Nuclear systems near the drip lines offer an exciting opportunity to advance our understanding of the interactions among nucleons, which has so far been mostly based on the study of stable nuclei. However, this is not a goal devoid of challenges. From a theoretical standpoint, it requires the capability to address within an ab initio framework not only bound, but also resonant and scattering states, all of which can be strongly coupled. In recent years, significant progress has been made in ab initio nuclear structure and reaction calculations based on input from Quantum Chromodynamics employing Hamiltonians constructed within chiral effective field theory. In this contribution, we present a brief overview of one of such methods, the ab initio no-core shell model with continuum, and its applications to nucleon and deuterium scattering on light nuclei. The first investigation of the low-lying continuum spectrum of 6 He within an ab initio framework that encompasses the 4 He+n+n three-cluster dynamics characterizing its lowest particle-decay channel will also be briefly presented.KEYWORDS: ab initio calculations, exotic nuclei, low-energy reactions
Ab initio nuclear theory including the continuumLight exotic nuclei offer an exciting opportunity to test our understanding of nuclear properties in terms of forces emerging from the underlying theory of Quantum Chromodynamics (QCD). This is not a goal devoid of challenges. Experimentally, the study of rare nuclei is challenged by their short half lives and minute production cross sections. A major stumbling block in nuclear theory has to deal with the low breakup thresholds, which can lead to multi-particle emissions, and can cause bound, resonant and scattering states to be strongly coupled. Even worse, many of the systems we are interested in are simply unbound. Hence, to achieve a fundamental understanding of exotic nuclei we need both advanced experimental techniques and an ab initio nuclear theory including the continuum. In the following we briefly outline the elements of one of such theories, the ab initio no-core shell model with continuum (NCSMC).
Nuclear forcesTo develop such an ab initio theory, we start from nuclear interactions grounded in the underlying theory of Quantum Chromodynamics via chiral effective field theory [1][2][3], where nucleons and pions are the only explicit degrees of freedom and the strong interaction is systematically expanded in terms of positive powers of small momenta Q (the generic momentum in the nuclear process or the pion mass) over the chiral symmetry breaking scale Λ ∼ 1 GeV. The nuclear forces emerging from such a procedure order by order are schematically represented by the diagrams of Fig. 1. In particular, we 1