Structure of the Pygmy Dipole Resonance

原子核的许多性质可以通过假设它的两个组成部分——质子和中子的对称行为来描述。因此，原子核的电偶极激发是一个有趣的可观察到的现象，因为它们需要打破质子-中子对称性。核的电偶极子响应受巨偶极子共振（GDR）支配，它处于较高的激发能，可以描述为所有中子对所有质子的非相振荡。近年来，在某些原子核中以低得多的激发能发现了大量的电偶极子强度，引起了人们的极大兴趣。这些激发通常被称为侏儒偶极子共振（PDR）。PDR的研究不仅与核结构物理有关，而且对态方程的对称能项和重核的合成也会产生影响。在拟议的研究项目中，我们计划完成我们目前关于PDR的非常零碎的知识，以便对理论描述进行严格的测试。我们想要解决的主要开放问题是激发态的衰变特性，可能分裂成主要的等标量分量和主要的等矢量分量，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装置上进行实验。这些实验将紧密地伴随着理论计算。

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.