and 1.3 (GeV/c)^2 with unprecedented precision. The polarization-transfer coefficients are found to differ from those of the 1H(e,e' p) reaction, contradicting a relativistic distorted-wave approximation, and favoring either the inclusion of medium-modified proton form factors predicted by the quark-meson coupling model or a spin-dependent charge-exchange final-state interaction. For the first time, the polarization-transfer ratio is studied as a function of the virtuality of the proton.

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arXiv2008

Spin asymmetry for the 16O(gamma,pi- p) reaction in the Delta(1232) region within an effective Lagrangian approach

The spin asymmetry of the photon in the exclusive A(gamma,pi N)A-1 reaction is computed employing a recently developed fully relativistic model based on elementary pion production amplitudes that include a consistent treatment of the spin-3/2 nucleon resonances. We compare the results of this model to the only available data on Oxygen [Phys. Rev. C 61 (2000) 054609] and find that, contrary to other models, the predicted spin asymmetry compares well to the available experimental data in the Delta(1232) region. Our results indicate that no major medium modifications in the Delta(1232) properties are needed in order to describe the measured spin asymmetries.

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arXiv2006

Gamow-Teller strength distributions in Xe isotopes

The energy distributions of the Gamow-Teller strength are studied for even-even Xe isotopes with mass numbers from 124 to 142. A self-consistent microscopic formalism is used to generate the single particle basis, using a deformed Skyrme Hartree-Fock mean field with pairing correlations in BCS approximation. The Gamow-Teller transitions are obtained within a quasiparticle random phase approximation (QRPA) approach using a residual spin-isospin interaction in the particle-hole and particle-particle channels. We then discuss the pairing BCS treatment and the determination of the ph and pp residual interaction coupling constants. We study the GT+ and GT- strength distributions for the equilibrium nuclear shapes, which are an essential information for studies of charge-exchange reactions and double-beta processes involving these isotopes.

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arXiv2004

Electromagnetic (${\vec e},e'{\vec p}$) Observables & Relativistic Nucleon Dynamics

At present there exists a great interest in the search for evidence of possible modification of the nucleon form factors inside the nuclear medium. Recent theoretical work predict changes in the form factors within the experimental limits. Importantly, the longitudinal to sideways transferred polarization ratio has been identified as being ideally suited for such studies, as these polarization observables are believed to be the least sensitive to most standard nuclear structure uncertainties while their ratio shows a high sensitivity to the ratio of the electric to magnetic form factors. The kinematic regime where the measurements have been undertaken is at relatively high energy and it is clear that relativistic effects in wave functions and operators are essential. In this work we focus on the analysis of polarized $^{16}O(\vec{e},e'\vec{p})^{15}N$ observables. Our aim is to explore a selected set of model dependences that could contaminate any attempt to infer medium modifications, mainly related to the description of FSI and to the role played by relativity.

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arXiv2003

Analysis of polarized $^{16}$O$(\vec{e},e'\vec{p})$ observables within the relativistic distorted wave impulse approximation

Recoil nucleon transferred polarization observables in coincidence quasielastic electron scattering are studied within the relativistic distorted wave impulse approximation. Results for response functions and polarization asymmetries are discussed for proton knockout from $p_{1/2}$, $p_{3/2}$ and $s_{1/2}$ shells in $^{16}$O. The impact of spinor distortion is examined by comparing the fully relativistic calculation with results obtained by projecting out the negative-energy components. In particular, a careful analysis of effects linked to the description of the bound and scattered relativistic nucleon wave functions is presented. The high sensitivity of some polarization observables to the dynamical enhancement of the lower components, already shown within the relativistic plane wave impulse approximation, is proven to be maintained in the relativistic distorted wave approach. Semi-relativistic approaches based on the effective momentum approximation are also studied. Finally, comparison with experimental data and a brief analysis of effects linked to medium modified form factors is presented.

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arXiv2002

Relativity in polarized electron scattering observables

Coincidence scattering of polarized electrons from nuclei with polarization transfer to outgoing nucleons is studied within the context of relativistic mean field theory. Effects introduced by the dynamical enhancement of the lower components of the bound and scattered nucleon wave functions are analyzed for the polarized response functions and transferred polarization asymmetries. Results obtained by projecting out the negative-energy components are compared with the fully-relativistic calculation. The crucial role played by the relativistic dynamics in some spin-dependent observables is clearly manifested even for low/medium values of the missing momentum. Kinematical relativistic effects are also analyzed. A discussion of the factorization approach and the mechanism for its breakdown is also briefly presented.

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arXiv2000

Polarization transfer in the $^{4}$He$(\vec{e},e' \vec{p}^{3}$H reaction

Polarization transfer in the 4He(e,e'p)3H reaction at a Q^2 of 0.4 (GeV/c)^2 was measured at the Mainz Microtron MAMI. The ratio of the transverse to the longitudinal polarization components of the ejected protons was compared with the same ratio for elastic ep scattering. The results are consistent with a recent fully relativistic calculation which includes a predicted medium modification of the proton form factor based on a quark-meson coupling model.

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