New test of time-reversal invariance inNe19beta decay

A new experimental upper limit on the "time-reversal" coefficient $D$ is reported for the allowed beta decay ${}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}\to {}_{}{}^{19}{}_{}{}^{}\mathrm{F}+e^{+}+{\nu }_e$. The result, expressed in terms of the relative phase angle between axial-vector ($A$) and vector ($V$) couplings, is ${\phi }_{A,V}\left({}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}\right)=180.2\mathrm{}\pm {0.4}^{\circ }$, consistent with $T$ invariance. A nuclear-spin-polarized atomic beam of ${}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}$ in which either of the magnetic substates $m_I=\pm \frac{1}{2}$ can be selected, is allowed to terminate its flight in a cell, where the ${}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}$ atoms remain for approximately four seconds without substantial spin depolarization. Delayed coincidences between positrons and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$-recoil ions from decays in the cell are observed and correlated with nuclear spin to determine $D$. The ${}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}$ nuclear polarization is monitored continuously by observation of the beta-decay asymmetry.