We investigate an anomalous collective motion in atomic nuclei observed in recent experiments, characterized by three distinct signatures: (1) an excitation energy ratio $R_{4/2}>2$, (2) sizable electric-quadrupole transition strengths $B\left(E2\right)$ in low-lying states, and (3) a suppressed ratio $B_{4/2}\equiv B(E2;4_1^{+}\to 2_1^{+})/B(E2;2_1^{+}\to 0_1^{+})<1$. This pattern challenges the prevailing understanding of nuclear theory because it contradicts all established collective models as well as microscopic models in nuclear physics. Using the full configuration-interaction shell model combined with the two-body random ensemble, we show that such an anomalous rotational mode emerges naturally in a small fraction of random many-body systems, providing critical insights into the connection between triaxiality and the emergent collectivity. The anomaly can arise from strong mixing between the $4^{+}$ states in the ground-state band and the triaxiality-induced $\gamma$ band in the SU(3) dynamical symmetry. These findings underscore the necessity of incorporating triaxial degrees of freedom in descriptions of nuclear collectivity and the critical role of the residual two-body interactions in inducing the rare collective mode with mixed nature.

Close

PrevNext

View figure in article

• Collective levels
• Electromagnetic transitions
• Nuclear structure & decays
• Multipole matrix elements
• Random matrix theory
• Shell model