Direct probe of spin polarization in potassium for beta decay

Motivated by the need for a more precise measurement of beta, neutrino and nuclear recoil asymmetries to probe new physics, we report on a new technique to measure the spin polarization of potassium by directly probing the 4s1/2 → 5p1/2 hyperfine transitions using a 405 nm π-light probe laser. Our previous method [B. Fenker et al. Phys Rev Lett 120 062502 (2018) Supp Mat] monitors the near-extinction of spontaneous fluorescence from 4S_1/2 → 4P_1/2 optical pumping as the atoms become polarized [B Fenker et al 2016 New J. Phys. 18 073028 (2016)] using the fluorescence data in a complex model to determine the average polarization of our atom cloud. With the new optical probing method we are able to directly probe the populations of the individual 4S_1/2 sublevels, resulting in a direct measurement of the sample polarization. This makes our new method a lot more flexible when it comes to different optical trap and pumping configurations, as there is no need to re-determine the parameters of the model. Using this new technique, we were able to directly measure potassium spectra with different optical pumping and MOT configurations.