Coincidence Measurement of the Nonmesonic Weak Decay of ¹²LambdaC

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We have measured the angular correlation of the pair nucleons np and nn emitted from the nonmesonic weak decay (NMWD) of ¹²LambdaC produced via the (pi+,K+) reaction in coincidence measurement. The Lambda p -> np and Lambda n -> 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 Gammaₙ/Gammaₚ of the partial decay widths Gammaₙ(Lambda n -> nn) and Gammaₚ(Lambda p -> np) of ¹²LambdaC 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 Gammaₙ/Gammaₚ ratio of ¹²LambdaC (p-shell) is close to that of ⁵LambdaHe (s-shell). The results are consistent with those of recent theoretical calculations.