l - ex / 0 60 10 29 v 1 2 1 Ja n 20 06 Coincidence Measurement of the Nonmesonic Weak Decay of 12 Λ C

We have measured the angular correlation of the pair nucleons np and nn emitted from the nonmesonic weak decay (NMWD) of 12 Λ C 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 12 Λ C was extracted to be 0.51 ± 0.13(stat)±0.04(syst); this Preprint submitted to Elsevier Science 9 March 2008 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 12 Λ C (p-shell) is close to that of 5 ΛHe (s-shell). The results are consistent with those of recent theoretical calculations.