Isoscalar dipole transition as a probe for asymmetric clustering

TL;DR

This paper demonstrates that isoscalar dipole transitions effectively probe asymmetric cluster states in light nuclei, providing theoretical predictions and clarifying the excitation mechanism, thus offering a new approach to studying nuclear clustering phenomena.

Contribution

The study analytically derives the excitation mechanism and performs nuclear model calculations predicting large isoscalar dipole transitions to asymmetric cluster states in specific nuclei.

Findings

Transition matrix is as large as Weisskopf estimate.

Transition matrix is amplified with cluster correlations.

Predicted large transitions to $ ext{α}+ ext{O}^{16}$ and $ ext{α}+ ext{Ca}^{40}$ cluster states.

Abstract

Background: The sharp $1^-$ resonances with enhanced isoscalar dipole transition strengths are observed in many light nuclei at relatively small excitation energies, but their nature was unclear. Purpose: We show those resonances can be attributed to the cluster states with asymmetric configurations such as $\alpha$+$^{16}{\rm O}$. We explain why asymmetric cluster states are strongly excited by the isoscalar dipole transition. We also provide a theoretical prediction of the isoscalar dipole transitions in $^{20}{\rm Ne}$ and $^{44}{\rm Ti}$. Method: The transition matrix is analytically derived to clarify the excitation mechanism. The nuclear model calculations by Brink-Bloch wave function and antisymmetrized molecular dynamics are also performed to provide a theoretical prediction for $^{20}{\rm Ne}$ and $^{44}{\rm Ti}$. Results: It is shown that the transition matrix is as large as the Weisskopf estimate even though the ground state is an ideal shell model state. Furthermore, it is considerably amplified if the ground state has cluster correlation. The nuclear model calculations predict large transition matrix to the $\alpha$+$^{16}{\rm O}$ and $\alpha$+$^{40}{\rm Ca}$ cluster states comparable with or larger than the Weisskopf estimate. Conclusion: We conclude that the asymmetric cluster states are strongly excited by the isoscalar dipole transition. Combined with the isoscalar monopole transition that populates the $0^+$ cluster states, the isoscalar transitions are promising probe for asymmetric clusters.