Isoscalar dipole transition as a probe for asymmetric clustering

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 has been unclear. Purpose: We show those resonances can be attributed to the cluster states with asymmetric configurations such as alpha + O-16. 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 Ne-20 and Ti-44. 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 Ne-20 and Ti-44. 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 a + O-16 and a + Ca-40 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.