We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay (NMWD) of Lambda(5)He and Lambda(12)C formed via the (pi(+), K+) reaction. In both of n + p and n + n pair coincidence spectra, we observed a clean back-to-back correlation coming from the two-body decay of Lambda p -> np and Lambda n -> nn, respectively. We obtained the ratio of the nucleon pair numbers, N-nn/N-np (Lambda(5)He) = 0.45 +/- 0.11 (stat) 0.03 +/- (syst) in the kinematic region of cos theta(NN)< -0.8. Since each decay mode was exclusively detected, the measure ratio should be close to the ratio of Gamma(Lambda p -> nn)/Gamma(Lambda n -> np). The Gamma(n)/Gamma(p) ratio was measured also for the NMWD of Lambda(12)C. It is also close to 0.5. Those ratios are consistent with recent theoretical calculations based on the heavy meson/direct quark exchange picture. (c) 2005 Elsevier B.V. All rights reserved.
展开▼
