This review aims at giving a critical description of the theoretical researches conducted on the low-lying dipole states traditionally denoted as Pygmy Dipole Resonances (PDR). A brief survey of the experimental techniques and recent experimental findings is presented as an introduction to the main part of the paper. The presence of the PDR on stable and unstable nuclei with neutron excess is well established in theoretical and experimental studies. The theoretical approaches are reviewed starting from the macroscopic collective models to the microscopic mean-field theories. The isospin mixed nature of the PDR – reproduced by all the microscopic approaches – allows to study the excitation with isovector and isoscalar probes. To draw a better picture on the structure of this mode is therefore important to complement the theoretical studies with detailed investigation on the reaction mechanism. To this mean, this paper gives specific focus to the description of the cross section calculations. The semiclassical Coupled Channel equations are shortly reviewed with particular attention to the construction of the nuclear potential and radial form factors with the microscopic transition densities. The interplay of Coulomb and nuclear contributions, their dependence on mass, charge and incident energy are analysed with the help of few selected examples. Most of the features of the PDR are well described by the theoretical approaches even though few open question remain to be clarified. Among them a discussion on the collectivity of the mode, isospin splitting and role of deformation is presented. Most of the theoretical works and the new experimental findings on the collective properties of the PDR jeopardise the common picture of this excitation mode as related to the oscillation of the neutron skin against an inert core The question on the influence of the neutron excess on other multipolarities is also reviewed.

本综述旨在对传统上称为侏儒偶极共振 (PDR) 的低位偶极态进行的理论研究进行批判性描述。对实验技术和最近的实验结果进行了简要概述，作为本文主要部分的介绍。PDR 在中子过剩的稳定和不稳定核上的存在在理论和实验研究中得到了很好的证实。回顾了从宏观集体模型到微观平均场理论的理论方法。PDR 的同位旋混合性质——通过所有显微方法再现——允许使用等向量和等量探针研究激发。因此，为了更好地了解这种模式的结构，重要的是通过对反应机理的详细研究来补充理论研究。为此，本文特别关注截面计算的描述。简要回顾了半经典耦合通道方程，特别关注具有微观跃迁密度的核势和径向形状因子的构造。库仑和核贡献的相互作用，它们对质量、电荷和入射能量的依赖性在几个选定的例子的帮助下进行了分析。尽管很少有悬而未决的问题有待澄清，但理论方法很好地描述了 PDR 的大部分特征。其中讨论了模式的集体性，介绍了同位旋分裂和变形的作用。大多数理论著作和关于 PDR 集体特性的新实验发现危及这种激发模式的普遍形象，因为它与中子皮对惰性核心的振荡有关。中子过剩对其他多极化的影响的问题也被审查。