Precision Measurement of the ß Asymmetry in Spin-Polarized

Using Triumf's neutral atom trap, Trinat, for nuclear ß decay, we have measured the ß asymmetry with respect to the initial nuclear spin in ^{37}K to be A_{ß}=-0.5707(13)_{syst}(13)_{stat}(5)_{pol}, a 0.3% measurement. This is the best relative accuracy of any ß-asymmetry measurement in a nucleus or the neutron, and is in agreement with the standard model prediction -0.5706(7). We compare constraints on physics beyond the standard model with other ß-decay measurements, and improve the value of V_{ud} measured in this mirror nucleus by a factor of 4.