Novel triaxiality-driven collective feature in atomic nuclei investigated via the two-body random ensemble

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 $B(E2)$ in low-lying states, and (3) a suppressed ratio ${B}_{4/2}\ensuremath{\equiv}B(E2;{4}_{1}^{+}\ensuremath{\rightarrow}{2}_{1}^{+})/B(E2;{2}_{1}^{+}\ensuremath{\rightarrow}{0}_{1}^{+})<1$. 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 $\ensuremath{\gamma}$ 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.