Impact of Clustering on the Li8 β Decay and Recoil Form Factors

We place unprecedented constraints on recoil corrections in the $\ensuremath{\beta}$ 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 $\ensuremath{\alpha}+\ensuremath{\alpha}$ system that is energetically accessible to $\ensuremath{\beta}$ decay but has not been observed in the experimental $^{8}\mathrm{Be}$ energy spectrum, and has an important effect on the recoil corrections and $\ensuremath{\beta}$ 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.