arXiv subjects

F. de Jong

Publications and source records attributed to F. de Jong.

8 recordsLinked to original sources

arXiv1997

Relativistic Brueckner-Hartree-Fock calculations with explicit intermediate negative energy states

In a relativistic Brueckner-Hartree-Fock calculation we include explicit negative-energy states in the two-body propagator. This is achieved by using the Gross spectator-equation, modified by medium effects. Qualitatively our results compare well with other RBHF calculations. In some details significant differences occur, e.g, our equation of state is stiffer and the momentum dependence of the self-energy components is stronger than found in a reference calculation without intermediate negative energy states.

nucl-th↗

arXiv1997

Asymmetric Nuclear Matter in the Relativistic Brueckner Hartree Fock approach

We present a calculation of asymmetric nuclear matter properties in a relativistic Brueckner Hartree Fock framework. Following other calculations the components of the self-energies are extracted by projecting on Lorentz invariant amplitudes. It is shown that for asymmetric nuclear matter one needs a sixth invariant. We present a set of invariants which in the limit of symmetric nuclear matter reduces to the conventional set. We argue that the existence of such a properly behaving set is also crucial for the application of the projection method in symmetric nuclear matter. Results for the equation of state and other observables are presented. Special attention is payed to an analysis in terms of mean-field effective coupling constants. Apart from the usual ones we also find significant strength in the isovector scalar channel, which can be interpreted as an effective $δ$-meson.

nucl-th↗

arXiv1997

Towards a fully self-consistent spectral function of the nucleon in nuclear matter

We present a calculation of nuclear matter which goes beyond the usual quasi-particle approximation in that it includes part of the off-shell dependence of the self-energy in the self-consistent solution of the single-particle spectrum. The spectral function is separated in contributions for energies above and below the chemical potential. For holes we approximate the spectral function for energies below the chemical potential by a $δ$-function at the quasi-particle peak and retain the standard form for energies above the chemical potential. For particles a similar procedure is followed. The approximated spectral function is consistently used at all levels of the calculation. Results for a model calculation are presented, the main conclusion is that although several observables are affected by the inclusion of the continuum contributions the physical consistency of the model does not improve with the improved self-consistency of the solution method. This in contrast to expectations based on the crucial role of self-consistency in the proofs of conservation laws.

nucl-th↗

arXiv1997

Lepton-Pair Production in Virtual Compton Scattering off the Proton

We show that lepton-pair production in Virtual Compton Scattering offers, through interference with the well-known Bethe-Heitler process, a sensitive probe to learn the longitudinal response of resonances and the electromagnetic nucleon form factors. This interference can be measured directly in terms of an asymmetry. The role of off-shell effects in the N-N-$γ$ vertices is investigated as well. An additional N-N-$γ-γ$ contact term in the amplitude, included to ensure gauge invariance of the model, cancels a substantial part of the off-shell effects.

nucl-th↗

arXiv1996

The effect of electro-magnetic formfactors on dilepton production off pp-collisions

We investigate the effect of the electric formfactor on dilepton production off pp-collisions. In the model we use, the formfactor is separated in a pion-cloud contribution and a Vector Meson Dominance part. We find that the dilepton spectrum is hardly affected by the pion-cloud part, but significantly by the VMD contribution. We point out that data with sufficient resolution would provide a crucial test of the validity of VMD for the nucleon formfactor in the timelike region.

nucl-th↗

arXiv1996

The influence of negative-energy states on proton-proton bremsstrahlung

We investigate the effect of negative-energy states on proton-proton bremsstrahlung using a manifestly covariant amplitude based on a T-matrix constructed in a spectator model. We show that there is a large cancellation among the zeroth-order, single- and double-scattering diagrams involving negative-energy nucleonic currents. We thus conclude that it is essential to include all these diagrams when studying effects of negative-energy states.

nucl-th↗

arXiv1996

Realistic Interactions and Dilepton production off pp-collisions

We present a model for dilepton production of proton-proton collisions using a realist T-matrix that by incorporating Delta-isobar degrees of freedom fits the NN-scattering data up to 2 GeV. The results we find differ in details from earlier work that use less sophisticated interactions but the overall agreement with these calculations is good.

nucl-th↗

arXiv1994

Proton-proton bremsstrahlung below and above pion-threshold: the influence of the $Δ$-isobar

The proton-proton bremsstrahlung is investigated within a coupled-channel model with the $Δ$ degree of freedom. The model is consistent with the $NN$ scattering up to 1 GeV and the $γNΔ$ vertex determined in the study of pion photoproduction reactions. It is found that the $Δ$ excitation can significantly improve the agreements with the $pp \rightarrow ppγ$ at $E_{lab}=280$ MeV. Predictions at $E_{lab}=550$ and $800$ MeV are presented for future experimental tests.

nucl-th↗