Hypernuclear weak decay of 12 Lambda C and 11 Lambda B.

The branching ratios of negative pions and protons due to the weak decays of {sup 12}{sub {Lambda}}C and {sup 11}{sub {Lambda}}B hypernuclei were measured. The negative pionic decay rates ({Gamma}{sub {pi}{sup {minus}}}), proton-induced nonmesonic decay rates ({Gamma}{sub {ital p}}), total nonmesonic decay rates ({Gamma}{sub {ital nm}}), and ratios of the {Lambda}{ital n}{r_arrow}{ital nn} process to the {Lambda}{ital p}{r_arrow}{ital pn} process ({Gamma}{sub {ital n}}/{Gamma}{sub {ital p}}) were derived from the present measurements combined with previous studies on the mesonic {pi}{sup 0} and total decay rates. The measured {Gamma}{sub {pi}{sup {minus}}} shows that Pauli blocking is less effective than expected due to pion distortion. We found that {Gamma}{sub {ital nm}} is nearly equal to unity in units of the free-space {Lambda}-decay width. The present results also indicate that {Gamma}{sub {ital n}} is almost twice that of {Gamma}{sub {ital p}}. This is in disagreement with calculations based on meson-exchange models in which {Gamma}{sub {ital n}} is strongly suppressed.