New Limit on Time-Reversal Violation in Beta Decay

We report the results of an improved determination of the triple correlation $D\mathbf{P}·({\mathbf{p}}_{\mathbf{e}}\times {\mathbf{p}}_{\mathbf{v}})$ that can be used to limit possible time-reversal invariance in the beta decay of polarized neutrons and constrain extensions to the standard model. Our result is $D=[-0.96\pm 1.89(\mathrm{stat})\pm 1.01(\mathrm{sys})]\times {10}^{-4}$. The corresponding phase between $g_A$ and $g_V$ is ${\varphi }_{AV}=180.013°\pm 0.028°$ (68% confidence level). This result represents the most sensitive measurement of $D$ in nuclear $\beta$ decay.

Figure 1

A schematic of the emiT detector illustrating the alternating electron and proton detector segments. The darker shaded proton detectors indicate the paired ring at $z=\pm 10\mathrm{cm}$. The cross section view illustrates, in an exaggerated manner, the effect of the magnetic field on the particle trajectories and average opening angle. A ${\mathrm{P}}_2{\mathrm{E}}_3$ coincidence event is shown.

Figure 2

Intensity log plot of SBD-scintillator coincidence data showing proton energy vs delay time. Events near $\Delta t=0$ are prompt events due primarily to beam-related backgrounds.

Figure 3

Solid (open) squares show the values of $v$ averaged over the four planes for proton cells on the even (odd) side of the proton-detection plane. Monte Carlo results for $B=560\mu \mathrm{T}$ are indicated by lines. The broken symmetry, due to the beam expansion, is evident in the shift in the crossing point from the detector center.

Figure 4

Results for $\tilde{D}$ by run subset. Uncertainties are statistical. The fully functioning paired rings used for each subset are indicated: 1–4 indicates all paired rings were used.