Showing papers by "Nils Paar published in 2014"

Neutron star structure and collective excitations of finite nuclei

Abstract: A method is introduced that establishes relations between properties of collective excitations in finite nuclei and the phase transition density ${n}_{t}$ and pressure ${P}_{t}$ at the inner edge separating the liquid core and the solid crust of a neutron star. A theoretical framework that includes the thermodynamic method, relativistic nuclear energy density functionals, and the quasiparticle random-phase approximation is employed in a self-consistent calculation of $({n}_{t},{P}_{t})$ and collective excitations in nuclei. Covariance analysis shows that properties of charge-exchange dipole transitions, isovector giant dipole and quadrupole resonances, and pygmy dipole transitions are correlated with the core-crust transition density and pressure. A set of relativistic nuclear energy density functionals, characterized by systematic variation of the density dependence of the symmetry energy of nuclear matter, is used to constrain possible values for $({n}_{t},{P}_{t})$. By comparing the calculated excitation energies of giant resonances, energy-weighted pygmy dipole strength, and dipole polarizability with available data, we obtain the weighted average values: ${n}_{t}=0.0955\ifmmode\pm\else\textpm\fi{}0.0007$ ${\mathrm{fm}}^{\ensuremath{-}3}$ and ${P}_{t}=0.59\ifmmode\pm\else\textpm\fi{}0.05$ MeV ${\mathrm{fm}}^{\ensuremath{-}3}$. This approach crucially depends on experimental results for collective excitations in nuclei and, therefore, accurate measurements are necessary to further constrain the structure of the crust of neutron stars.

Optimizing relativistic energy density functionals: covariance analysis

Abstract: The stability of model parameters for a class of relativistic energy density functionals, characterized by contact (point-coupling) effective inter-nucleon interactions and density-dependent coupling parameters, is analyzed using methods of statistical analysis. A set of pseudo-observables in infinite and semi-infinite nuclear matter is used to define a quality measure $\chi^2$ for subsequent analysis. We calculate uncertainties of model parameters and correlation coefficients between parameters, and determine the eigenvectors and eigenvalues of the matrix of second derivatives of $\chi^2$ at the minimum. This allows to examine the stability of the density functional in nuclear matter, and to deduce weakly and strongly constrained combinations of parameters. In addition, we also compute uncertainties of observables that are not included in the calculation of $\chi^2$: binding energy of asymmetric nuclear matter, surface thickness of semi-infinite nuclear matter, binding energies and charge radii of finite nuclei.

Probing the neutron skin thickness in collective modes of excitation

Abstract: Nuclear collective motion provides valuable constraint on the size of neutron- skin thickness and the properties of nuclear matter symmetry energy. By employing relativistic nuclear energy density functional (RNEDF) and covariance analysis related to 2 fitting of the model parameters, relevant observables are identified for dipole excita- tions, which strongly correlate with the neutron-skin thickness (rnp), symmetry energy at saturation density (J) and slope of the symmetry energy (L). Using the RNEDF frame- work and experimental data on pygmy dipole strength ( 68 Ni, 132 Sn, 208 Pb) and dipole polarizability ( 208 Pb), it is shown how the values of J, and L, and rnp are constrained. The isotopic dependence of moments associated to dipole excitations in 116 136 Sn shows that the low-energy dipole strength and polarizability in neutron-rich nuclei display strong sensitivity to the symmetry energy parameter J, more pronounced than in isotopes with moderate neutron-to-proton number ratios.

Probing the neutron skin thickness in collective modes of excitation

Abstract: Nuclear collective motion provides valuable constraint on the size of neutron-skin thickness and the properties of nuclear matter symmetry energy. By employing relativistic nuclear energy density functional (RNEDF) and covariance analysis related to $\chi^2$ fitting of the model parameters, relevant observables are identified for dipole excitations, which strongly correlate with the neutron-skin thickness $(r_{np})$, symmetry energy at saturation density $(J)$ and slope of the symmetry energy $(L)$. Using the RNEDF framework and experimental data on pygmy dipole strength ($^{68}$Ni, $^{132}$Sn, $^{208}$Pb) and dipole polarizability ($^{208}$Pb), it is shown how the values of $J$, and $L$, and $r_{np}$ are constrained. The isotopic dependence of moments associated to dipole excitations in $^{116-136}$Sn shows that the low-energy dipole strength and polarizability in neutron-rich nuclei display strong sensitivity to the symmetry energy parameter $J$, more pronounced than in isotopes with moderate neutron-to-proton number ratios.

Covariance analysis for Energy Density Functionals and instabilities

Abstract: We present the covariance analysis of two successful nuclear energy density functionals, (i) a non-relativistic Skyrme functional built from a zero-range effective interaction, and (ii) a relativistic nuclear energy density functional based on density dependent meson-nucleon couplings. The covariance analysis is a useful tool for understanding the limitations of a model, the correlations between observables and the statistical errors. We show, for our selected test nucleus 208Pb, that when the constraint on a property A included in the fit is relaxed, correlations with other observables B become larger; on the other hand, when a strong constraint is imposed on A, the correlations with other properties become very small. We also provide a brief review, partly connected with the covariance analysis, of some instabilities displayed by several energy density functionals currently used in nuclear physics.

Nuclear Symmetry Energy: constraints from Giant Quadrupole Resonances and Parity Violating Electron Scattering

Abstract: Experimental and theoretical efforts are being devoted to the study of observables that can shed light on the properties of the nuclear symmetry energy. We present our new results on the excitation energy [1] and polarizability of the Isovector Giant Quadrupole Resonance (IVGQR), which has been the object of new experimental investigation[2]. We also present our theoretical analysis on the parity violating asymmetry at the kinematics of the Lead Radius Experiment (PREx [3]) and highlight its relation with the density dependence of the symmetry energy [4].

Stellar electron-capture rates on nuclei based on Skyrme functionals

Abstract: In this work, electron-capture rates on nuclei for stellar conditions are calculated for Ni isotopes, using a self-consistent microscopic model based on the finite temperature Skyrme Hartree-Fock plus finite-temperature charge-exchange random phase approximation approach. The results of the calculations show that electron-capture rates obtained either with different Skyrme sets or with different available models can differ by up to a few orders of magnitude.

Resolving neutrino mass hierarchy from supernova (anti)neutrino-nucleus reactions

Abstract: We introduce a hybrid method to determine neutrino mass hierarchy by simultaneous measurements of detector responses induced by antineutrino and neutrino fluxes from accretion and cooling phase of type II supernova. The (anti)neutrino-nucleus cross sections for $^{12}$C, $^{16}$O, $^{56}$Fe and $^{208}$Pb are calculated in the framework of relativistic nuclear energy density functional and weak Hamiltonian, while the cross sections for inelastic scattering on free protons, $p(\bar{ u}_{e},e^{+})n$, are obtained using heavy-baryon chiral perturbation theory. The simulations of (anti)neutrino fluxes emitted from a protoneutron star in a core-collapse supernova include collective and Mickheev-Smirnov-Wolfenstein effects inside star. The emission rates of elementary decay modes of daughter nuclei are calculated for normal and inverted neutrino mass hierarchy. It is shown that simultaneous use of (anti)neutrino detectors with different target material and time dependence of the signal allow to determine the neutrino mass hierarchy from the ratios of $ u_e / $ $\bar{ u}_e$ induced particle emissions. The hybrid method favors detectors with heavier target nuclei ($^{208}$Pb) for the neutrino sector, while for antineutrinos the use of free protons and light nuclei ($\text{H}_2\text{O}$ or $\text{-CH}_2\text{-}$) represent appropriate choice.