arXiv subjects

B. I. S. van der Ventel

Publications and source records attributed to B. I. S. van der Ventel.

11 recordsLinked to original sources

arXiv2018

Calculation of a complete set of spin observables for proton elastic scattering from stable and unstable nuclei

A microscopic study of proton elastic scattering from unstable nuclei at intermediate energies using a relativistic formalism is presented. We have employed both the original relativistic impulse approximation (IA1) and the generalised impulse approximation (IA2) formalisms to calculate the relativistic optical potentials, with target densities derived from relativistic mean field (RMF) theory using the NL3 and FSUGold parameter sets. Comparisons between the optical potentials computed using both IA1 and IA2 formalisms, and the different RMF Lagrangians are presented for both stable and unstable targets. The comparisons are required to study the effect of using IA1 versus IA2 optical potentials, with different RMF parameter sets, on elastic scattering observables for unstable targets at intermediate energies. We also study the effect of full-folding versus the factorized form of the optical potentials on elastic scattering observables. As with the case for stable nuclei, we found that the use of the full-folding optical potential improves the scattering observables (especially spin observables) at low intermediate energy (e.g. 200MeV). No discernible difference is found at a projectile incident energy of 500 MeV. To check the validity of using localized optical potential, we calculate the scattering observables using non-local potentials by solving the momentum space Dirac equation. The Dirac equation is transformed to two coupled Lippmann-Schwinger equations, which are then numerically solved to obtain elastic scattering observables. The results are discussed and compared to calculations involving local coordinate-space optical potentials.

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arXiv2011

Electromagnetic production of hypernuclei

A formalism for the electromagnetic production of hypernuclei is developed where the cross section is written as a contraction between a leptonic tensor and a hadronic tensor. The hadronic tensor is written in a model-independent way by expanding it in terms of a set of five nuclear structure functions. These structure functions are calculated by assuming that the virtual photon interacts with only one bound nucleon. We use the most recent model for the elementary current operator which gives a good description of the experimental data for the corresponding elementary process. The bound state wave functions for the bound nucleon and hyperon are calculated within a relativistic mean-field model. We calculate the unpolarized triple differential cross section for the hypernuclear production process e + 12C --> e + K+ + 12_Lambda{B} as a function of the kaon scattering angle. The nuclear structure functions are calculated within a particle-hole model. The cross section displays a characteristic form of being large for small values of the kaon scattering angle with a smooth fall-off to zero with increasing angle. The shape of the cross section is essentially determined by the nuclear structure functions. In addition, it is found that for the unpolarized triple differential cross section one structure function is negligible over the entire range of the kaon scattering angle.

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arXiv2008

Photo- and Electroproduction of the Hypertriton on 3He

Differential cross sections of the photo- and electroproduction of the hypertriton have been calculated by utilizing modern nuclear wave functions and the elementary operator of KAON-MAID. It is found that a proper treatment of Fermi motion is essential for the two processes. While the average momentum approximation can partly simulate the Fermi motion in the process, the ``frozen nucleon'' assumption yields very different results, especially at lower energies. The Coulomb effect induced by the interaction between the positively charged kaon and the hypertriton is found to be negligible. The influence of higher partial waves is also found to be relatively small, in contrast to the finding of the previous work. The off-shell assumption is found to be very sensitive in the case of electroproduction rather than in photoproduction. It is shown that the few available experimental data favor the assumption that the initial nucleon is off-shell and the final hyperon is on-shell. This seems to be reasonable, since the hyperon in the hypertriton is less bound than the nucleon in the initial 3He nucleus. The effect of the missing resonance D_{13}(1895) is more profound in the longitudinal cross sections. Excluding this resonance reduces the longitudinal cross sections by one order of magnitude, but does not change the effects of various off-shell assumptions on the cross sections.

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arXiv2006

Relativistic predictions of polarization phenomena in exclusive proton-induced proton-knockout reactions

Whereas a nonrelativistic distorted wave model fails to quantitatively describe analyzing power data for exclusive proton-induced proton-knockout from the 3s_{1/2} state in Pb-208 at 202 MeV, the corresponding relativistic prediction provides a perfect description, thus suggesting that the Dirac equation is the more appropriate underlying dynamical equation. We check the consistency of this rsult by comparing predictions for both dynamical models to new high resolution data for 3s_{1/2} knockout in Pb-208 at a higher incident energy of 392 MeV.

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arXiv2005

Strange-quark contribution to the ratio of neutral- to charged-current cross sections in neutrino-nucleus scattering

A formalism based on a relativistic plane wave impulse approximation is developed to investigate the strange-quark content ($g_{A}^{s}$) of the axial-vector form factor of the nucleon via neutrino-nucleus scattering. Nuclear structure effects are incorporated via an accurately calibrated relativistic mean-field model. The ratio of neutral- to charged-current cross sections is used to examine the sensitivity of this observable to $g_{A}^{s}$. For values of the incident neutrino energy in the range proposed by the FINeSSE collaboration and by adopting a value of $g_{A}^{s}=-0.19$, a 30% enhancement in the ratio is observed relative to the $g_{A}^{s}=0$ result.

