Limit on Tensor Currents from ${}^8\mathrm{Li}$ $\beta$ 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 work, 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 ($C_T=-C_T^{\prime }$) limits the tensor fraction to $|C_T/C_A{|}^2<0.011$ (95.5% C.L.). The measurement confirms the ${}^6\mathrm{He}$ result using a different nuclear system and employing modern ion-trapping techniques subject to different systematic uncertainties.

Figure 1

Cross-sectional view of the BPT and detector system in the rf plane. The direction of the emitted $\alpha$’s and $\beta$’s is determined by the vector between the trap center and detector pixels.

Figure 2

Spectra from events with $\beta$ and $\alpha$ particles detected on the top and bottom detector. (a) Energy difference along with the fit to the simulated spectrum and the normalized residual. (b) The reconstructed $\cos ({\theta }_{\beta \nu })$ spectrum for the same events with a fit to the simulated spectrum and the normalized residual. The gray curves show the expected spectra for a pure $T$ interaction.