Measurement of the neutrino asymmetry in the β decay of laser-cooled, polarized 37K

A measurement of the neutrino asymmetry is presented which represents the first search for new physics using polarized radioactive atoms initially cooled and confined in a magneto-optical trap. Optical pumping and photoionization techniques are used to generate and measure, in situ, a highly spin-polarized (96.5(0.8)%) sample of the short-lived β+-emitter 37K. The angular distribution of neutrinos from this polarized decay, inferred from the daughter recoil asymmetry, is used to search for a hypothetical V+A current in the weak interaction. We find the ν asymmetry parameter to be Bν=−0.755±0.020(stat)±0.013(syst), in agreement with the standard model's purely V-A interaction.