Limit on Tensor Currents from Li 8 β Decay
In the standard model, the weak interaction is formulated with a purely vector-axial-vector ($V\text{-}A$) structure. Without restriction on the chirality of the neutrino, the most general limits on tensor currents from nuclear ${\beta}$ decay are dominated by a single measurement of the ${\beta}\text{-}\overline{{\nu}}$ correlation in $^{6}\mathrm{He}$ ${\beta}$ decay dating back over a half century. In the present paper, the ${\beta}\text{-}\overline{{\nu}}\text{-}{\alpha}$ correlation in the ${\beta}$ decay of $^{8}\mathrm{Li}$ and subsequent ${\alpha}$-particle breakup of the ${^{8}\mathrm{Be}}^{*}$ daughter was measured. The results are consistent with a purely $V\text{-}A$ interaction and in the case of couplings to right-handed neutrinos (${C}_{T}={-}{C}_{T}^{{'}}$) limits the tensor fraction to $|{C}_{T}/{C}_{A}{|}^{2}<0.011$ (95.5% C.L.). Finally, the measurement confirms the $^{6}\mathrm{He}$ result using a different nuclear system and employing modern ion-trapping techniques subject to different systematic uncertainties.