Progress Towards a Measurement of the Fierz Interference in 37K Beta Decay

The TRIUMF Neutral Atom Trap provides a unique environment for precision tests of the standard model.  By observing the decay products emerging from a small cloud of cold, spin-polarized 37K, angular correlations can be constructed to measure, eg, the beta asymmetry coefficient [PRL 120, 062502].  This measurement can be generalized to consider scalar and tensor currents, which give rise to the Fierz interference (b_Fierz). While b_Fierz is zero under the standard model, a non-zero b_Fierz would introduce a new beta energy dependence. By constructing the beta asymmetry with the super-ratio technique, many systematic effects cancel out at leading order, while the resulting combination of data remains sensitive to b_Fierz.  Because the sensitivity to b_Fierz under this construction scales with the size of the asymmetry parameter, 37K provides significantly better sensitivity than either 19Ne or the free neutron. The collected data carries sufficient statistical weight to determine b_Fierz to within 0.04 of its true value, and an investigation into systematic effects is ongoing.