pi^- decay rates of p-shell hypernuclei revisited

Explicit expressions for the parity-violating s-wave and the parity-conserving p-wave contributions to pi- weak-decay rates of Lambda hypernuclei in the 1p shell are given in the weak-coupling limit, to update previous shell-model calculations and to compare with recent pi- spectra and total decay rates measured by the FINUDA Collaboration. A useful sum rule for the summed strength of Lambda_{1s} --> pi- + proton_{1p} weak decays is derived. Fair agreement between experiment and calculations is reached, using the primary s-wave amplitude and Cohen-Kurath nuclear wavefunctions. The role of the p-wave amplitude is studied in detail for Lambda-15N and found to be secondary. Previous assignments of ground-state spin-parity values are confirmed and a new assignment is made, spin-parity 3/2+ for Lambda-15N, based on the substantial suppression calculated here for spin-parity 1/2+.