arXiv subjects

T. W. Donnelly

Publications and source records attributed to T. W. Donnelly.

At least 109 records · Page 6Linked to original sources

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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arXiv2004

Role of 2p-2h MEC excitations in superscaling

Following recent studies of inclusive electron scattering from nuclei at high energies which focused on two-nucleon emission mediated by meson-exchange currents, in this work the superscaling behavior of such contributions is investigated. Comparisons are made with existing data below the quasielastic peak where at high momentum transfers scaling of the second kind is known to be excellent and scaling of the first kind is good, in the proximity of the peak where both 1p-1h and 2p-2h contributions come into play, and above the peak where inelasticity becomes important and one finds scaling violations of the two kinds.

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arXiv2004

Inelastic electron-nucleus scattering and scaling at high inelasticity

Highly inelastic electron scattering is analyzed within the context of the unified relativistic approach previously considered in the case of quasielastic kinematics. Inelastic relativistic Fermi gas modeling that includes the complete inelastic spectrum - resonant, non-resonant and Deep Inelastic Scattering - is elaborated and compared with experimental data. A phenomenological extension of the model based on direct fits to data is also introduced. Within both models, cross sections and response functions are evaluated and binding energy effects are analyzed. Finally, an investigation of the second-kind scaling behavior is also presented.

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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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arXiv2003

The 2p-2h electromagnetic response in the quasielastic peak and beyond

The contribution to the nuclear transverse response function R_T arising from two particle-two hole (2p-2h) states excited through the action of electromagnetic meson exchange currents (MEC) is computed in a fully relativistic framework. The MEC considered are those carried by the pion and by Delta degrees of freedom, the latter being viewed as a virtual nucleonic resonance. The calculation is performed in the relativistic Fermi gas model in which Lorentz covariance can be maintained. All 2p-2h many-body diagrams containing two pionic lines that contribute to R_T are taken into account and the relative impact of the various components of the MEC on R_T is addressed. The non-relativistic limit of the MEC contributions is also discussed and compared with the relativistic results to explore the role played by relativity in obtaining the 2p-2h nuclear response.

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arXiv2003

Nuclear response functions for the N-N*(1440) transition

Parity-conserving and -violating response functions are computed for the inclusive electroexcitation of the N*(1440)(Roper) resonance in nuclear matter modeled as a relativistic Fermi gas. Using various empirical parameterizations and theoretical models of the N-N*(1440) transition form factors, the sensitivity of the response functions to details of the structure of the Roper resonance is investigated. The possibility of disentangling this resonance from the contribution of Delta electroproduction in nuclei is addressed. Finally, the contributions of the Roper resonance to the longitudinal scaling function and to the Coulomb sum rule are also explored.

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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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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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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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arXiv2002

RPWIA analysis of dynamical relativistic contributions in $^{16}O(\vec{e},e'\vec{p})$ reactions

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 nucleon wave function are analyzed for the polarized response functions and transferred polarization asymmetries assuming the relativistic plane-wave impulse approximation (RPWIA). Results obtained by projecting out the negative-energy components are compared with the fully-relativistic calculation for proton knockout from $p_{1/2}$ and $p_{3/2}$ shells in $^{16}O$ for a variety of kinematic situations. 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. The degree to which knowledge about nucleon form factors can be extracted from analyses of this type of process is also discussed.

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arXiv2002

Kinematical relativistic effects in $(\vec{e},e'\vec{N})$ reactions

Kinematical relativistic effects are analyzed within the plane-wave impulse approximation for outgoing nucleon polarized responses in coincidence electron scattering. Following recent approaches for non-relativistic reductions of the electromagnetic current operator, here an exact treatment of the problem for the transferred energy and momentum is presented. Rather than employing existing non-relativistic expressions for the single-nucleon matrix elements, in this work the response functions are expanded directly. Results are compared for different kinematical situations with the fully-relativistic PWIA calculation as well as with previous non-relativistic expansions. Off- and `on-shell' prescriptions are also studied in detail, and a systematic analysis of the various kinematical variables involved in the process is presented.

