Isoscalar monopole excitations in $^{16}$O: $\alpha$-cluster states at low energy and mean-field-type states at higher energy

TL;DR

This paper investigates isoscalar monopole excitations in $^{16}$O, revealing two distinct excitation types: low-energy cluster states modeled by a 4α cluster framework and higher-energy mean-field states described by RPA and QRPA, linked to the dual nature of the ground state.

Contribution

It demonstrates the coexistence of cluster and mean-field monopole excitations in $^{16}$O, highlighting the dual nature of its ground state and providing a unified understanding of its excitation spectrum.

Findings

Low-energy monopole structures are well reproduced by the 4α cluster model.

Higher-energy bump structures are consistent with mean-field calculations like RPA and QRPA.

The dual nature of the ground state underpins the coexistence of different excitation types.

Abstract

Isoscalar monopole strength function in $^{16}$O up to $E_{x}\simeq40$ MeV is discussed. We found that the fine structures at the low energy region up to $E_{x} \simeq 16$ MeV in the experimental monopole strength function obtained by the $^{16}$O$(\alpha,\alpha^{\prime})$ reaction can be rather satisfactorily reproduced within the framework of the $4\alpha$ cluster model, while the gross three bump structures observed at the higher energy region ($16 \lesssim E_{x} \lesssim 40$ MeV) look likely to be approximately reconciled by the mean-field calculations such as RPA and QRPA. In this paper, it is emphasized that two different types of monopole excitations exist in $^{16}$O; one is the monopole excitation to cluster states which is dominant in the lower energy part ($E_{x} \lesssim 16$ MeV), and the other is the monopole excitation of the mean-field type such as one-particle one-hole ($1p1h$) which {is attributed} mainly to the higher energy part ($16 \lesssim E_{x} \lesssim 40$ MeV). It is found that this character of the monopole excitations originates from the fact that the ground state of $^{16}$O with the dominant doubly closed shell structure has a duality of the mean-field-type {as well as} $\alpha$-clustering {character}. This dual nature of the ground state seems to be a common feature in light nuclei.