Three-nucleon scattering and neutron-rich nuclei
The research in this proposal strives to make significant advances in the theoretical description of atomic nuclei. In particular, we will study the possibilities to constrains the three-nucleon interactions in chiral effective field theory using three-nucleon scattering observables as well as recent isovector observables. This will play a crucial role for the description of unstable neutron-rich isotopes. We will employ chiral effective field theory to analyze the strong force between nucleons. This approach offers a systematic description of the nuclear interaction in terms of a series of pion-exchanges and contact interactions, with the well-known one-pion exchange at the leading order. In the past decade very precise models of the strong force resulted from this procedure by going to next-to-next-to-next-to-leading order, and atomic nuclei have indeed been computed from scratch based on this approach. In these models, three-nucleon forces play a smaller but pivotal role. Furthermore, the strength of the corresponding amplitudes, in particular of the three-neutron part, is currently very weakly constrained. This will have a dramatic impact on the correlations that determine the stability of neutron-rich matter. The computational problem corresponds to: - very fast computation of three-nucleon scattering observables (repeated matrix-vector products and matrix diagonalizations). - many-parameter optimization (up to 40 parameters) - large-scale matrix diagonalization (to solve the quantum mechanical many-body problem with strong interactions). Sparse MatVec and large VecVec operations. Our collaboration, that includes researchers in Scandinavia and the US, are currently pioneering such efforts and use state-of-the-art optimization methods at next-to-next-to-leading order to describe atomic nuclei. [G. Hagen et al. Nature Physics 12, 186–190 (2016) , R. F. Garcia Ruiz et al. Nature Physics. (2016) advanced publication online doi:10.1038/nphys3645]. In 2014-15 we have also implemented optimization with regards to nucleon-nucleon and pion-nucleon scattering observables, extracted covariance matrices and performed error propagation in the few-body sector using valuable computer time provided by SNIC/triolith. [B. D. Carlson et al, Phys. Rev. X 6, 011019] The next steps in this ambitious research project is to drastically improve the description of three-nucleon forces in the chiral expansion, and furthering the computationally demanding task of error propagation to the many-body sector.