Impact of clustering on the $^8$Li $\beta$ decay and recoil form factors

TL;DR

This paper uses ab initio calculations to constrain recoil corrections in $^8$Li beta decay, revealing new states and correlations that improve understanding of weak interactions and BSM physics.

Contribution

It introduces a novel ab initio approach to constrain recoil corrections and predicts a new $2^+$ state affecting $A=8$ beta decay analysis.

Findings

Strong correlation between recoil corrections and quadrupole moment.

Prediction of an unobserved $2^+$ state in $^8$Be.

Enhanced constraints for high-precision weak interaction experiments.

Abstract

We place unprecedented constraints on recoil corrections in the $\beta$ 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 $\alpha+\alpha$ system that is energetically accessible to $\beta$ decay but has not been observed in the experimental $^8$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.