Speaker
Description
Unraveling the nature (soft vs. rigid) and origin of triaxiality at low spin in nuclei, is a long standing challenge. Recent experimental and theoretical developments have spurred renewed interest in this topic. A detailed analysis of E2 matrix elements extracted from Coulomb excitation experiments provided evidence for rigid triaxiality in $^{76}$Ge [1,2,3], and data collected in ultra-relativistic heavy-ion collisions was shown to exhibit evidence for prominent triaxiality in $^{129}$Xe [4,5]. On the theoretical side, large-scale Monte Carlo shell model calculations of $^{166}$Er have questioned its traditional interpretation as axially-deformed, replacing it with triaxial rigidity [6], and leading to claims for prevailing triaxial shapes in nuclei [7]. These observations serve as the motivation for the study reported in the present contribution.
Nuclear triaxiality is traditionally described using two simple models within the framework of the Bohr Hamiltonian. The rigid triaxial model of Davydov and Filippov (DF) [8,9] has a well-defined potential minimum at a nonzero value of $\gamma$ , whereas the $\gamma$-soft model of Wilets and Jean (WJ) [10] incorporates a $\gamma$-independent potential. Nuclei are mesoscopic systems, with a finite number of constituents, hence it is important to understand how these two paradigms of triaxiality emerge in such environment. We address this issue in the framework of the interacting boson model (IBM) which describes quadrupole collective states in even-even nuclei in terms of N monopole ($s$) and quadrupole ($d$) bosons, representing valence nucleon pairs. An IBM Hamiltonian appropriate for the dynamics of a rigid triaxial shape is constructed, whose spectrum resembles that of a rigid-triaxial rotovibrator with families of $L=0,2^2,3,4^3,5^2,6^4,\ldots$ states arranged in ground and excited bands, and its classical energy surface accommodates a global deformed minimum at $(\beta>0,\gamma=30^{\circ})$. The rotational states of the ground band are obtained by angular momentum projection from a triaxial intrinsic state and closed expressions for their wave-functions are derived. The Hamiltonian has a partial SO(6) symmetry which ensures a good SO(6) quantum number for these selected states, but with broken SO(5) symmetry. The finiteness of $N$ and a conserved discrete $d$-parity govern the SO(5) admixtures in the states. For a fixed Hamiltonian, as N is varied, the system undergoes a transition from the $\gamma$-unstable limit (the algebraic analog of the WJ model) with a high SO(5) purity for small N, to the rigid-triaxial limit (the algebraic analog of the DF model with $\gamma=30^{\circ}$) exhibiting substantial SO(5) mixing for large N. Such structural changes are reflected in the evolution with N of energy and B(E2) ratios and of the odd-even staggering of the $L=2^{+}_2,3^{+}_1,4^{+}_2,5^{+}_1,6^{+}_2,\ldots$ states, that can distinguish between $\gamma$-soft and $\gamma$-rigid types of triaxiality. Attention is paid to properties of interband E2 transitions between states in the ground band and single-phonon excited beta and gamma vibrational bands.
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[1] Y. Toh et al., Phys. Rev. C 87, 041304(R) (2013).
[2] A. D. Ayangeakaa et al., Phys. Rev. Lett. 123, 102501 (2019).
[3] A. D. Ayangeakaa et al., Phys. Rev. C 107, 044314 (2023).
[4] B. Bally, M. Bender, G. Giacalone, V. Soma, Phys. Rev. Lett. 128, 082301 (2022).
[5] S. Zhao et al., Phys. Rev. Lett. 133, 192301 (2024).
[6] Y. Tsunoda and T. Otsuka, Phys. Rev. C 103, L021303 (2021).
[7] T. Otsuka et al., Eur. Phys. J. A 61, 126 (2025).
[8] A. S. Davydov and G. F. Filippov, Nucl. Phys. 8, 237 (1958).
[9] A. S. Davydov and V. S. Rostovsky, Nucl. Phys. 12, 58 (1959).
[10] L. Wilets and M. Jean, Phys Rev. 102, 788 (1956).
| Contribution category | Theory |
|---|---|
| Presenter status | Faculty/Staff |