Search for an Electroweak Tensor Coupling with Trapped ⁸Li β-decay

The Beta-decay Paul Trap (BPT) measures the beta-neutrino angular correlation coefficient aᵦᵥ in the Gamow-Teller β-decays of ⁸Li and ⁸B (decaying to ⁸Be* → 2α) to search for an electroweak tensor coupling, a beyond-Standard Model possibility. The new BPT Mk IV trap reduces β scattering by a factor of 4, a key source of systematic uncertainty, and has recently been commissioned with ⁸Li with 2.7 million triple coincidence events. In addition, a complete detector characterization has been performed with an α beam to reduce systematic uncertainties associated with the silicon strip detector response. The new high-statistics ⁸Li data set, coupled with the experimental advancements, will allow for improved sensitivity to a tensor contribution once data analysis is completed. In the near future, the BPT should be able to reach an uncertainty of Δ|CT/CA|² ≦ 10⁻³, though the theoretical understanding of the nuclear structure still requires improvement.