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.
@article{arxiv.1706.00414,
title = {Precision measurement of the $\beta$-asymmetry in spin-polarized $^{37}\mathrm{K}$ decay},
author = {B. Fenker and A. Gorelov and D. Melconian and J. A. Behr and M. Anholm and D. Ashery and R. S. Behling and I. Cohen and I. Craiciu and G. Gwinner and J. McNeil and M. Mehlman and K. Olchanski and P. D. Shidling and S. Smale and C. L. Warner},
journal= {arXiv preprint arXiv:1706.00414},
year = {2018}
}