Mesonic enhancement of the weak axial-vector current evaluated from β decay in the lead region

A shell-model study is made of first-forbidden β decay in A=205–212 nuclei. A least-squares fit for eighteen ΔJ=0 and 1 decays gave a scaling factor for the rank-one contributions of 0.97±0.06, i.e., agreement with experiment, and an enhancement factor ${\mathrm{\epsilon }}_{\mathrm{MEC}}$ for the rank-zero matrix element of ${\gamma }_5$ of 2.01±0.05, indicating an enhancement by 100% over the impulse approximation. A 40% effect is predicted from meson exchange. Thus, a deficiency in the meson-exchange calculation or some further unforeseen effect is suggested.