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

Abstract

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

Figures (5)

• Figure 1: (a) The results for the $R_{4/2}$ ratio derived from the consistent-$Q$ Hamiltonian ($N_\pi=3$ and $N_\nu=5$) for different $\eta$ are presented as a function of $\chi_\mathrm{c}$ (see the tex for its definition). (b) The same as in (a) but for the $B(E2)$ ratio $B_{4/2}$. (c) The results for the $R_{4/2}$ ratio derived from the Hamiltonian for different $\chi_\mathrm{c}$ are presented as a function of $\eta$. (d) The same as in (c) but for the $B(E2)$ ratio $B_{4/2}$.
• Figure 2: (a) Evolution of the $B_{4/2}$ ratio solved from the case with $\eta=0.11$ given in Fig. \ref{['F1']}(b) but for different effective charges. (b) The same as in (a) but for the case with $\chi_\mathrm{c}=-1.2$ derived from Fig. \ref{['F1']}(d). (c) Evolution of typical energy ratios solved from the case (a). (d) Evolution of typical energy ratios solved from the case (b).
• Figure 3: (a) The low-lying level pattern derived from the consistent-$Q$ Hamiltonian ($N_\pi=3$ and $N_\nu=5$) with $\eta=0.11$ and $\chi_\nu=-\chi_\pi=-1.2$. (b) The same as in (a) but for that derived from the Hamiltonian with $\eta=0.19$.
• Figure 4: The ratios of $R_{4/2}$ and $B_{4/2}$ in $^{166}$W Saygi2017, $^{168,170}$Os Grahn2016Goasduff2019, and $^{172}$Pt Cederwall2018 are shown as a function of the mass number $A$ to compare with the calculated results with the parameters in the consistent $Q$ Hamiltonian adopted as $\eta=0.036$ and $\chi_\pi=-\chi_\nu=1.18$.
• Figure 5: The level energies of yrast states in $^{172}$Pt, $^{168,170}$Os and $^{166}$W are presented as a function of the mass number $A$ to compare with the model results. In the calculations, the parameters are taken as same as those adopted in Fig. \ref{['F3']} except that an overall scale factor $\varepsilon_0$ in the Hamiltonian (\ref{['H']}) is additionally adjusted to reproduce the experimental $E(2_1^+)$ value for each nucleus.