Alpha clustering and alpha-capture reaction rate from ab initio symmetry-adapted description of ^20Ne

We introduce a new framework for studying clustering and for calculating alpha partial widths using ab initio wave functions. We demonstrate the formalism for ^20Ne, by calculating the overlap between the ^16O+α cluster configuration and states in ^20Ne computed in the ab initio symmetry-adapted no-core shell model. We present spectroscopic amplitudes and spectroscopic factors, and compare those to no-core symplectic shell-model results in larger model spaces, to gain insight into the underlying physics that drives alpha-clustering. Specifically, we report on the alpha partial width of the lowest 1^- resonance in ^20Ne, which is found to be in good agreement with experiment. We also present first no-core shell-model estimates for asymptotic normalization coefficients for the ground state, as well as for the first excited 4^+ state in ^20Ne that lies in a close proximity to the ^16O+α threshold. This outcome highlights the importance of correlations for developing cluster structures and for describing alpha widths. The widths can then be used to calculate alpha-capture reaction rates for narrow resonances of interest to astrophysics. We explore the reaction rate for the alpha-capture reaction ^16O(α,γ)^20Ne at astrophysically relevant temperatures and determine its impact on simulated X-ray burst abundances.