First-forbidden β decay: Meson-exchange enhancement of the axial charge atA∼16

Calculations are presented for four relatively strong first-forbidden β decays in the region A=11–16 in order to study the very large mesonic-exchange-current enhancement of the rank-zero components. The ${\mathrm{\mu }}^{\mathrm{-}}$ capture on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ is considered on the same footing. The wave functions utilized include up to 4ħω excitations. Two-body exchange-current matrix elements are calculated as well as one-body impulse-approximation matrix elements. The resultant enhancement factor that multiplies the impulse-approximation axial-charge matrix element is thereby determined by comparison to experiment to be ${\mathrm{\epsilon }}_{\exp }$=1.61±0.03 from three ${\mathrm{\beta }}^{\mathrm{-}}$ decays and ${\mathrm{\mu }}^{\mathrm{-}}$ capture on ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$, which is in excellent agreement with meson-exchange calculations in the soft-pion approximation.