Precision Measurement of the β Asymmetry in Spin-Polarized 37K Decay.

Using Triumf's neutral atom trap, Trinat, for nuclear β decay, we have measured the β asymmetry with respect to the initial nuclear spin in 37K to be Aβ=-0.5707(13)syst(13)stat(5)pol, a 0.3% measurement. This is the best relative accuracy of any β-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 β-decay measurements, and improve the value of Vud measured in this mirror nucleus by a factor of 4. [ABSTRACT FROM AUTHOR]