Measurement of the β-ν correlation of Na21 using shakeoff electrons

The β-ν correlation coefficient, a_βν, is measured in Na21 by detecting the time of flight of the recoil nucleus detected in coincidence with the atomic electrons shaken off in β decay. The sample of Na21 is confined in a magneto-optic trap. High detection efficiency allows low trap density, which suppresses the photoassociation of molecular sodium, which can cause a large systematic error. Suppressing the fraction of trapped atoms in the excited state by using a dark trap also reduces the photoassociation process, and data taken with this technique are consistent. The main remaining systematic uncertainties come from the measurement of the position and size of the atom trap and the subtraction of background. We find a_βν=0.5502(60), in agreement with the Standard Model prediction of a_βν=0.553(2), and disagreeing with a previous measurement, which was susceptible to an error introduced by the presence of molecular sodium.