Ab initio nuclear shell model calculations of some light nuclei with a three-nucleon force

A consistent microscopic theory of the atomic nucleus is necessary to explain a wide range of nuclear phenomena and to predict nuclear behavior that cannot be measured experimentally. Over the past two decades, aided by the increase in available computational resources, nuclear theory has fundamentally shifted from phenomenological models to first principles (ab initio ) methods for characterizing nuclei. The No-Core Shell Model (NCSM) is an ab initio approach with success in describing light nuclei (A ≤ 16). Previous studies of light nuclei show improvements using a three-nucleon force over the traditional two-nucleon force. The NCSM solves the nuclear quantum many-body problem through diagonalization of a Hamiltonian matrix, which is large, sparse, and irregular. Scaling a shell model code for larger nuclei and including higher body forces poses a complex computational challenge due to its extreme memory requirements. We describe here the NCSM aproach and code improvements to achieve ab initio calculations of light nuclei including three-nucleon forces. These improvements allow for the first calculations of 9Be, using an ab initio three-nucleon interaction derived from chiral effective field theory (chiEFT). Consistent with previous work, our data show the addition of the three-nucleon force improves the binding energy and spectra for 9Be and the three-body effective interaction speeds up the convergence. Surprisingly, preliminary calculations of higher p-shell nuclei (15O and 16O) do not appear to benefit from the same improvements from the three-nucleon force and three-body effective interactions at the attainable model spaces.