Clustering and alpha-capture reaction rates from first-principle structure calculations for nucleosynthesis

We outline a new formalism that begins with first-principles structure calculations to describe alpha-clustering, and ultimately leads to a description of alpha-capture reaction rates and impacts on abundance patterns from x-ray burst (XRB) nucle-osynthesis. We utilize a symmetry-adapted basis, which allows us to extend traditional ab initio calculations into the larger model spaces needed for the development of collectivity and clustering in nuclei. In particular, the use of symplectic symmetry allows us to describe spatially expansive states in nuclei – including the Hoyle state of 12C, its 2+ excitation, and B(E2) transitions – with only one or a few basis configurations. For narrow resonances, coupling to the continuum is weak and the number of competing channels is greatly reduced, so most of the physics of the system is described through the overlap of a wave function for the complete A-particle system, computed with a single symplectic configuration (consisting of several hundreds of basis states), and a cluster basis for a single cluster partitioning. This proves to be a very powerful tool for estimating spectroscopic amplitudes, decay widths, and nuclear reaction rates, with the ability to push toward nuclear reactions involving exotic nuclei that cannot currently be measured. We show preliminary results for the 16O(α, γ)20Ne reaction rate, and consider the implications for abundance patterns determined from XRB nucleosynthesis simulations.