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arXiv2002

Gauge and Lorentz invariant one-pion exchange currents in electron scattering from a relativistic Fermi gas

A consistent analysis of relativistic pionic correlations and meson-exchange currents for electroweak quasielastic electron scattering from nuclei is carried out. Fully-relativistic one-pion-exchange electromagnetic operators are developed for use in one-particle emission electronuclear reactions within the context of the relativistic Fermi gas model. Then the exchange and pionic correlation currents are set up fully respecting the gauge invariance of the theory. Emphasis is placed on the self-energy current which, being infinite, needs to be renormalized. This is achieved starting in the Hartree-Fock framework and then expanding the Hartree-Fock current to first order in the square of the pion coupling constant to obtain a truly, gauge invariant, one-pion-exchange current. The model is applied to the calculation of the parity-conserving (PC) and parity-violating (PV) inclusive responses of nuclei. Interestingly, in the pionic correlations terms exist which arise uniquely from relativity, although their impact on the responses is found to be modest.

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arXiv2001

Electron helicity-dependence in (e,e'p) reactions with polarized nuclei and the fifth response function

The cross section for electron-induced proton knockout reactions with polarized beam and target is computed in a DWIA model. The analysis is focused on the electron helicity-dependent (HD) piece of the cross section. For the case of in-plane emission a new symmetry is found in the HD cross section, which takes opposite values for specific opposing pairs of nuclear orientations, explaining why the fifth response (involving polarized electrons, but unpolarized nuclei) is zero for this choice of kinematics. Even when this symmetry breaks down in the case of nucleon emission out of the scattering plane, it is shown that it is still present in the separate response functions. A physical explanation of these results is presented using a semi-classical model of the reaction based on the PWIA and on the concept of a nucleon orbit with its implied expectation values of position and spin.

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arXiv2001

Extended Superscaling of Electron Scattering from Nuclei

An extended study of scaling of the first and second kinds for inclusive electron scattering from nuclei is presented. Emphasis is placed on the transverse response in the kinematic region lying above the quasielastic peak. In particular, for the region in which electroproduction of resonances is expected to be important, approximate scaling of the second kind is observed and the modest breaking of it is shown probably to be due to the role played by an inelastic version of the usual scaling variable.

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arXiv2001

Kinematical versus dynamical relativistic effects in A(\vec{e},e'\vec{p})B

The relativistic mean field approach is applied to the description of coincidence $A(e,e'p)B$ reactions where both the incident electron beam and knockout proton are polarized. Effects introduced by the dynamical enhancement of the lower components of the bound nucleon wave function are analyzed within RPWIA for the polarized responses and transferred polarizations. Results obtained by projecting out the negative-energy components are also compared with various nonrelativistic reductions.

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arXiv2001

Consistent one-pion exchange currents in electron scattering from a relativistic Fermi gas

A set of fully relativistic one-pion-exchange electromagnetic operators is developed for use in one-particle emission reactions induced by electrons. To preserve the gauge invariance of the theory additional pionic correlation operators are required beyond the usual meson-exchange current operators. Of these, in the present work emphasis is placed on the self-energy current which is infinite and needs to be renormalized. The renormalized current is expanded to first order to obtain a genuine one-pion-exchange contribution to the inclusive responses.

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arXiv2001

Relativistic pionic effects in quasielastic electron scattering

The impact of relativistic pionic correlations and meson-exchange currents on the response functions for electromagnetic quasielastic electron scattering from nuclei is studied in detail. Results in first-order perturbation theory are obtained for one-particle emission electronuclear reactions within the context of the relativistic Fermi gas model. Improving upon previous analyses where non-relativistic reductions of the currents were performed, here a fully relativistic analysis in which both forces and currents are treated consistently is presented. Lorentz covariance is shown to play a crucial role in enforcing the gauge invariance of the theory. Effects stemming uniquely from relativity in the pionic correlations are identified and, in particular, a comprehensive study of the self-energy contributions and of the currents associated with the pion is presented. First- and second-kind scaling for high momentum transfer is investigated.

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arXiv2000

Contrasting N Delta and N N parity-violating asymmetries in nuclei

Several perspectives that can be opened by studying the electroexcitation of the delta via parity-violating electron scattering from nuclei are examined, working within the context of the relativistic Fermi gas model. A strong enhancement of the left-right asymmetry in the delta sector compared with that in the quasi-elastic regime is found and the potential to find clear signatures for the axial-vector contributions of the nucleon-to-delta transition to the asymmetry identified at specific low momentum transfer kinematics. Possibilities of probing the deformation of the delta are also explored, and using both the proton and nuclei as targets, the abilitiy to study the asymmetry on neutrons is studied.

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