The isoscalar giant monopole resonance (ISGMR) and isovector giant dipole resonance (IVGDR) in finite nuclei are studied within the framework of a relativistic transport approach. Small-amplitude oscillations are analyzed with the relativistic Vlasov equation, i.e., neglecting nucleon collision terms. The excitation energies of ISGMR and IVGDR are obtained for spherical nuclei with various sets of Lagrangian parameters. In the case of (208)Pb we study in detail the dependence of the monopole response on the effective mass and symmetry energy at saturation a(4) given by the used covariant effective interaction. We show that a reduced m(*) and a larger a(4) can compensate for the effect on the ISGMR energy centroid of a much larger compressibility modulus K(nm). The role of the symmetry energy is confirmed by the observation of reduced corrections for the more symmetric (90)Zr. This result is important for overcoming the conflicting determination of nuclear compressibility between nonrelativistic and relativistic effective interactions. For symmetry-energy dynamical effects, we analyze the influence of the inclusion of an effective isovector-scalar channel, delta-meson field, with constant- and density-dependent couplings. We show that the delta-meson contribution, keeping fixed the equilibrium a(4) value, leads to a reduction in the centroid energy of the ISGMR and IVGDR for (208)Pb. The same mechanism can account for the apparent paradox of a decrease of the frequency of a IVGDR mode in heavy nuclei (of volume type) even in presence of a larger a(4). All that in fact reveals the sensitivity of the ISGMR and IVGDR collective motions for neutron-rich systems on the slope (or pressure) of the symmetry energy at saturation. Density-dependent vertices do not much affect our conclusions. Following as a guidance the dispersion relations in nuclear matter, we see two main reasons for that: The smoothness of the density dependences around saturation and the presence of compensation effects coming from rearrangement terms.

Relativistic transport approach to collective nuclear dynamics

GRECO, VINCENZO
2005

Abstract

The isoscalar giant monopole resonance (ISGMR) and isovector giant dipole resonance (IVGDR) in finite nuclei are studied within the framework of a relativistic transport approach. Small-amplitude oscillations are analyzed with the relativistic Vlasov equation, i.e., neglecting nucleon collision terms. The excitation energies of ISGMR and IVGDR are obtained for spherical nuclei with various sets of Lagrangian parameters. In the case of (208)Pb we study in detail the dependence of the monopole response on the effective mass and symmetry energy at saturation a(4) given by the used covariant effective interaction. We show that a reduced m(*) and a larger a(4) can compensate for the effect on the ISGMR energy centroid of a much larger compressibility modulus K(nm). The role of the symmetry energy is confirmed by the observation of reduced corrections for the more symmetric (90)Zr. This result is important for overcoming the conflicting determination of nuclear compressibility between nonrelativistic and relativistic effective interactions. For symmetry-energy dynamical effects, we analyze the influence of the inclusion of an effective isovector-scalar channel, delta-meson field, with constant- and density-dependent couplings. We show that the delta-meson contribution, keeping fixed the equilibrium a(4) value, leads to a reduction in the centroid energy of the ISGMR and IVGDR for (208)Pb. The same mechanism can account for the apparent paradox of a decrease of the frequency of a IVGDR mode in heavy nuclei (of volume type) even in presence of a larger a(4). All that in fact reveals the sensitivity of the ISGMR and IVGDR collective motions for neutron-rich systems on the slope (or pressure) of the symmetry energy at saturation. Density-dependent vertices do not much affect our conclusions. Following as a guidance the dispersion relations in nuclear matter, we see two main reasons for that: The smoothness of the density dependences around saturation and the presence of compensation effects coming from rearrangement terms.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11769/13504
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