Above threshold s-wave resonances illustrated by the 1/2$^+$ states in $^9$Be and $^9$B

TL;DR

This paper resolves the structure of the 1/2$^+$ resonance in $^9$Be, demonstrating it as a genuine three-body resonance with a unique decay mechanism, and extends findings to the isobaric analog in $^9$B.

Contribution

It provides a novel three-body resonance interpretation for the 1/2$^+$ state in $^9$Be and predicts similar states in $^9$B, clarifying previous misconceptions.

Findings

The 1/2$^+$ state in $^9$Be is a genuine three-body resonance.

The decay width is approximately half of previous estimates.

Predicted similar resonances in $^9$B near 2.0 MeV energy.

Abstract

We solve the persistent problem of the structure of the lowest $1/2^+$ resonance in $^9$Be which is important to bridge the A=8 gap in nucleosynthesis in stars. We show that the state is a genuine three-body resonance even though it decays entirely into neutron-$^8$Be relative s-waves. The necessary barrier is created by "dynamical" evolution of the wave function as the short-distance $\alpha$-$^5$He structure is changed into the large-distance n-$^8$Be structure. This decay mechanism leads to a width about two times smaller than table values. The previous interpretations as a virtual state or a two-body resonance are incorrect. The isobaric analog 1/2$^+$ state in $^9$B is found to have energy and width in the vicinity of 2.0 MeV and 1.5 MeV, respectively. We also predict another 1/2$^+$ resonance in $^9$B with similar energy and width.