Constraints on parity-mixing matrix elements from hard-pion exchange corrections to first-forbidden beta decays

Abstract

The author concentrates on some sources of uncertainty contained in the procedure for extracting parity-mixing matrix elements from meson-exchange corrections to beta decays of neighbouring isobar-analogue states. He shows that non-soft pion corrections to the weak axial vector current destroy the analogy between the pi -exchange part of the parity non-conserving weak NN potential and the nuclear two-body chiral charge density. Further, he summarises the work of several authors in calculating first-forbidden beta -decay transitions to argue that the dependence of the scaling parameter on the radial shape of the single-particle wavefunctions can introduce an error of approximately 10%. In combining both effects a summed uncertainty of about 30% appears in extracting the upper bound for the weak isovector ( pi NN) coupling constant mod fpi 1 mod . Finally, the importance of the heavy-meson exchanges in the weak parity-violating NN potential is discussed in order to understand qualitatively more recent experimental results for the parity-mixing matrix element (18F(0-, T=0; 1.081 MeV) mod VPNC( Delta T=1) mod 18F(0+, T=1; 1.042 MeV)) reported to be a great deal smaller than the earlier data.