Non-mesonic weak decays of {sub {lambda}}{sup 5}He and {sub {lambda}}{sup 12}C hypernuclei using the ({pi}{sup +},K{sup +}) reactions

We have measured for the first time yields of both neutron-proton and neutron-neutron pairs emitted from the non-mesonic weak decay of the {sub {lambda}}{sup 5}He and {sub {lambda}}{sup 12}C hypernuclei, produced via the ({pi}{sup +},K{sup +}) reaction. From the yields of both the back-to-back np-pair events and nn-pair events, we have directly determined the ratio of the neutron- and proton-induced decay widths, {gamma}{sub n}({lambda}n{yields}nn)/{gamma}{sub p}({lambda}p{yields}np). Meanwhile, we have also determined the quenching of the observed numbers of single nucleons and coincident nucleon pairs compared with theoretical calculations. Our results suggest that contributions from the two-nucleon-induced three-body process {lambda}NN{yields}NNN are significant.