Exclusive Measurement of the Nonmesonic Weak Decay of ^{5}_{\Lambda}He   Hypernucleus
  We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay (NMWD) of ^{5}_{\Lambda}He formed via the ^{6}Li(\pi^+,K^+) reaction. The energies of two nucleons and the pair number distributions in the opening angle between them were measured. In both np and nn pairs, we observed a clean back-to-back correlation coming from the two-body decay of \Lambda p --> n p and \Lambda n --> n n, respectively. The ratio of the nucleon pair numbers was N_{nn}/N_{np}=0.45 \pm 0.11(stat)\pm 0.03(syst) in the kinematic region of cos(theta_{NN}) < -0.8. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of \Gamma(\Lambda p --> np)/\Gamma(\Lambda n --> nn).   The ratio is consistent with recent theoretical calculations based on the heavy meson/direct quark exchange picture. 
B.H.Kang, S.Ajimura, K.Aoki, A.Banu, H.Bhang, T.Fukuda, O.Hashimoto,   J.I.Hwang, S.Kameoka, E.H.Kim, J.H.Kim, M.J.Kim, T.Ma...