Spectral shapes of forbidden argon beta decays as background component for rare-event searches

摘要

The spectral shape of the electrons from the two first-forbidden unique beta(-) decays of Ar-39 and Ar-42 were calculated for the first time to the next-to-leading order. Especially the spectral shape of the Ar-39 decay can be used to characterize this background component for dark matter searches based on argon. Alternatively, due to the low thresholds of these experiments, the spectral shape can be investigated over a wide energy range with high statistics and thus allow a sensitive comparison with the theoretical predictions, in particular at low electron energies where the shape of the computed beta spectrum has a slight dependence on the value of the weak axial-vector coupling constant.