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

The positron-neutrino correlation in the 0(+) --> 0(+) beta -decay of Ar-32 was measured at ISOLDE by analyzing the effect of lepton recoil on the sha pe of the narrow proton group following the super-allowed decay. Our result is consistent with the standard model prediction; for vanishing Fierz inte rference we find a = 0.9989 +/- 0.0052 +/- 0.0039. Our result leads to impr oved constraints on scalar weak interactions. The positron-neutrino correla tion in Ar-33 decay was measured in the same experiment; for vanishing Fier z interference we find a = 0.944 +/- 0.002 +/- 0.003. The Ar-32 and Ar-33 c orrelations, in combination with precision measurements of the half-lives, super-allowed branching ratios and beta endpoint energies, will determine t he 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.