Angular Correlations in the $\beta$ Decay of ${}^8\mathrm{B}$: First Tensor-Current Limits from a Mirror-Nucleus Pair

We present the first measurement of the $\alpha \text{−}\beta \text{−}\nu$ angular correlation in the Gamow-Teller ${\beta }^{+}$ decay of ${}^8\mathrm{B}$. This was accomplished using the Beta-decay Paul Trap, expanding on our previous work on the ${\beta }^{-}$ decay of ${}^8\mathrm{Li}$. The ${}^8\mathrm{B}$ result is consistent with the $V\text{−}A$ electroweak interaction of the standard model and, on its own, provides a limit on the exotic right-handed tensor current relative to the axial-vector current of $|C_T/C_A{|}^2<0.013$ at the 95.5% confidence level. This represents the first high-precision angular correlation measurements in mirror decays and was made possible through the use of an ion trap. By combining this ${}^8\mathrm{B}$ result with our previous ${}^8\mathrm{Li}$ results, we demonstrate a new pathway for increased precision in searches for exotic currents.

Figure 1

Cross-sectional view of the BPT and detector system in the rf plane. The directions of the $\alpha$ and $\beta$ particles are determined by the vectors between the trap center and detector pixels.

Figure 2

Energy difference spectrum along with the fit, shown in black, to the simulated spectrum and normalized residuals. Shown in blue is the expected result for a pure tensor interaction. The number of events $N$, the fitted ${|C_T/C_A|}^2$, and the fit ${\chi }^2$ per number of degrees of freedom are also shown.

Figure 3

The 95.5% confidence level regions for ${}^8\mathrm{Li}$ (blue [21], orange [22]), the present ${}^8\mathrm{B}$ measurement (green), and the joint probability distribution (black).