Improved Limit on Tensor Currents in the Weak Interaction from ${}^8\mathrm{Li}$ $\beta$ Decay

The electroweak interaction in the standard model is described by a pure vector-axial-vector structure, though any Lorentz-invariant component could contribute. In this Letter, we present the most precise measurement of tensor currents in the low-energy regime by examining the $\beta \text{−}\overline{\nu }$ correlation of trapped ${}^8\mathrm{Li}$ ions with the Beta-decay Paul Trap. We find $a_{\beta \nu }=-0.3325\pm 0.0013_{\mathrm{stat}}\pm 0.0019_{\mathrm{syst}}$ at $1\sigma$ for the case of coupling to right-handed neutrinos ($C_T=-C_T^{\prime }$), which is consistent with the standard model prediction.

Figure 1

(a) The $\beta$-decay scheme of ${}^8\mathrm{Li}$ (with $E_x$, $J^{\pi }$, and T listed from left to right) [17]. Sample ${}^8\mathrm{Be}$ excitation energy $E_x$ spectrum R-matrix fits (black) with approximate individual state contributions from (b) the 3-MeV resonance (blue) and doublet states (red) and (c) with an intruder state (green) centered at $\sim 9\mathrm{MeV}$ added. The region highlighted in blue is the $E_x$ range used in our analysis.

Figure 2

Radial-plane cross-sectional view of the BPT showing a typical triple event.

Figure 3

Measured $\mathrm{\Delta }{E}_{\alpha }$ spectrum fit to a linear combination of simulated pure $A$ and $T$ interactions (black curve) with the normalized residuals below. For comparison, an example pure $T$ interaction spectrum is plotted in blue.