Coincidence measurement of the nonmesonic weak decay of 12ΛC

We have measured the angular correlation of the pair nucleons np and nn emitted from the nonmesonic weak decay (NMWD) of 12ΛC 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 12ΛC was extracted to be 0.51±0.13(stat)±0.05(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 12ΛC (p-shell) is close to that of 5ΛHe (s-shell). The results are consistent with those of recent theoretical calculations.