We place unprecedented constraints on recoil corrections in the $\beta$ decay of ${}^8\mathrm{Li}$, by identifying a strong correlation between them and the ${}^8\mathrm{Li}$ 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. In addition, our calculations predict a $2^{+}$ state of the $\alpha +\alpha$ system that is energetically accessible to $\beta$ decay but has not been observed in the experimental ${}^8\mathrm{Be}$ 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.

• Received 15 August 2021
• Accepted 4 April 2022

DOI:https://doi.org/10.1103/PhysRevLett.128.202503