Improved Limit on Tensor Currents in the Weak Interaction from 8Li and 8B Beta Decays

The electroweak interaction in the Standard Model is described by a pure vector–axial-vector structure, though any Lorentz-invariant term could contribute. A series of high-precision measurements of ⁸Li and ⁸B beta decays have set the most stringent constraint in the low-energy domain on the possible contributions of Lorentz-invariant tensor currents in the weak interaction. One of the largest sources of uncertainties in these measurements arise due to higher-order (called, recoil-order) nuclear beta decay terms that become non-negligible in such high-precision experiments. We discuss ab initio calculations of recoil-order terms in 8Li and 8B beta decays essential for improving the sensitivity of high-precision experiments that probe the weak interaction theory. The large-scale calculations lead to precise prediction of recoil-order form factors by identifying strong correlations between them and the ⁸Li and ⁸B ground state quadrupole moments. In addition, our calculations find a 2⁺ state of the alpha + alpha system that is energetically accessible to beta decay but has not been observed in the experimental ⁸Be energy spectrum and has an important effect on the recoil-order corrections and beta decay for the A = 8 systems.