Impact of clustering on the ${}^8$Li ${\hat{I}}^2$ decay and recoil form factors

Abstract

We place unprecedented constraints on recoil corrections in the ${\hat{I}}^2$ decay of ${}^8$Li, by identifying a strong correlation between them and the ${}^8$Li 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 (BSM). In addition, our calculations predict a $2^{+}$ state of the $\hat{I}\pm +\hat{I}\pm$ system that is energetically accessible to ${\hat{I}}^2$ decay but has not been observed in the experimental ${}^8$Be energy spectrum, and has an important effect on the recoil corrections and ${\hat{I}}^2$ 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.

Accepted version