$B(E2)$ Anomaly in Asymmetric Two-Fluid Coupling Systems

Abstract A theoretical investigation into the potential correlation between the $B(E2)$ anomaly and intrinsic two-fluid characters in nuclear systems has been carried out within the proton-neutron interacting boson model. Our analysis demonstrates that the hallmark signature of this anomaly, the reduced $E2$ transitional ratio $B_{4/2}\equiv B(E2;4_1^+\rightarrow2_1^+)/B(E2;2_1^+\rightarrow0_1^+)<1.0$, can be naturally produced in a weakly coupled two-fluid boson system, wherein the proton and neutron boson subsystems occupy distinct collective configurations. We provide a symmetry-based analytical elucidation of the underlying $B(E2)$ anomaly mechanism and apply this framework to describe the structural evolution across the Os isotopes near $A=170$, thereby offering new insights into the observed $B(E2)$ anomaly phenomena.