Three-cluster dynamics within the ab initio Dno-core shell model with continuum: How many-body correlations and α clustering shape 6He

We realize the treatment of bound and continuum nuclear systems in the proximity of a three-body breakup threshold within the ab initio framework of the no-core shell model with continuum. Many-body eigenstates obtained from the diagonalization of the Hamiltonian within the harmonic-oscillator expansion of the no-core shell model are coupled with continuous microscopic three-cluster states to correctly describe the nuclearwave function both in the interior and asymptotic regions. We discuss the formalism in detail and give algebraic expressions for the case of core + n + n systems. Using similarity-renormalization-group evolved nucleon-nucleon interactions, we analyze the role of He-4 + n + n clustering andmany-body correlations in the ground and low-lying continuum states of the Borromean He-6 nucleus, and study the dependence of the energy spectrum on the resolution scale of the interaction. We show that He-6 small binding energy and extended radii compatible with experiment can be obtained simultaneously, without resorting to extrapolations. We also find that a significant portion of the ground-state energy and the narrow width of the first 2(+) resonance stem from many-body correlations that can be interpreted as core-excitation effects.