Abstract
We discuss peculiarities of open-quantum systems, as compared to closed-quantum systems. We emphasize the importance of taking continuum degrees of freedom into account when dealing with systems with a tendency to decay through emission of fragments. In this context, we introduce the coupled-cluster theory and argue that this method allows for an accurate description of such systems starting from nucleon-nucleon degrees of freedom. We present ab-initio coupled-cluster calculations with singles and doubles excitations (CCSD) for the ground states of the helium isotopes {^3-10}He. The correlated many-body wave function is built from a single-particle basis which treats bound-, resonant-, and non-resonant continuum states on equal footing, which is a Berggren basis. In order to keep the basis size manageable, we use a renormalized interaction of the low-momentum type derived from the N{^3}LO nucleon-nucleon interaction. The calculated masses and decay widths are in semi-quantitative agreement with experiment. The discrepancy with experiment is suspected to be attributed to the three-nucleon force (3NF) which is not included at this point.
