Positron-neutrino correlations in Ar-32 and Ar-33 decays: Probes of scalar weak currents and nuclear isospin mixing

Abstract

The positron-neutrino correlation in the 0(+) --> 0(+) β-decay of Ar-32 was measured at ISOLDE by analyzing the effect of lepton recoil on the shape of the narrow proton group following the super-allowed decay. Our result is consistent with the standard model prediction; for vanishing Fierz interference we find a = 0.9989 ± 0.0052 ± 0.0039. Our result leads to improved constraints on scalar weak interactions. The positron-neutrino correlation in Ar-33 decay was measured in the same experiment; for vanishing Fierz interference we find a = 0.944 ± 0.002 ± 0.003. The Ar-32 and Ar-33 correlations, in combination with precision measurements of the half-lives, super-allowed branching ratios and beta endpoint energies, will determine the isospin impurities of the super-allowed transitions. These will provide useful tests of isospin-violation corrections used in deducing |V-ud| which currently indicates non-unitarity of the KM matrix.