Hypernuclear weak decay experiments at KEK; nucleon inclusive measurement

Recent high quality neutron spectrum of _Λ¹²C made it possible to derive the Γ_n/Γ_p ratio directly comparing its yield to that of the previous proton spectrum. The derived ratio, (0.45 - 0.51) ± 0.15, was significantly smaller than unity showing the dominance of the proton induced NMWD channel for the first time and agreed well with the recent theoretical calculations. However, this ratio still has inherent ambiguity due to the two nucleon induced NMWD and the nuclear final state interaction (FSI). Recently exclusive coincidence measurements of each NMWD channel of _Λ⁵He and _Λ¹²C were made in order to reduce such ambiguities and confirmed the proton channel dominance in both inclusive and exclusive data. Important progress also has been made in the measurement of the decay asymmetry parameter, α_p^NM, of the NMWD of _Λ⁵ _Λ¹²C with much improved statistics over previous experiments. We have obtained small α_p^NM values for both _Λ⁵He and _Λ¹²C, close to zero within one a sigma level. The nearly identical α_p^NM values of _Λ⁵He and _Λ¹²C indicates that the NMWD mechanisms of s- and p-shell hypernuclei are similar. However, the theoretical results for α_p^NM in the range of -0.6-(-0.7). Though these new results seem to indicate that the long standing inconsistency between the Γ_n/Γ_n values of experiments and theories has almost been resolved, the decay asymmetry still remains to be understood.