Isoscalar dipole transition as a probe for asymmetric clustering

Abstract

Background: The sharp ${1}^{\ensuremath{-}}$ resonances with enhanced isoscalar dipole transition strengths are observed in many light nuclei at relatively small excitation energies, but their nature has been unclear.Purpose: We show those resonances can be attributed to the cluster states with asymmetric configurations such as $\ensuremath{\alpha}+^{16}\mathrm{O}$. 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 $^{20}\mathrm{Ne}$ and $^{44}\mathrm{Ti}$.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 $^{20}\mathrm{Ne}$ and $^{44}\mathrm{Ti}$.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 $\ensuremath{\alpha}+^{16}\mathrm{O}$ and $\ensuremath{\alpha}+^{40}\mathrm{Ca}$ cluster states comparable with or larger than the Weisskopf estimate.Conclusions: 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 probes for asymmetric clusters.