Precision measurement of the $\beta$-asymmetry in spin-polarized ${}^{37}\mathrm{K}$ decay

Abstract

Using TRIUMF's neutral atom trap, TRINAT, for nuclear $\beta$ decay, we have measured the $\beta$ asymmetry with respect to the initial nuclear spin in ${}^{37}\mathrm{K}$ to be $A_{\beta }=-0.5707(13{)}_{\mathrm{syst}}(13{)}_{\mathrm{stat}}(5{)}_{\mathrm{pol}}$, a 0.3% measurement. This is the best relative accuracy of any $\beta$-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 $\beta$-decay measurements, and improve the value of $V_{\mathrm{ud}}$ measured in this mirror nucleus by a factor of 4.