Precise Measurement of the Positron Asymmetry in the Decay of Spin-polarized $^{37}$K

Precise low-energy measurements in nuclear $\beta$-decay provide constraints on possible physics beyond the standard model complementary to high-energy collider experiments. We report the most precise measurement of the positron asymmetry from a polarized nucleus to-date. At the TRIUMF Neutral Atom Trap, atoms of the positron emitter $^{37}$K are confined in an alternating-current magneto-optical trap and spin-polarized to $99.13\pm0.09\%$ via optical pumping. The use of atom-trapping techniques allows for an exceptionally open geometry with the decay products escaping the trapping region unperturbed by the trapping potential. We detect the emitted positrons in a pair of symmetric detectors placed along the polarization axis to measure the asymmetry \emph{in situ}. The analysis was performed blind and considers $\beta$-scattering and other systematic effects. The results place limits on the mass of a hypothetical $W$ boson coupling to right-handed neutrinos as well as contribute to an independent determination of the $V_{ud}$ element of the CKM matrix.