Speaker
Description
Reflection-symmetry-breaking nuclear octupole deformation is a phenomenon of significant interest due to its connection with fundamental symmetry considerations ($\mathcal{C}$, $\mathcal{P}$, and $\mathcal{T}$) and its relevance in nuclear structure studies. Substantial experimental evidence indicates that a few nuclei exhibit a pear-like octupole deformation [1], whereas global theoretical studies predict the existence of a dozen or even more such nuclei [2]. Meanwhile, the nuclear electromagnetic response can provide insight into the collective properties of the nucleus, such as deformation [3,4]. However, the impact of octupole deformation on the nuclear electromagnetic response remains less studied and is the focus of the present work.
We have studied the effect of octupole deformation on the nuclear electromagnetic response using nuclear density functional theory (DFT) combined with linear response theory. We employed the Skyrme-Hartree-Fock-Bogoliubov (HFB) model to determine two distinct deformed ground-state solutions for the studied nuclei: one with conserved and the other with broken reflection symmetry. Based on these two HFB ground-state solutions, we performed finite amplitude method (FAM) [5] calculations to solve quasiparticle random phase approximation (QRPA)-type equations and obtain transition strength functions. In the calculations, zero-energy linear and rotational momentum spurious modes associated with the broken translational and rotational symmetries, respectively, were removed where relevant. The resulting transition strength functions and selected sum rules were then compared between the two deformed HFB solutions.
We calculated electric and magnetic transition strength functions for different multipolarities ($E1$, $E2$, $E3$, and $M1$) of expected octupole-deformed even-even nuclei around the actinide region. Our results indicate three key aspects of the effect of octupole deformation on the nuclear electromagnetic response. Firstly, octupole deformation appears to have only a modest impact on the transition strengths in the resonance region. Secondly, at low excitation energies (< 8 MeV) of $M1$ transitions, especially around the expected scissors resonance [4], octupole deformation has a stronger impact on transition strengths. This effect even leads to a violation of the expected correlation [4] between the non-energy weighted sum rule and the quadrupole deformation parameter. Thirdly, our analysis confirms that octupole-deformed solutions can exhibit a significant rotational spurious contribution to the isoscalar $E3$ transition strength (its $K=1^-$ mode), which is consistent with the non-conservation of parity in these solutions. Therefore, both the rotational and linear-momentum spurious modes were removed from the calculated isoscalar $E3$ transition strengths. These results motivate further investigation into the impact of octupole deformation on low-energy $M1$ transitions and highlight the importance of accounting for spurious mode contributions arising from broken parity symmetry.
[1] P. A. Butler. Proc. R. Soc. A 476, 20200202 (2020).
[2] Y. Cao et al. Phys. Rev. C 102, 024311 (2020).
[3] B. L. Berman and S. C. Fultz. Rev. Mod. Phys. 47, 713 (1975).
[4] K. Heyde, P. von Neumann-Cosel, and A. Richter. Rev. Mod. Phys. 82, 2365 (2010).
[5] P. Avogadro and T. Nakatsukasa. Phys. Rev. C 84, 014314 (2011).