Impact of Clustering on the Li-8 beta Decay and Recoil Form Factors

We place unprecedented constraints on recoil corrections in the beta decay of Li-8, by identifying a strong correlation between them and the Li-8 ground state quadrupole moment in large-scale ab initio calculations. The results are essential for improving the sensitivity of high-precision experiments that probe the weak interaction theory and test physics beyond the standard model. In addition, our calculations predict a 2(+) state of the a + a system that is energetically accessible to beta decay but has not been observed in the experimental Be-8 energy spectrum, and has an important effect on the recoil corrections and beta decay for the A = 8 systems. This state and an associated 0(+) state are notoriously difficult to model due to their cluster structure and collective correlations, but become feasible for calculations in the ab initio symmetry-adapted no-core shell-model framework.