Recent progresses in the study of nonmesonic weak decay of λ hypernuclei

Fundamental interest on Non Mesonic Weak Decay (NMWD) is that it provides practically the only means to study the elementary S = 1 baryon-baryon weak interaction process, ΛN→NN (Λp → np and Λn → nn), since it is difficult to be realized in the free space. Thanks to the recent developments in experimental and theoretical studies, the relative strength of two channels, the Γ_n/Γ_p ratios whose experimental and theoretical values have showed a long standing discrepancy have been converged to ∼ 0.5 and the discrepancy problem finally has been solved.¹³ However, despite the progresses, the decay interaction of NMWD is not yet well established and there remain the important issues such as the discrepancy of asymmetry parameter between experimental and theoretical values, confirmation of 3-body NMWD process, and whether the I = 1/2 rule for S = 1 decay would hold or not in NMWD. Especially the recent experimental indication of the 3-body decay process ΛNN→NNN whose contribution was predicted to be significant in the theoretical calculations seems to be strong and showed a surprisingly large contribution in the quenching of singles and coincidence nucleon yields in NMWD. Recent results of the asymmetry parameter α^nm of NMWD showed small values for both s-shell Λ⁵ He and p-shell Λ¹²C and consistent with each other. However, the discrepancy of the theoretical values from those of experimental ones remains to be understood. It now becomes one of the urgent issues in NMWD study to determine the contribution of the 3-body NMWD channel experimentally, in order to understand the discrepancy of the asymmetry papameter, and to test the I = 1/2 rule for NMWD which are the main purposes of J-PARC 50 GeV PS experiment, E18 and E22.