Self-consistent description of electromagnetic excitations for astrophysical applications

• Applicants: Professor Dr. Karlheinz Langanke; Professor Dr. Gabriel Martinez-Pinedo; Professor Dr. Paul-Gerhard Reinhard; Professor Dr. Josef Speth
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2021 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448588010

Microscopic nuclear models based on energy-density functionals (EDF) have acquired over the past decades a high level of descriptive power. This development was motivated and challenged, first, by remarkable experimental progress in accessing new exotic nuclei, and second, by astrophysical demands calling for high quality input from nuclear models. However, there is still one aspect which is not yet well under control by self-consistent microscopic models: thedescription of electromagnetic excitations. They are fundamental for the determination of radiative neutron capture rates for r-process nucleosynthesis calculations.The proposed project aims at an extension of existing EDFs in two directions: one is to provide a pertinent description of magnetic excitation modes and the other is to consider open shell spherical nuclei. Formally, the description of magnetic excitations uses the same approaches as for electrical modes (natural parity). Basis is the widely used Random-Phase-Approximation (RPA) which produces the gross structure of strength distributions. The fine structure isobtained by phonon-coupling within the Time-Blocking Approximation (TBA). Actually, we use recent implementation of TBA which includes the single-particle continuum, the quasiparticle-phonon interaction, and renormalization of phonons within the framework of a nonlinearmodel, which ensures a natural convergence of the method with increasing number of phonons. A further new feature, aimed at in this project, is the extension to quasi-particle RPA (QRPA) which extends the range of applicability also to open-shell nuclei. The outcome willbe a more reliable and more complete description of electromagnetic excitation cross sections that delivers improved input for nucleosynthesis calculations.The project proceeds in four steps:1. we scrutinize all possible electrical and magnetic modes in 208Pb including high-spin channels.2. we use the results to modify the nuclear EDF to accommodate magnetic modes as well as the electrical ones which means mainly calibration of hitherto undetermined spin parameters in the EDF.3. having now a reliable description of magnetic modes, we will include these modes into the phonon space for TBA which will further refine the fine structure of spectral distributions4. Finally the approach will be extended to describe spherical open-shell nuclei. This means first to extend RPA to QRPA and after that to scrutinize the effects if the now larger basis of phonons in TBA.In a first phase, we consider non-relativistic Skyrme EDF for which we have already a considerable body of experience. In the second phase, we step forward into the relativistic domain, first by adding explicit treatment of pi- and rho- meson exchange in the magnetic channel, and second by applying the above sequence of steps to relativistic EDFsincluding also those which are derived from local chiral potentials.

• DFG Programme: Research Grants
• International Connection: Russia
• Partner Organisation: Russian Foundation for Basic Research
• Cooperation Partners: Dr. Nikolay Lyutorovich; Dr. Victor Tselyaev