Search for right-handed currents in the \boldmath$\beta^+$ decay of laser-cooled, polarized $^{37}$K

We have finished analyzing a measurement of the neutrino asymmetry parameter of the decay of polarized $^{37}$K, which is sensitive to physics outside of the Standard Model. The atom cloud, initially cooled and confined by a magneto-optic trap, was optically pumped to achieve $(96.5\pm0.7)\%$ nuclear polarization. We non-destructively monitored this polarization and the cloud characteristics using a novel photoionization technique. The observed recoil asymmetry, directly related to the neutrino asymmetry, yielded $B_\nu=-0.771\pm0. 020(\mathrm{stat})\pm0.011(\mathrm{syst})$. This initial measurement --- the first in a system other than the neutron --- is in agreement with the Standard Model prediction of $-0. 7692(13)$. In the manifest left-right symmetric model, this limits the mass of a possible right-handed boson to be $\stackrel{>}{\sim}$180$~GeV/c$^2$ (90\%~CL). Though this is not yet competitive with limits from other nuclear, neutron and $\mu$ decay experiments, systematics of this first measurement have been identified and can be reduced. Modest improvements of this method will reach an accuracy of $<0.5\%$, at which point it becomes complementary to other experiments. Supported by NSERC, NRC through {\sc Triumf}, and CIPI.