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arXiv2003

Sensitivity of exclusive proton knockout spin observables to different Lorentz invariant representations of the NN interaction

Within the framework of the relativistic plane wave impulse approximation, we study the observable consequences of employing a complete Lorentz invariant representation of the NN scattering matrix in terms of 44 independent amplitudes, as opposed to the previously-employed, but ambiguous, five-term Lorentz invariant parametrization of the NN scattering matrix, for the prediction of complete sets of exclusive ($\vec{p},2 \vec{p} $) polarization transfer observables. Two kinematic conditions are considered, namely proton knockout from the $3s_{1/2}$ state of $^{208}$Pb at an incident energy of 202 MeV for coplanar scattering angles ($28.0^{\circ}, -54.6^{\circ}$), as well as an incident energy of 392 MeV for the angle pair ($32.5^{\circ}, -80.0^{\circ}$). The results indicate that certain spin observables are ideal for discriminating between the two representations.

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arXiv2003

Quasielastic neutrino-nucleus scattering

We study the sensitivity of neutral-current neutrino-nucleus scattering to the strange-quark content of the axial-vector form factor of the nucleon. A model-independent formalism for this reaction is developed in terms of eight nuclear structure functions. Taking advantage of the insensitivity of the ratio of proton $(ν,ν' p)$ to neutron $(ν,ν' n)$ yields to distortion effects, we compute all structure functions in a relativistic plane wave impulse approximation approach. Further, by employing the notion of a bound-state nucleon propagator, closed-form, analytic expressions for all nuclear-structure functions are developed in terms of an accurately calibrated relativistic mean-field model. Using a strange-quark contribution to the axial-vector form factor of $g_{A}^{s}=-0.19$, a significant enhancement in the proton-to-neutron yields is observed relative to one with $g_{A}^{s}=0$.

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arXiv2003

Quasifree eta photoproduction from nuclei and medium modifications of resonances

We investigate the sensitivity of the differential cross section, recoil nucleon polarization and the photon asymmetry to changes in the elementary amplitude, medium modifications of the resonance $(S_{11},D_{13})$ masses, as well as nuclear target effects. All calculations are performed within a relativistic plane wave impulse approximation formalism resulting in analytical expressions for all observables. The spin observables are shown to be unique tools to study subtle effects that are not accessible by only looking at the unpolarized differential cross section.

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arXiv2003

Relativistic predictions of spin observables for exclusive proton knockout reactions

Within the framework of the relativistic distorted wave impulse approximation (DWIA), we investigate the sensitivity of complete sets of polarization transfer observables for exclusive proton knockout from the 3s$_{1/2}$, 2d$_{3/2}$ and 2d$_{5/2}$ states in $^{208}$Pb, at an incident laboratory kinetic energy of 202 MeV, and for coincident coplanar scattering angles ($28.0^{\circ}$, $-54.6^{\circ}$), to different distorting optical potentials, finite-range (FR) versus zero-range (ZR) approximations to the DWIA, as well as medium-modified meson-nucleon coupling constants and meson masses. Results are also compared to the nonrelativistic DWIA predictions based on the Schrödinger equation.

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arXiv2000

Relativistic predictions of quasielastic proton-nucleus spin observables based on a complete Lorentz invariant representation of the NN scattering matrix

Within the framework of the relativistic plane wave impulse approximation (RPWIA), complete sets of quasielastic $(\vec{p}, \vec{p}^{'})$ and $(\vec{p}, \vec{n})$ spin observables are calculated employing a general Lorentz invariant representation of the NN scattering matrix (referred to as the IA2 representation). The use of a complete representation eliminates the arbitrariness of a previously-used five-term parameterization (commonly called the IA1 representation) and allows for the correct incorporation of effective-mass-type medium effects within the RPWIA framework and within the context of the Walecka model. For quasielastic scattering from a $^{40}$Ca target at incident proton energies between 200 and 500 MeV, we investigate the sensitivity of complete sets of spin observables to effective nucleon masses for both IA1 and IA2 representations. In general it is seen that the IA1 representation may overestimate the importance of nuclear medium effects, whereas the IA2-based predictions nearly correspond to values for free nucleon-nucleon scattering.

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arXiv1999

Polarization transfer observables for quasielastic proton-nucleus scattering in terms of a complete Lorentz invariant representation of the NN scattering matrix

For the calculation of polarization transfer observables for quasielastic scattering of protons on nuclei, a formalism in the context of the Relativistic Plane Wave Impulse Approximation is developed, in which the interaction matrix is expanded in terms of a complete set of 44 independent invariant amplitudes. A boson-exchange model is used to predict the 39 amplitudes which were omitted in the formerly used five-term parameterization(the SPVAT form) of the nucleon-nucleon scattering matrix. Use of the complete set of amplitudes eliminates the arbitrariness of the five-term representation.

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