Recent experimental results of weak decay of Λ hypernuclei and the long standing Γ_n/Γ_p puzzle

In spite of notable progresses achieved at BNL and KEK during last decade, a consistent understanding of nonmesonic weak decay (NMWD) of Λ hypernuclei is yet to be achieved. Until recently, all the experimental data since the days of bubble chamber and emulsion showed the dominance of the neutron induced channel (Γ_n) over the proton induced one (Γ_p) in the NMWD of Λ hypernuclei, while the theoretical calculation predicted the predominance of the proton induced one. This inconsistency has been known as the famous 'Γ_n/Γ_p puzzle' and the central focus in the study of the decay mechanism of the Λ hypernuclei and the baryonic weak interaction. However, important progresses have been made recently in both the experimental and theoretical sides. In this paper, the progresses in the experimental data on NMWD will be presented. The direct comparison of the recent proton and neutron spectra showed the Γ_n/Γ_p ratio, 0.45 - 0.51 ± 0.15 (stat.) much less than unity. This ratio agrees well with the recent theoretical calculations. Though this outcome seems to indicate that the famous long standing puzzle has almost been resolved, the decay asymmetry still remains to be understood.