Structure of the Pygmy Dipole Resonance

原子核的许多性质可以用它的两个成分质子和中子的对称行为的假设来描述。因此，原子核的电偶极激发是一个有趣的可观察性，因为它们需要打破质子-中子对称性。原子核的电偶极响应是由巨偶极子共振(GDR)主导的，GDR具有较高的激发能，可以描述为所有中子对所有质子的异相振荡。最近，在某些原子核中发现了激发能低得多的电偶极子强度，这引起了人们的极大兴趣。这些激发通常被称为俾格米偶极子共振(PDR)。PDR的研究不仅与核结构物理有关，而且还可能对状态方程中的对称能项以及重核的合成产生影响。在拟议的研究项目中，我们计划完善我们目前对PDR的非常零碎的知识，以便对理论描述进行严格的检验。我们想要解决的主要问题是激发的衰变性质，可能分裂成主要的等标量分量和主要的等向量分量，以及潜在的单粒子结构。为此，我们计划在德国和国外的不同设置进行实验。实验将与理论计算紧密相伴。

Many properties of atomic nuclei can be described by the assumption of a symmetric behavior of its two constituents, the protons and neutrons. Therefore, electric dipole excitations of the nucleus are an interesting observable, since they require a breaking of the proton-neutron symmetry. The electric dipole response of a nucleus is dominated by the Giant Dipole Resonance (GDR) which lies at relatively high excitation energy and can be described as an out-of-phase oscillation of all neutrons against all protons. Recently the discovery of substantial electric dipole strength at much lower excitation energy in certain nuclei has attracted a lot of interest. These excitations are often denoted as Pygmy Dipole Resonance (PDR). The study of the PDR is not only relevant for nuclear structure physics, but can as well have an impact on the symmetry energy term in the equation of state, and on the synthesis of heavy nuclei.Within the proposed research project we plan to complete our presently very fragmentary knowledge about the PDR to allow stringent test of theoretical descriptions. Major open question we want to address are the decay properties of the excitations, a possible splitting into a dominantly isoscalar and a dominantly isovector component, and the underlying single particle structure. For this purpose we plan experiments at different setups in Germany and abroad. The experiments will be closely accompanied by theoretical calculations.