关于原子核中同量异位态的反转
On the inversion of isobaric-analogue states in nuclei
摘要：
同位旋是原子核中的近似对称性，源于质子和中子相当相似的性质。也许原子核内的同位旋最明显的表现是镜像核中激发态的近乎相同的结构：质子数和中子数相反的原子核。由于电磁相互作用和上下夸克的质量差，同位旋对称性被打破，因此镜像对称性也被打破。霍夫和合作者最近的一项研究提供了证据，表明$^{73}$ Sr的基态自旋与其镜像$^{73}$ Br的基态自旋不同，这是由于基态和第一激发态的反转，在$^{73}$ Br系统中仅分离27 keV。在这个简短的说明中，我们把这个反演放在过去半个世纪的实验和理论工作的必要背景下，并表明它与正常行为完全一致，并且没有提供对同位旋对称性破缺的新见解。重要的一点是，由于库仑相互作用的同位旋破缺效应经常变化，从一个给定的中等质量的核内的水平，高达200千电子伏的水平。任何比这更小的能级分裂都容易在镜像伙伴中表现出能级反转，如果不与适当的核模型相矛盾，这不会对我们的理解提出挑战。虽然我们注意到原子核基态反转的新颖性，但我们要强调的是，在同位旋的背景下，关于基态或能级反转并没有什么特别的说明。
英文摘要：
Isospin is an approximate symmetry in atomic nuclei, arising from the rather similar properties of protons and neutrons. Perhaps the clearest manifestation of isospin within nuclei is in the near-identical structure of excited states in mirror nuclei: nuclei with inverted numbers of protons and neutrons. Isospin symmetry, and therefore mirror-symmetry, is broken by electromagnetic interactions and the difference in the masses of the up and down quarks. A recent study by Hoff and collaborators presented evidence that the ground-state spin of $^{73}$Sr is different from that of its mirror, $^{73}$Br, due to an inversion of the ground- and first-excited states, separated by only 27 keV in the $^{73}$Br system. In this brief note, we place this inversion within the necessary context of the past half-century of experimental and theoretical work, and show that it is entirely consistent with normal behaviour, and affords no new insight into isospin-symmetry breaking. The essential point is that isospin-breaking effects due to the Coulomb interaction frequently vary from level to level within a given medium-mass nucleus by as much as 200 keV. Any level splitting smaller than this is liable to manifest a level inversion in the mirror partner which, absent disagreement with an appropriate nuclear model, does not challenge our understanding. While we note the novelty of an inversion in nuclear ground states, we emphasize that in the context of isospin there is nothing specifically illuminating about the ground state, or a level inversion.
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