Exclusive measurement of the nonmesonic weak decay of the lambda(5)He hypernucleus.

We performed a coincidence measurement of two nucleons emitted from the nonmesonic weak decay of {sub {lambda}}{sup 5}He formed via the {sup 6}Li({pi}{sup +},K{sup +}) reaction. The energies of the 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 weak reactions of {lambda}p{yields}np and {lambda}n{yields}nn, respectively. The ratio of the nucleon pair numbers was N{sub nn}/N{sub np}=0.45{+-}0.11(stat){+-}0.03(syst) in the kinematic region of cos{theta}{sub NN}<-0.8. Since each decay mode was exclusively detected, the measured ratio should be close to the ratio of {gamma}({lambda}p{yields}np)/{gamma}({lambda}n{yields}nn). The ratio is consistent with recent theoretical calculations based on the heavy meson and/or direct-quark exchange picture.