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

Abstract

Novel collective modes characterized by a $B_{4/2}$ ratio ($\equiv B(E2;4_1^{+}\to 2_1^{+})/B(E2;2_1^{+}\to 0_1^{+})$) 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 $B_{4/2}$ ratio observed in ¹⁶⁶W, ^168,170Os, and ¹⁷²Pt, offering new insights into the $B\left(E2\right)$ anomaly phenomenon in neutron-deficient nuclei.