Nucleon-nucleon coincidence measurement in the non-mesonic weak decay of   5_Lambda-He and 12_Lambda-C hypernuclei
  We have measured both yields of neutron-proton and neutron-neutron pairs emitted from the non-mesonic weak decay process of 5_Lambda-He and 12_Lambda-C hypernuclei produced via the (pi^+,K^+) reaction for the first time. We observed clean back-to-back correlation of the np- and nn-pairs in the coincidence spectra for both hypernuclei. The ratio of those back-to-back pair yields, Nnn / Nnp, must be close to the ratio of neutron- and proton-induced decay widths of the decay, Gn(Lambda n -> nn)/Gp(Lambda p -> np). The obtained ratios for each hypernuclei support recent calculations based on short-range interactions. 
S. Okada, S. Ajimura, K. Aoki, A. Banu, H.C. Bhang, T. Fukuda, O.   Hashimoto, J.I. Hwang, S. Kameoka, B.H. Kang, E.H. Kim, ...