Anomalous collective modes in atomic nuclei within the proton-neutron interacting boson model

Novel collective modes characterized by a $B_{4\mathrm{/}2}$ ratio ($\equiv B(E2;4_1^{+}\to 2_1^{+})\mathrm{/}B(E2;2_1^{+}\to 0_1^{+})$) less than 1.0 that were observed recently have been identified within the proton-neutron interacting boson model (IBM-2) using the consistent-$Q$ Hamiltonian. These modes are shown to give rise to triaxial spectral features, including significant band mixing. The results provide a compelling explanation for the deeply suppressed $B_{4\mathrm{/}2}$ ratio observed in ${}^{166}$W, ${}^{168,170}$Os, and ${}^{172}$Pt, offering new insights into the $B(E2)$ anomaly phenomenon in neutron-deficient nuclei.