First-Forbidden beta decay: Medium enhancement of the axial charge at A=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 µ − capture on 16 O is considered on the same footing. The wave functions utilized include up to 4¯ hω 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 ǫ exp = 1.61 ± 0.03 from three β − decays and µ − capture on 16 O, which is in excellent agreement with meson-exchange calculations in the soft-pion approximation.