Conservation laws are deeply related to any symmetry present in a physical system^1,2. Analogously to electrons in atoms exhibiting spin symmetries³, it is possible to consider neutrons and protons in the atomic nucleus as projections of a single fermion with an isobaric spin (isospin) of t = 1/2 (ref. ⁴). Every nuclear state is thus characterized by a total isobaric spin T and a projection T_z—two quantities that are largely conserved in nuclear reactions and decays^5,6. A mirror symmetry emerges from this isobaric-spin formalism: nuclei with exchanged numbers of neutrons and protons, known as mirror nuclei, should have an identical set of states⁷, including their ground state, labelled by their total angular momentum J and parity π. Here we report evidence of mirror-symmetry violation in bound nuclear ground states within the mirror partners strontium-73 and bromine-73. We find that a J^π = 5/2⁻ spin assignment is needed to explain the proton-emission pattern observed from the T = 3/2 isobaric-analogue state in rubidium-73, which is identical to the ground state of strontium-73. Therefore the ground state of strontium-73 must differ from its J ^π = 1/2⁻ mirror bromine-73. This observation offers insights into charge-symmetry-breaking forces acting in atomic nuclei.

^{守恒定律与物理系统1,2}中存在的任何对称性密切相关。^{与表现出自旋对称性3}的原子中的电子类似，可以将原子核中的中子和质子视为等压自旋 (isospin) 为t  = 1/2 的单个费米子的投影（参考文献 ⁴）。因此，每个核状态都以总等压自旋T和投影T _z为特征——这两个量在核反应和衰变中基本上是守恒的^5,6。这种等压自旋形式出现了镜像对称性：具有交换数量的中子和质子的原子核，称为镜像原子核，应该具有相同的状态集⁷，包括它们的基态，由它们的总角动量J和奇偶校验π标记。在这里，我们报告了镜像伙伴 strontium-73 和 bromine-73 内束缚核基态中镜像对称违反的证据。我们发现需要一个J ^π  = 5/2 ^-自旋分配来解释从铷 73 中的T  = 3/2 等压模拟状态观察到的质子发射模式，这与锶 73 的基态相同. 因此锶 73 的基态必须不同于其J ^π  = 1/2 ^- 镜子溴73。这一观察结果提供了对作用于原子核中的电荷对称破坏力的见解。