44 research outputs found
Self-consistent Green's function method for nuclei and nuclear matter
No full textRecent results obtained by applying the method of self-consistent Green's functions to nuclei and nuclear matter are reviewed. Particular attention is given to the description of experimental data obtained from the (e,e′p) and (e,e′2N) reactions that determine one- and two-nucleon removal probabilities in nuclei since the corresponding amplitudes are directly related to the imaginary parts of the single-particle and two-particle propagators. For this reason and the fact that these amplitudes can now be calculated with the inclusion of all the relevant physical processes, it is useful to explore the efficacy of the method of self-consistent Green's functions in describing these experimental data. Results for both finite nuclei and nuclear matter are discussed with particular emphasis on clarifying the role of short-range correlations in determining various experimental quantities. The important role of long-range correlations in determining the structure of low-energy correlations is also documented. For a complete understanding of nuclear phenomena it is therefore essential to include both types of physical correlations. We demonstrate that recent experimental results for these reactions combined with the reported theoretical calculations yield a very clear understanding of the properties of all protons in the nucleus. We propose that this knowledge of the properties of constituent fermions in a correlated many-body system is a unique feature of nuclear physics
Extension of the random phase approximation including the self-consistent coupling to two-phonon contributions
No full textA microscopic formalism is developed that includes the coupling to two particle-hole phonons in the particle-hole propagator by extending the dressed random phase approximation (DRPA) equation for a finite system. The resulting formalism is applied to study the low-lying excitation spectrum of [Formula Presented] It is observed that the coupling to two-phonon states at low energy generates excited states with quantum numbers that cannot be obtained in the DRPA approach. Nevertheless, the two-phonon states mix weakly with particle-hole configurations and participate only partially in the formation of the lowest-lying positive-parity excited states. The stability of the present calculation is tested vs the truncation of model space. It is demonstrated that when single-particle strength fragmentation is properly considered, the present formalism exhibits convergence with respect to the chosen model space within the confines of the chosen approximation scheme
Spectroscopic factors in 16O and nucleon asymmetry
No full textThe self-consistent Green's functions method is employed to study the spectroscopic factors of quasiparticle states around 16,28O and 40,60Ca. The Faddeev random phase approximation (FRPA) is used to account for the coupling of particles with collective excitation modes. Results for 16O are reviewed first. The same approach is applied to isotopes with large proton-neutron asymmetry to estimate its effect on spectroscopic factors. The results, based on the chiral N3LO force, exhibit an asymmetry dependence similar to that observed in heavy-ion knockout experiments but weaker in magnitude
Faddeev description of two-hole-one-particle motion and the single-particle spectral function
No full textThe Faddeev technique is employed to address the problem of describing the influence of both particle-particle and particle-hole phonons on the single-particle self-energy. The scope of the few-body Faddeev equations is extended to describe the motion of two-hole-one-particle (two-particle-one-hole) excitations. This formalism allows one to sum both particle-particle and particle-hole phonons, obtained separately in the random phase approximation. The appearance of spurious solutions for the present application of the Faddeev method is related to the inclusion of a consistent set of diagrams. The formalism presented here appears practical for finite nuclei and achieves a simultaneous inclusion of particle-particle and particle-hole phonons to all orders while the spurious solutions are properly eliminated
Faddeev treatment of long-range correlations and the one-hole spectral function of O-16
No full textThe Faddeev technique is employed to study the influence of both particle-particle and particle-hole phonons on the one-hole spectral function of 16O. Collective excitations are accounted for at a random phase approximation level and subsequently summed to all orders by the Faddeev equations to obtain the nucleon self-energy. An iterative procedure is applied to investigate the effects of the self-consistent inclusion of the fragmentation in the determination of the phonons and the corresponding self-energy. The present results indicate that the characteristics of hole fragmentation are related to the low-lying states of 16O
Self-consistent Green's function calculations of 16O at small missing energies
No full textCalculations of the one-hole spectral function of 16O for small missing energies are reviewed. The self-consistent Green's function approach is employed together with the Faddeev equations technique in order to study the coupling of both particle-particle and particle-hole phonons to the single-particle motion. The results indicate that the characteristics of hole fragmentation are related to the low-lying states of 16O and an improvement of the description of this spectrum, beyond the random phase approximation, is required to understand the experimental strength distribution. A first calculation in this direction that accounts for two-phonon states is discussed
Quasiparticles in neon using the Faddeev random-phase approximation
No full textThe spectral function of the closed-shell neon atom is computed by expanding the electron self-energy through a set of Faddeev equations. This method describes the coupling of single-particle degrees of freedom with correlated two-electron, two-hole, and electron-hole pairs. The excitation spectra are obtained using the random-phase approximation (RPA), rather than the Tamm-Dancoff framework employed in the third-order algebraic diagrammatic construction method. The difference between these two approaches is studied, as well as the interplay between ladder and ring diagrams in the self-energy. Satisfactory results are obtained for the ionization energies as well as the energy of the ground state with the Faddeev RPA scheme, which is also appropriate for the high-density electron gas
Effects of Nuclear Correlations on the 16O(e,e'pN) Reactions to Discrete Final States
No full textCalculations of the 16O(e,e′p N) cross sections to the ground state and first excited levels of the 14C and 14N nuclei are presented. The effects of nuclear fragmentation have been obtained in a self-consistent approach and are accounted for in the determination of the two-nucleon removal amplitudes. The Hilbert space is partitioned in order to compute the contribution of both long- and short-range effects in a separate way. Both the two-proton and the proton-neutron emission cross sections have been computed within the same model for the nuclear structure as well as the same treatment of the reaction mechanism, with the aim of better comparing the differences between the two physical processes. The 16O(e,e′ pp) reaction is found to be sensitive to short-range correlations, in agreement with previous results. The 16O(e,e′ pn) cross section to 1+ final states is dominated by the Δ current and tensor correlations. For both reactions, the interplay between collective (long-range) effects and short-range and tensor correlations plays an important role. This suggests that the selectivity of (e,e′pN) reactions to the final state can be used to probe correlations also beyond short-range effects
Toward a Global Dispersive Optical Model for the Driplines
No full textA dispersive-optical-model analysis has been performed for both protons and neutrons on 40, 42, 44, 48Ca isotopes. The fitted potentials describe accurately both scattering and bound quantities and extrapolate well to other stable nuclei. Further experimental information will be gathered to constrain extrapolations toward the driplines
Toepassen of aanpassen: Borgen van kwaliteit door resultaatverantwoordelijke onderwijsteams
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