$^4{\rm He}$+$n$+$n$ continuum within an ab initio framework

The low-lying continuum spectrum of the $^6{\rm He}$ nucleus is investigated for the first time within an ab initio framework that encompasses the $^4{\rm He}$+$n$+$n$ three-cluster dynamics characterizing its lowest decay channel. This is achieved through an extension of the no-core-shell model combined with the resonating-group method, in which energy-independent non-local interactions among three nuclear fragments can be calculated microscopically starting from realistic nucleon-nucleon interactions and consistent ab initio many-body wave functions of the clusters. The three-cluster Schrödinger equation is solved with three-body scattering boundary conditions by means of the hyperspherical-harmonic method on a Lagrange mesh. Using a soft similarity-renormalization-group evolved chiral nucleon-nucleon potential, we find the known $J^π= 2^+$ resonance as well as a result consistent with a new low-lying second $2^+$ resonance recently observed at GANIL at $2.6$ MeV above the $^6$He ground state. We also find resonances in the $2^-$, $1^+$ and $0^-$ channels, while no low-lying resonances are present in the $0^+$ and $1^-$ channels.

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