Measurement of the Standard Model Beta Asymmetry Parameter, Aß, in 37K

Abstract

Low energy nuclear β-decay experiments have been instrumental in shaping our understanding of the weak interaction and the development of the standard model. Today these experiments continue to help our understanding evolve by measuring the predictions of the standard model to even greater levels of accuracy. Recently an experiment to measure the standard model β-asymmetry correlation parameter, Aβ , was carried out at TRIUMF.

This experiment is the first time that a measurement of Aβ has been made in 37K. Additionally it is the first time that Aβ, in any isotope, has ever been measured making use of a source of cold atoms provided by a magneto-optical trap. The experiment was carried out by the TRINAT (TRIUMF neutral atom trap) collaboration.

In order to make this measurement the 37K atoms had to be highly spin polarized and this was done via optical pumping. In the course of the data analysis a large difference between the response of the rate-equation model and the optical Bloch model to the presence of misaligned magnetic field was observed. The resolution of this model difference is ongoing and a well motivated approximate polarization of 0:99±0:01 has been used as a place holder to allow the analysis to be carried through to completion.

Assuming this final number for the polarization for the atoms we find Aβ(0) =-0:5639±0:0094, in agreement with the standard model predicted value of -0:5706±0:0007. If we do not assume the polarization number is correct our result will be PAβ = -0:5583±0:0109 This value of Aβ corresponds to a ρ value of ρ = 0:553+0:034-0:021. Combining this measurement with measurements from other T=1/2 mirror transitions a value for |Vud| = 0:9723±0:0017 is calculated. This measurement interpreted in the framework of the manifest-left right symmetric model excludes masses of M2 above 230 GeV at the 90% confidence level at ζ = 0.