Anomalous low-energy $E2$-related behavior in triaxial nuclei

The anomalous low-energy $E2$-related behavior of a triaxially-deformed nucleus bas been identified and analyzed based on the SU(3) algebraic theory within the framework of the interacting boson model. The results show striking features that include a $B(E2;4_1^{+}\to 2_1^{+})\mathrm{/}B(2_1^{+}\to 0_1^{+})<1.0$ transition rate ratio and a $E(4_1^{+})\mathrm{/}E(2_1^{+})>2.0$ excitation energy ratio that can be tracked back to a finite-$N$ effect, which in a large-$N$ limit of the theory yields normal results for a stable $\gamma$-deformation. This description is shown to be able to explain observed $E2$ anomalous phenomenon in neutron-deficient nuclei such as ${}^{172}$Pt and ${}^{168}$Os, and in so doing yields a deeper understanding of the physical features of a soft triaxially-deformed nucleus.