Coincidence Exclusive Measurement of the Nonmesonic Weak Decay of Λ12C

We have measured the angular correlation of the pair nucleons np and nn emitted from the nonmesonic weak decay (NMWD) of Λ12C produced via the (π+,K+) reaction in coincidence measurement. The Λp → np and Λn → nn modes were clearly identified by measuring the back-to-back correlation of the emitted nucleon pairs which is the characteristic of two-body kinematics. From the measured nucleon pair numbers Nnn and Nnp, the ratio Γn/Γp of the partial decay widths Γn(Λn → nn) and Γp(Λp → np) of Λ12C was extracted to be 0.51 ± 0.13(stat)±0.04(syst); this result is almost free from the ambiguity due to the nuclear final state interaction and 3-body decay process, which were inherent in the previous results. The obtained Γn/Γp ratio of Λ12C (p-shell) is close to that of Λ5He (s-shell). The results are consistent with those of recent theoretical calculations