Self-consistent description of electromagnetic excitations for astrophysical applications

在过去的几十年里，基于能量密度泛函（EDF）的微观核模型已经获得了很高的描述能力。这一发展的动机和挑战，首先是由于在获取新的奇异核方面取得了显著的实验进展，其次是由于天体物理学要求从核模型中获得高质量的输入。然而，仍有一个方面尚未得到自洽微观模型的很好控制：电磁激励的描述。它们是确定r过程核合成计算的辐射中子俘获率的基础。提出的项目旨在从两个方向扩展现有的edf：一是提供磁激发模式的相关描述，另一个是考虑开壳球核。形式上，磁激励的描述使用与电模式（自然宇称）相同的方法。基是广泛使用的随机相位近似（RPA），它产生强度分布的总体结构。在时间阻塞近似（TBA）内通过声子耦合获得了精细结构。实际上，我们使用了最近的TBA实现，其中包括单粒子连续体，准粒子-声子相互作用，以及非线性模型框架内声子的重正化，这确保了该方法随着声子数量的增加而自然收敛。本项目的另一个新特点是将其扩展到准粒子RPA (QRPA)，从而将其适用范围也扩展到开壳核。结果将是一个更可靠和更完整的电磁激励截面描述，为核合成计算提供改进的输入。该项目分四个步骤进行：1。我们仔细研究了208Pb中所有可能的电和磁模式，包括高自旋通道。我们利用这些结果来修改核EDF以适应磁模式和电模式，这主要意味着校准EDF中迄今尚未确定的自旋参数。现在有了对磁模的可靠描述，我们将把这些磁模纳入TBA的声子空间，这将进一步细化谱分布的精细结构。最后将该方法推广到球形开壳核。这意味着首先将RPA扩展到QRPA，然后仔细检查TBA中现在更大的声子基础的影响。在第一阶段，我们考虑非相对论的Skyrme EDF，我们已经有了相当多的经验。在第二阶段，我们将进入相对论领域，首先通过明确地处理磁通道中的介子交换和介子交换，然后将上述步骤序列应用于相对论edf，包括那些由局部手性势推导出来的edf。

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.